[Illustration: _Dunes at Ipswich Light, Massachusetts. Note theeffect of bushes in arresting the movement of the wind-blown sand. _]

 OUTLINES OF THE EARTH'S HISTORY

 A POPULAR STUDY IN PHYSIOGRAPHY

 BY

 NATHANIEL SOUTHGATE SHALER

 PROFESSOR OF GEOLOGY IN HARVARD UNIVERSITY DEAN OF LAWRENCE SCIENTIFIC SCHOOL

 ILLUSTRATED WITH INDEX

 NEW YORK AND LONDON D. APPLETON AND COMPANY

 1898, 1910

 PREFACE. 

The object of this book is to provide the beginner in the study of theearth's history with a general account of those actions which can bereadily understood and which will afford him clear understandings asto the nature of the processes which have made this and othercelestial spheres. It has been the writer's purpose to select thoseseries of facts which serve to show the continuous operations ofenergy, so that the reader might be helped to a truer conception ofthe nature of this sphere than he can obtain from ordinary text-books. 

In the usual method of presenting the elements of the earth's historythe facts are set forth in a manner which leads the student toconceive that history as in a way completed. The natural prepossessionto the effect that the visible universe represents something done, rather than something endlessly doing, is thus re-enforced, with theresult that one may fail to gain the largest and most educativeimpression which physical science can afford him in the sense of theswift and unending procession of events. 

It is well known to all who are acquainted with the history of geologythat the static conception of the earth--the idea that its existingcondition is the finished product of forces no longer in action--ledto prejudices which have long retarded, and indeed still retard, theprogress of that science. This fact indicates that at the outset of astudent's work in this field he should be guarded against suchmisconceptions. The only way to attain the end is by bringing to theunderstanding of the beginner a clear idea of successions of eventswhich are caused by the forces operating in and on this sphere. Of allthe chapters of this great story, that which relates to the history ofthe work done by the heat of the sun is the most interesting andawakening. Therefore an effort has been made to present the greatsuccessive steps by which the solar energy acts in the processes ofthe air and the waters. 

The interest of the beginner in geology is sure to be aroused when hecomes to see how very far the history of the earth has influenced thefate of men. Therefore the aim has been, where possible, to show theways in which geological processes and results are related toourselves; how, in a word, this earth has been the well-appointednursery of our kind. 

All those who are engaged in teaching elementary science learn theneed of limiting the story they have to tell to those truths which canbe easily understood by beginners. It is sometimes best, as in statingsuch difficult matters as those concerning the tides, to giveexplanations which are far from complete, and which, as to their modeof presentation, would be open to criticism were it not for the factthat any more elaborate statements would most likely beincomprehensible to the novice, thus defeating the teacher's aim. 

It will be observed that no account is here given of the geologicalages or of the successions of organic life. Chapters on these subjectswere prepared, but were omitted for the reason that they made thestory too long, and also because they carried the reader into a fieldof much greater difficulty than that which is found in the physicalhistory of the earth. 

 N. S. S. _March, 1898. _

 CONTENTS. 

 CHAPTER PAGE

 I. --INTRODUCTION TO THE STUDY OF NATURE 1 II. --WAYS AND MEANS OF STUDYING NATURE 9 III. --THE STELLAR REALM 31 IV. --THE EARTH 81 V. --THE ATMOSPHERE 97 VI. --GLACIERS 207 VII. --THE WORK OF UNDERGROUND WATER 250 VIII. --THE SOIL 313 IX. --THE ROCKS AND THEIR ORDER 349

 LIST OF FULL-PAGE ILLUSTRATIONS. 

 FACING PAGE

 Dunes at Ipswich Light, Massachusetts _Frontispiece_ Seal Rocks near San Francisco, California 33 Lava stream, in Hawaiian Islands, flowing into the sea 72 Waterfall near Gadsden, Alabama 90 South shore, Martha's Vineyard, Massachusetts 121 Pocket Creek, Cape Ann, Massachusetts 163 Muir Glacier, Alaska 207 Front of Muir Glacier 240 Mount Ætna, seen from near Catania 201 Mountain gorge, Himalayas, India 330

 OUTLINES OF THE EARTH'S HISTORY. 

 CHAPTER I. 

 AN INTRODUCTION TO THE STUDY OF NATURE. 

The object of this book is to give the student who is about to enteron the study of natural science some general idea as to the conditionsof the natural realm. As this field of inquiry is vast, it will bepossible only to give the merest outline of its subject-matter, notingthose features alone which are of surpassing interest, which aredemanded for a large understanding of man's place in this world, orwhich pertain to his duties in life. 

In entering on any field of inquiry, it is most desirable that thestudent should obtain some idea as to the ways in which men have beenled to the knowledge which they possess concerning the world aboutthem. Therefore it will be well briefly to sketch the steps by whichnatural science has come to be what it is. By so doing we shallperceive how much we owe to the students of other generations; and bynoting the difficulties which they encountered, and how they avoidedthem, we shall more easily find our own way to knowledge. 

The primitive savages, who were the ancestors of all men, howevercivilized they may be, were students of Nature. The remnants of theselowly people who were left in different parts of the world show usthat man was not long in existence before he began to devise someexplanation concerning the course of events in the outer world. Seeing the sun rise and set, the changes of the moon, the alternationof the seasons, the incessant movement of the streams and sea, and theother more or less orderly successions of events, our primitiveforefathers were driven to invent some explanation of them. This, independently, and in many different times and places, they did in asimple and natural way by supposing that the world was controlled by ahost of intelligent beings, each of which had some part in orderingmaterial things. Sometimes these invisible powers were believed to bethe spirits of great chieftains, who were active when on earth, andwho after death continued to exercise their power in the larger realmsof Nature. Again, and perhaps more commonly, these movements of Naturewere supposed to be due to the action of great though invisiblebeasts, much like those which the savage found about him. Thus amongour North American Indians the winds are explained by the suppositionthat the air is fanned by the wings of a great unseen bird, whose dutyit is to set the atmosphere into motion. That no one has ever seen thebird doing the work, or that the task is too great for any conceivablebird, is to the simple, uncultivated man no objection to this view. Itis long, indeed, before education brings men to the point where theycan criticise their first explanations of Nature. 

As men in their advance come to see how much nobler are their ownnatures than those of the lower animals, they gradually put aside theexplanation of events by the actions of beasts, and account for theorder of the world by the supposition that each and every importantdetail is controlled by some immortal creature essentially like a man, though much more powerful than those of their own kind. This stage ofunderstanding is perhaps best shown by the mythology of the Greeks, where there was a great god over all, very powerful but notomnipotent; and beneath him, in endless successions of command, subordinate powers, each with a less range of duties and capacitiesthan those of higher estate, until at the bottom of the system therewere minor deities and demigods charged with the management of thetrees, the flowers, and the springs--creatures differing little fromman, except that they were immortal, and generally invisible, thoughthey, like all the other deities, might at their will displaythemselves to the human beings over whom they watched, and whose pathin life they guided. 

Among only one people do we find that the process of advance ledbeyond this early and simple method of accounting for the processes ofNature, bringing men to an understanding such as we now possess. Thisgreat task was accomplished by the Greeks alone. About twenty-fivehundred years ago the philosophers of Greece began to perceive thatthe early notion as to the guidance of the world by creaturesessentially like men could not be accepted, and must be replaced bysome other view which would more effectively account for the facts. This end they attained by steps which can not well be related here, but which led them to suppose separate powers behind each of thenatural series--powers having no relation to the qualities of mankind, but ever acting to a definite end. Thus Plato, who represents mostclearly this advance in the interpretation of facts, imagined thateach particular kind of plant or animal had its shape inevitablydetermined by something which he termed an idea, a shape-giving powerwhich existed before the object was created, and which would remainafter it had been destroyed, ever ready again to bring matter to theparticular form. From this stage of understanding it was but a shortstep to the modern view of natural law. This last important advancewas made by the great philosopher Aristotle, who, though he died abouttwenty-two hundred years ago, deserves to be accounted the first andin many ways the greatest of the ancient men of science who wereinformed with the modern spirit. 

With Aristotle, as with all his intellectual successors, theoperations of Nature were conceived as to be accounted for by theaction of forces which we commonly designate as natural laws, of whichperhaps the most familiar and universal is that of gravitation, whichimpels all bodies to move toward each other with a degree of intensitywhich is measured by their weight and the distance by which they areseparated. 

For many centuries students used the term law in somewhat the same wayas the more philosophical believers in polytheism spoke of their gods, or as Plato of the ideas which he conceived to control Nature. We seeby this instance how hard it is to get rid of old ways of thinking. Even when the new have been adopted we very often find that somethingof the ancient and discarded notions cling in our phrases. The moreadvanced of our modern philosophers are clear in their mind that allwe know as to the order of Nature is that, given certain conditions, certain consequences inevitably follow. 

Although the limitations which modern men of science perceive to beput upon their labours may seem at first sight calculated to confineour understanding within a narrow field of things which can be seen, or in some way distinctly proved to exist, the effect of thislimitation has been to make science what it is--a realm of thingsknown as distinct from things which may be imagined. All thedifference between ancient science and modern consists in the factthat in modern science inquirers demand a businesslike method in theinterpretation of Nature. Among the Greeks the philosopher who taughtexplanations of any feature in the material world which interested himwas content if he could imagine some way which would account for thefacts. It is the modern custom now to term the supposition of anexplanation a _working hypothesis_, and only to give it the name oftheory after a very careful search has shown that all the facts whichcan be gathered are in accordance with the view. Thus when Newton madehis great suggestion concerning the law of gravitation, which was tothe effect that all bodies attracted each other in proportion totheir masses, and inversely as the square of their distance from eachother, he did not rest content, as the old Greeks would have done, with the probable truth of the explanation, but carefully explored themovements of the planets and satellites of the solar system to see ifthe facts accorded with the hypothesis. Even the perfectcorrespondence which he found did not entirely content inquirers, andin this century very important experiments have been made which haveserved to show that a ball suspended in front of a precipice will beattracted toward the steep, and that even a mass of lead some tons inweight will attract toward itself a small body suspended in the mannerof a pendulum. 

It is this incessant revision of the facts, in order to see if theyaccord with the assumed rule or law, which has given modern sciencethe sound footing that it lacked in earlier days, and which haspermitted our learning to go on step by step in a safe way up theheights to which it has climbed. All explanations of Nature begin withthe work of the imagination. In common phrase, they all are guesseswhich have at first but little value, and only attain importance inproportion as they are verified by long-continued criticism, which hasfor its object to see whether the facts accord with the theory. It isin this effort to secure proof that modern science has gathered theenormous store of well-ascertained facts which constitutes its truewealth, and which distinguishes it from the earlier imaginative and toa great extent unproved views. 

In the original state of learning, natural science was confounded withpolitical and social tradition, with the precepts of duty whichconstitute the law of the people, as well as with their religion, thewhole being in the possession of the priests or wise men. So long asnatural action was supposed to be in the immediate control of numerousgods and demigods, so long, in a word, as the explanation of Naturewas what we term polytheistic, this association of science with otherforms of learning was not only natural but inevitable. Gradually, however, as the conception of natural law replaced the earlier idea asto the intervention of a spirit, science departed from other forms oflore and came to possess a field to itself. At first it was one bodyof learning. The naturalists of Aristotle's time, and from his daydown to near our own, generally concerned themselves with the wholefield of Nature. For a time it was possible for any one able andlaborious man to know all which had been ascertained concerningastronomy, chemistry, geology, as well as the facts relating to livingbeings. The more, however, as observation accumulated, and the storeof facts increased, it became difficult for any one man to know thewhole. Hence it has come about that in our own time natural learningis divided into many distinct provinces, each of which demands alifetime of labour from those who would know what has already beendone in the field, and what it is now important to do in the way ofnew inquiries. 

The large divisions which naturalists have usually made of their tasksrest in the main on the natural partitions which we may readilyobserve in the phenomenal world. First of all comes astronomy, including the phenomena exhibited in the heavens, beyond the limits ofthe earth's atmosphere. Second, geology, which takes account of allthose actions which in process of time have been developed in our ownsphere. Third, physics, which is concerned with the laws of energy, orthose conditions which affect the motion of bodies, and the changeswhich are impressed upon them by the different natural forces. Fourth, chemistry, which seeks to interpret the principles which determine thecombination of atoms and the molecules which are built of them underthe influence of the chemical affinities. Fifth, biology, or the lawsof life, a study which pertains to the forms and structures of animalsand plants, and their wonderful successions in the history of theworld. Sixth, mathematics, or the science of space and number, thatdeals with the principles which underlie the order of Nature asexpressed at once in the human understanding and in the materialuniverse. By its use men were made able to calculate, as inarithmetic, the problems which concern their ordinary business, aswell as to compute the movements of the celestial bodies, and a hostof actions which take place on the earth that would be inexplicableexcept by the aid of this science. Last of all among the primarysciences we may name that of psychology, which takes account of mentaloperations among man and his lower kindred, the animals. 

In addition to the seven sciences above mentioned, which rest in agreat measure on the natural divisions of phenomena, there are many, indeed, indefinitely numerous, subdivisions which have been made tosuit the convenience of students. Thus astronomy is often separatedinto physical and mathematical divisions, which take account either ofthe physical phenomena exhibited by the heavenly bodies or of theirmotions. In geology there are half a dozen divisions relating toparticular branches of that subject. In the realm of organic life, inchemistry, and in physics there are many parts of these sciences whichhave received particular names. 

It must not be supposed that these sciences have the independence ofeach other which their separate names would imply. In fact, thestudent of each, however, far he may succeed in separating his fieldfrom that of the other naturalists, as we may fitly term all studentsof Nature, is compelled from time to time to call in the aid of hisbrethren who cultivate other branches of learning. The modernastronomer needs to know much of chemistry, or else he can notunderstand many of his observations on the sun. The geologists have toshare their work with the student of animal and vegetable life, withthe physicists; they must, moreover, know something of the celestialspheres in order to interpret the history of the earth. In fact, dayby day, with the advance of learning, we come more clearly toperceive that all the processes of Nature are in a way related to eachother, and that in proportion as we understand any part of the greatmechanism, we are forced in a manner to comprehend the whole. In otherwords, we are coming to understand that these divisions of the fieldof science depend upon the limitations of our knowledge, and not uponthe order of Nature itself. For the purposes of education it isimportant that every one should know something of the great truthswhich each science has disclosed. No mortal man can compass the wholerealm of this knowledge, but every one can gain some idea of thelarger truths which may help him to understand the beauty and grandeurof the sphere in which he dwells, which will enable him the better tomeet the ordinary duties of life, that in almost all cases are relatedto the facts of the world about us. It has been of late the custom toterm this body of general knowledge which takes account of the moreevident facts and important series of terrestrial actionsphysiography, or, as the term implies, a description of Nature, withthe understanding that the knowledge chosen for the account is thatwhich most intimately concerns the student who seeks information thatis at once general and important. Therefore, in this book the effortis made first to give an account as to the ways and means which haveled to our understanding of scientific problems, the methods by whicheach person may make himself an inquirer, and the outline of theknowledge that has been gathered since men first began to observe andcriticise the revelations the universe may afford them. 

 CHAPTER II. 

 WAYS AND MEANS OF STUDYING NATURE. 

It is desirable that the student of Nature keep well in mind the meanswhereby he is able to perceive what goes on in the world about him. Heshould understand something as to the nature of his senses, and theextent to which these capacities enable him to discern the operationsof Nature. Man, in common with his lower kindred, is, by the mechanismof the body, provided with five somewhat different ways by which hemay learn something of the things about him. The simplest of thesecapacities is that of touch, a faculty that is common to the generalsurface of the body, and which informs us when the surface is affectedby contact with some external object. It also enables us to discerndifferences of temperature. Next is the sense of taste, which islimited to the mouth and the parts about it. This sense is in a wayrelated to that of touch, for the reason that it depends on thecontact of our body with material things. Third is the sense of smell, so closely related to that of taste that it is difficult to draw theline between the two. Yet through the apparatus of the nose we canperceive the microscopically small parts of matter borne to us throughthe air, which could not be appreciated by the nerves of the mouth. Fourth in order of scope comes the hearing, which gives us an accountof those waves of matter that we understand as sound. This power ismuch more far ranging than those before noted; in some cases, as inthat of the volcanic explosions from the island of Krakatoa, in theeruption of 1883, the convulsions were audible at the distance ofmore than a thousand miles away. The greater cannon of modern days maybe heard at the distance of more than a hundred miles, so that whilethe sense of touch, taste, and smell demand contact with the bodieswhich we appreciate, hearing gives us information concerning objectsat a considerable distance. Last and highest of the senses, vastly themost important in all that relates to our understanding of Nature, issight, or the capacity which enables us to appreciate the movement ofthose very small waves of ether which constitute light. The eminentpeculiarity of sight is that it may give us information concerningthings which are inconceivably far away; it enables us to discern thelight of suns probably millions of times as remote from us as is thecentre of our own solar system. 

Although much of the pleasure which the world affords us comes throughthe other senses, the basis of almost all our accurate knowledge isreported by sight. It is true that what we have observed with our eyesmay be set forth in words, and thus find its way to the understandingthrough the ears; also that in many instances the sense of touchconveys information which extends our perceptions in many importantways; but science rests practically on sight, and on the insight thatcomes from the training of the mind which the eyes make possible. 

The early inquirers had no resources except those their bodiesafforded; but man is a tool-making creature, and in very early days hebegan to invent instruments which helped him in inquiry. The earliestdeliberate study was of the stars. Science began with astronomy, andthe first instruments which men contrived for the purpose ofinvestigation were astronomical. In the beginning of this search thestars were studied in order to measure the length of the year, andalso for the reason that they were supposed in some way to control thefate of men. So far as we know, the first pieces of apparatus for thispurpose were invented in Egypt, perhaps about four thousand yearsbefore the Christian era. These instruments were of a simple nature, for the magnifying glass was not yet contrived, and so the telescopewas impossible. They consisted of arrangements of straight edges anddivided circles, so that the observers, by sighting along theinstruments, could in a rough way determine the changes in distancebetween certain stars, or the height of the sun above the horizon atthe various seasons of the year. It is likely that each of the greatpyramids of Egypt was at first used as an observatory, where thepriests, who had some knowledge of astronomy, found a station for theapparatus by which they made the observations that served as a basisfor casting the horoscope of the king. 

In the progress of science and of the mechanical invention attendingits growth, a great number of inventions have been contrived whichvastly increase our vision and add inconceivably to the precision itmay attain. In fact, something like as much skill and labour has beengiven to the development of those inventions which add to our learningas to those which serve an immediate economic end. By far the greatestof these scientific inventions are those which depend upon the lens. By combining shaped bits of glass so as to control the direction inwhich the light waves move through them, naturalists have been able tocreate the telescope, which in effect may bring distant objects somethousand times nearer to view than they are to the naked eye; and themicroscope, which so enlarges minute objects as to make them visible, as they were not before. The result has been enormously to increaseour power of vision when applied to distant or to small objects. Infact, for purposes of learning, it is safe to say that those toolshave altogether changed man's relation to the visible universe. Thenaked eye can see at best in the part of the heavens visible from anyone point not more than thirty thousand stars. With the telescopesomewhere near a hundred million are brought within the limits ofvision. Without the help of the microscope an object a thousandth ofan inch in diameter appears as a mere point, the existence of which wecan determine only under favourable circumstances. With thatinstrument the object may reveal an extended and complicated structurewhich it may require a vast labour for the observer fully to explore. 

Next in importance to the aid of vision above noted come thescientific tools which are used in weighing and measuring. Thesebalances and gauges have attained such precision that intervals sosmall as to be quite invisible, and weights as slight as aten-thousandth of a grain, can be accurately measured. From theseinstruments have come all those precise examinations on which theaccuracy of modern science intimately depends. All these instrumentsof precision are the inventions of modern days. The simplesttelescopes were made only about two hundred and fifty years ago, andthe earlier compound microscopes at a yet later date. Accuratebalances and other forms of gauges of space, as well as good means ofdividing time, such as our accurate astronomical clocks andchronometers, are only about a century old. The instruments have madescience accurate, and have immensely extended its powers in nearly allthe fields of inquiry. 

Although the most striking modern discoveries are in the field whichwas opened to us by the lens in its manifold applications, it is inthe chemist's laboratory that we find that branch of science, longcultivated, but rapidly advanced only within the last two centuries, which has done the most for the needs of man. The ancients guessedthat the substances which make up the visible world were morecomplicated in their organization than they appear to our vision. Theyeven suggested the great truth that matter of all kinds is made up ofinconceivably small indivisible bits which they and we term atoms. Itis likely that in the classic days of Greece men began to make simpleexperiments of a chemical nature. A century or two after the time ofMohammed, the Arabians of his faith, a people who had acquired Greekscience from the libraries which their conquests gave them, conductedextensive experiments, and named a good many familiar chemicalproducts, such as alcohol, which still bears its Arabic name. 

These chemical studies were continued in Europe by the alchemists, aname also of Arabic origin, a set of inquirers who were to a greatextent drawn away from scientific studies by vain though unendingefforts to change the baser metals into gold and silver, as well as tofind a compound which would make men immortal in the body. By theinvention of the accurate balance, and by patient weighing of thematters which they submitted to experiment, by the invention ofhypotheses or guesses at truth, which were carefully tested byexperiment, the majestic science of modern chemistry has come forthfrom the confused and mystical studies of the alchemists. We havelearned to know that there are seventy or more primitive or apparentlyunchangeable elements which make up the mass of this world, andprobably constitute all the celestial spheres, and that these elementsin the form of their separate atoms may group themselves in almostinconceivably varied combinations. In the inanimate realm theseassociations, composed of the atoms of the different substances, forming what are termed molecules, are generally composed of but fewunits. Thus carbonic-acid gas, as it is commonly called, is made up ofan aggregation of molecules, each composed of one atom of carbon andtwo of oxygen; water, of two atoms of hydrogen and one of oxygen;ordinary iron oxide, of two atoms of iron and three of oxygen. In therealm of organic life, however, these combinations become vastly morecomplicated, and with each of them the properties of the substancethus produced differ from all others. A distinguished chemist hasestimated that in one group of chemical compounds, that of carbon, itwould be possible to make such an array of substances that it wouldrequire a library of many thousand ordinary volumes to contain theirnames alone. 

It is characteristic of chemical science that it takes account ofactions which are almost entirely invisible. No contrivances have beenor are likely to be invented which will show the observer what takesplace when the atoms of any substance depart from their previouscombination and enter on new arrangements. We only know that undercertain conditions the old atomic associations break up, and new onesare formed. But though the processes are hidden, the results aremanifest in the changes which are brought about upon the masses ofmaterial which are subjected to the altering conditions. Gradually thechemists of our day are learning to build up in their laboratoriesmore and more complicated compounds; already they have succeeded inproducing many of the materials which of old could only be obtained byextracting them from plants. Thus a number of the perfumes of flowers, and many of the dye-stuffs which a century ago were extracted fromvegetables, and were then supposed to be only obtainable in that way, are now readily manufactured. In time it seems likely that importantarticles of food, for which we now depend upon the seeds of plants, may be directly built up from the mineral kingdom. Thus the result ofchemical inquiry has been not only to show us much of the vast realmof actions which go on in the earth, but to give us control of many ofthese movements so that we may turn them to the needs of man. 

Animals and plants were at an early day very naturally the subjects ofinquiry. The ancients perceived that there were differences of kindamong these creatures, and even in Aristotle's time the sciences ofzoölogy and botany had attained the point where there wereconsiderable treatises on those subjects. It was not, however, until alittle more than a century ago that men began accurately to describeand classify these species of the organic world. Since the time ofLinnæus the growth of our knowledge has gone forward with amazingswiftness. Within a century we have come to know perhaps a hundredtimes as much concerning these creatures as was learned in all theearlier ages. This knowledge is divisible into two main branches: inone the inquirers have taken account of the different species, genera, families, orders, and classes of living forms with such effect thatthey have shown the existence at the present time of many hundredthousand distinct species, the vast assemblage being arranged in aclassification which shows something as to the relationship which theforms bear to each other, and furthermore that the kinds now livinghave not been long in existence, but that at each stage in the historyof the earth another assemblage of species peopled the waters and thelands. 

At first naturalists concerned themselves only with the external formsof living creatures; but they soon came to perceive that the way inwhich these organisms worked, their physiology, in a word, affordedmatters for extended inquiry. These researches have developed thescience of physiology, or the laws of bodily action, on many accountsthe most modern and extensive of our new acquisitions of naturallearning. Through these studies we have come to know something of thelaws or principles by which life is handed on from generation togeneration, and by which the gradations of structure have beenadvanced from the simple creatures which appear like bits of animatedjelly to the body and mind of man. 

The greatest contribution which modern naturalists have made toknowledge concerns the origin of organic species. The students of acentury ago believed that all these different kinds had been suddenlycreated either through natural law or by the immediate will of God. Wenow know that from the beginning of organic life in the remote past tothe present day one kind of animal or plant has been in a natural andessentially gradual way converted into the species which was to be itssuccessor, so that all the vast and complicated assemblage of kindswhich now exists has been derived by a process of change from theforms which in earlier ages dwelt upon this planet. The exact mannerin which these alterations were produced is not yet determined, but inlarge part it has evidently been brought about by the method indicatedby Mr. Darwin, through the survival of the fittest individuals in thestruggle for existence. 

Until men came to have a clear conception as to the spherical form ofthe earth, it was impossible for them to begin any intelligentinquiries concerning its structure or history. The Greeks knew theearth to be a sphere, but this knowledge was lost among the earlyChristian people, and it was not until about four hundred years agothat men again came to see that they dwelt upon a globe. On the basisof this understanding the science of geology, which had in a way beenfounded by the Greeks, was revived. As this science depends upon theknowledge which we have gained of astronomy, physics, chemistry, andbiology, all of which branches of learning have to be used inexplaining the history of the earth, the advance which has been madehas been relatively slow. Geology as a whole is the least perfectlyorganized of all the divisions of learning. A special difficultypeculiar to this science has also served to hinder its development. All the other branches of learning deal mainly, if not altogether, with the conditions of Nature as they now exist. In this alone is itnecessary at every step to take account of actions which have beenperformed in the remote past. 

It is an easy matter for the students of to-day to imagine that theearth has long endured; but to our forefathers, who were educated inthe view that it had been brought from nothingness into existenceabout seven thousand years ago, it was most difficult and for a timeimpossible to believe in its real antiquity. Endeavouring, as theynaturally did, to account for all the wonderful revolutions, thehistory of which is written in the pages of the great stone book, theearly geologists supposed this planet to have been the seat offrequent and violent changes, each of which revolutionized its shapeand destroyed its living tenants. It was only very gradually thatthey became convinced that a hundred million years or more haveelapsed since the dawn of life on the earth, and that in this vastperiod the march of events has been steadfast, the changes takingplace at about the same rate in which they are now going on. As yetthis conception as to the history of our sphere has not become thegeneral property of the people, but the fact of it is recognised byall those who have attentively studied the matter. It is now as wellascertained as any of the other truths which science has disclosed tous. 

It is instructive to note the historic outlines of scientificdevelopment. The most conspicuous truth which this history disclosesis that all science has had its origin and almost all its developmentamong the peoples belonging to the Aryan race. This body of folkappears to have taken on its race characteristics, acquired itsoriginal language, its modes of action, and the foundations of itsreligion in that part of northern Europe which is about the BalticSea. Thence the body of this people appear to have wandered towardcentral Asia, where after ages of pastoral life in the high tablelands and mountains of their country it sent forth branches to India, Asia Minor and Greece, to Persia, and to western Europe. It seems everto have been a characteristic of these Aryan peoples that they had anextreme love for Nature; moreover, they clearly perceived the need ofaccounting for the things that happened in the world about them. Ingeneral they inclined to what is called the pantheistic explanation ofthe universe. They believed a supreme God in many different forms tobe embodied in all the things they saw. Even their own minds andbodies they conceived as manifestations of this supreme power. Amongthe Aryans who came to dwell in Europe and along the easternMediterranean this method of explaining Nature was in time changed toone in which humanlike gods were supposed to control the visible andinvisible worlds. In that marvellous centre of culture which wasdeveloped among the Greeks this conception of humanlike deities was intime replaced by that of natural law, and in their best days theGreeks were men of science essentially like those of to-day, exceptthat they had not learned by experience how important it was tocriticise their theories by patiently comparing them with the factswhich they sought to explain. The last of the important Greek men ofscience, Strabo, who was alive when Christ was born, has left uswritings which in quality are essentially like many of the able worksof to-day. But for the interruption in the development of Greeklearning, natural science would probably have been fifteen hundredyears ahead of its present stage. This interruption came in two ways. In one, through the conquest of Greece and the destruction of itsintellectual life by the Romans, a people who were singularlyincapable of appreciating natural science, and who had no otherinterest in it except now and then a vacant and unprofitable curiosityas to the processes of the natural world. A second destructiveinfluence came through the fact that Christianity, in its energeticprotest against the sins of the pagan civilization, absolutelyneglected and in a way despised all forms of science. 

The early indifference of Christians to natural learning is partly tobe explained by the fact that their religion was developed among theHebrews, a people remarkable for their lack of interest in thescientific aspects of Nature. To them it was a sufficient explanationthat one omnipotent God ruled all things at his will, the heavens andthe earth alike being held in the hollow of his hand. 

Finding the centre of its development among the Romans, Christianitycame mainly into the control of a people who, as we have beforeremarked, had no scientific interest in the natural world. Thiscondition prolonged the separation of our faith from science forfifteen hundred years after its beginning. In this time the records ofGreek scientific learning mostly disappeared. The writings ofAristotle were preserved in part for the reason that the Churchadopted many of his views concerning questions in moral philosophy andin politics. The rest of Greek learning was, so far as Europe wasconcerned, quite neglected. 

A large part of Greek science which has come down to us owes itspreservation to a very singular incident in the history of learning. In the ninth century, after the Arabs had been converted toMohammedanism, and on the basis of that faith had swiftly organized agreat and cultivated empire, the scholars of that folk became deeplyinterested in the remnants of Greek learning which had survived in themonastic and other libraries about the eastern Mediterranean. Sogreatly did they prize these records, which were contemned by theChristians, that it was their frequent custom to weigh the oldmanuscripts in payment against the coin of their realm. In astronomy, mathematics, chemistry, and geology the Arabian students, building onthe ancient foundations, made notable and for a time most importantadvances. In the tenth century of our era they seemed fairly in theway to do for science what western Europe began five centuries laterto accomplish. In the fourteenth century the centre of Mohammedanstrength was transferred from the Arabians to the Turks, from a peoplenaturally given to learning to a folk of another race, who despisedall such culture. Thenceforth in place of the men who had treasuredand deciphered with infinite pains all the records of earlierlearning, the followers of Mohammed zealously destroyed all therecords of the olden days. Some of these records, however, survivedamong the Arabs of Spain, and others were preserved by the Christianscholars who dwelt in Byzantium, or Constantinople, and were broughtinto western Europe when that city was captured by the Turks in thefifteenth century. 

Already the advance of the fine arts in Italy and the general tendencytoward the study of Nature, such as painting and sculpture indicate, had made a beginning, or rather a proper field for a beginning, ofscientific inquiry. The result was a new interest in Greek learning inall its branches, and a very rapid awakening of the scientific spirit. At first the Roman Church made no opposition to this new interestwhich developed among its followers, but in the course of a few years, animated with the fear that science would lead men to doubt many ofthe dogmas of the Church, it undertook sternly to repress the work ofall inquirers. 

The conflict between those of the Roman faith and the men of sciencecontinued for above two hundred years. In general, the part which theChurch took was one of remonstrance, but in a few cases the spirit offanaticism led to the persecution of the men who did not obey itsmandates and disavow all belief in the new opinions which were deemedcontrary to the teachings of Scripture. The last instance of suchoppression occurred in France in the year 1756, when the great Buffonwas required to recant certain opinions concerning the antiquity ofthe earth which he had published in his work on Natural History. Thishe promptly did, and in almost servile language withdrew all theopinions to which the fathers had objected. A like conflict betweenthe followers of science and the clerical authorities occurred inProtestant countries. Although in no case were the men of sciencephysically tortured or executed for their opinions, they werenevertheless subjected to great religious and social pressure: theywere almost as effectively disciplined as were those who fell underthe ban of the Roman Church. 

Some historians have criticised the action of the clerical authoritiestoward science as if the evil which was done had been performed in ourown day. It should be remembered, however, that in the earliercenturies the churches regarded themselves as bound to protect all menfrom the dangers of heresy. For centuries in the early history ofChristianity the defenders of the faith had been engaged in alife-and-death struggle with paganism, the followers of which held allthat was known of Nature. Quite naturally the priestly class fearedthat the revival of scientific inquiry would bring with it the evilsfrom which the world had suffered in pagan times. There is no doubtthat these persecutions of science were done under what seemed theobligations of duty. They may properly be explained particularly bymen of science as one of the symptoms of development in the day inwhich they were done. It is well for those who harshly criticise therelations of the Church to science to remember that in our owncountry, about two centuries ago, among the most enlightened andreligious people of the time, Quakers were grievously persecuted, andwitches hanged, all in the most dutiful and God-fearing way. Inconsidering these relations of science to our faith, the matter shouldbe dealt with in a philosophical way, and with a sense of thedifferences between our own and earlier ages. 

To the student of the relations between Christianity and science itmust appear doubtful whether the criticism or the other consequenceswhich the men of science had to meet from the Church was harmful totheir work. The early naturalists, like the Greeks whom they followed, were greatly given to speculations concerning the processes of Nature, which, though interesting, were unprofitable. They also showed acurious tendency to mingle their scientific speculations with ancientand base superstitions. They were often given to the absurditycommonly known as the "black art, " or witchcraft, and held to thepreposterous notions of the astrologists. Even the immortal astronomerKepler, who lived in the sixteenth century, was a professionalastrologer, and still held to the notion that the stars determined thedestiny of men. Many other of the famous inquirers in those yearswhich ushered in modern science believed in witchcraft. Thus for atime natural learning was in a way associated with ancient andpernicious beliefs which the Church was seeking to overthrow. Oneresult of the clerical opposition to the advancement of science wasthat its votaries were driven to prove every step which led to theirconclusions. They were forced to abandon the loose speculation oftheir intellectual guides, the Greeks, and to betake themselves toobservation. Thus a part of the laborious fact-gathering habit onwhich the modern advance of science has absolutely depended was due tothe care which men had to exercise in face of the religiousauthorities. 

In our own time, in the latter part of the nineteenth century, theconflict between the religious authority and the men of science haspractically ceased. Even the Roman Church permits almost everywhere anuntrammelled teaching of the established learning to which it was atone time opposed. Men have come to see that all truth is accordant, and that religion has nothing to fear from the faithful and devotedstudy of Nature. 

The advance of science in general in modern times has been greatly dueto the development of mechanical inventions. Among the ancients, thetools which served in the arts were few in number, and these ofexceeding simplicity. So far as we can ascertain, in the five hundredyears during which the Greeks were in their intellectual vigour, notmore than half a dozen new machines were invented, and these wereexceedingly simple. The fact seems to be that a talent for mechanicalinvention is mainly limited to the peoples of France, Germany, and ofthe English-speaking folk. The first advances in these contrivanceswere made in those countries, and all our considerable gains have comefrom their people. Thus, while the spirit of science in general isclearly limited to the Aryan folk, that particular part of the motivewhich leads to the invention of tools is restricted to western andnorthern Europe, to the people to whom we give the name of Teutonic. 

Mechanical inventions have aided the development of our sciences inseveral ways. They have furnished inquirers with instruments ofprecision; they have helped to develop accuracy of observation; bestof all, they have served ever to bring before the attention of men aspectacle of the conditions in Nature which we term cause and effect. The influence of these inventions on the development of learning hasbeen particularly great where the machines, such as our wind and watermills, and our steam engine, make use of the forces of Nature, subjugating them to the needs of man. Such instruments give anunending illustration as to the presence in Nature of energy. Theyhave helped men to understand that the machinery of the universe ispropelled by the unending application of power. It was, in fact, through such machines that men of science first came to understandthat energy, manifested in the natural forces, is something thateternally endures; that we may change its form in our arts as its formis changed in the operations of Nature, but the power endures forever. 

It is interesting to note that the first observation which led to thismost important scientific conclusion that energy is indestructiblehowever much it may change its form, was made by an American, BenjaminThompson, who left this country at the time of the Revolution, andafter a curious life became the executive officer, and in effect king, of Bavaria. While engaged in superintending the manufacture of cannon, he observed that in boring out the barrel of the gun an amount of heatwas produced which evaporated a certain amount of water. He thereforeconcluded that the energy required to do the boring of the metalpassed into the state of heat, and thus only changed its state, in nowise disappearing from the earth. Other students pursuing the sameline of inquiry have clearly demonstrated what is called the law ofthe conservation of energy, which more than anything has helped us tounderstand the large operations of Nature. Through these studies wehave come to see that, while the universe is a place of ceaselesschange, the quantities of energy and of matter remain unaltered. 

The foregoing brief sketch, which sets forth some of the importantconditions which have affected the development of science, may in away serve to show the student how he can himself become an interpreterof Nature. The evidence indicates that the people of our race havebeen in a way chosen among all the varieties of mankind to lead inthis great task of comprehending the visible universe. The facts, moreover, show that discovery usually begins with the interest whichmen feel in the world immediately about them, or which is presented totheir senses in a daily spectacle. Thus Benjamin Franklin, in themidst of a busy life, became deeply interested in the phenomena oflightning, and by a very simple experiment proved that this wonder ofthe air was due to electrical action such as we may arouse by rubbinga stick of sealing-wax or a piece of amber with a cloth. Alldiscoveries, in a word, have had their necessary beginnings in aninterest in the facts which daily experience discloses. This desire toknow something more than the first sight exhibits concerning theactions in the world about us is native in every human soul--at least, in all those who are born with the heritage of our race. It iscommonly strong in childhood; if cultivated by use, it will growthroughout a lifetime, and, like other faculties, becomes the strongerand more effective by the exertions which it inspires. It is thereforemost important that every one should obey this instinctive command toinquiry, and organize his life and work so that he may not lose butgain more and more as time goes on of this noble capacity tointerrogate and understand the world about him. 

It is best that all study of Nature should begin not in laboratories, nor with the things which are remote from us, but in the field ofNature which is immediately about us. The student, even if he dwell inthe unfavourable conditions of a great city, is surrounded by theworld which has yielded immeasurable riches in the way of learning, which he can appropriate by a little study. He can readily come toknow something of the movements of the air; the buildings will givehim access to a great many different kinds of stone; the smallestpark, a little garden, or even a few potted plants and captiveanimals, may tell him much concerning the forms and actions of livingbeings. By studying in this way he can come to know something of thedifferences between things and their relations to each other. He willthus have a standard by which he can measure and make familiar thebody of learning concerning Nature which he may find in books. Fromprinted pages alone, however well they be written, he can never hopeto catch the spirit that animates the real inquirer, the true lover ofNature. 

On many accounts the most attractive way of beginning to form thehabit of the naturalist is by the study of living animals and plants. To all of us life adds interest, and growth has a charm. Therefore itis well for the student to start on the way of inquiry by watching theactions of birds and insects or by rearing plants. It is fortunate ifhe can do both these agreeable things. When the habit of taking anaccount of that most important part of the world which is immediatelyabout him has been developed in the student, he may profitably proceedto acquire the knowledge of the invisible universe which has beengathered by the host of inquirers of his race. However far hejourneys, he should return to the home world that lies immediately andfamiliarly about him, for there alone can he acquire and preserve thatpersonal acquaintance with things which is at once the inspiration andthe test of all knowledge. 

Along with this study of the familiar objects about us the student maywell combine some reading which may serve to show him how others havebeen successful in thus dealing with Nature at first hand. For thispurpose there are, unfortunately, but few works which are wellcalculated to serve the needs of the beginner. Perhaps the bestnaturalist book, though its form is somewhat ancient, is White'sNatural History of Selborne. Hugh Miller's works, particularly his OldRed Sandstone and My Schools and Schoolmasters, show well how a manmay become a naturalist under difficulties. Sir John Lubbock's studieson Wasps, and Darwin's work on Animals and Plants under Domesticationare also admirable to show how observation should be made. Dr. AsaGray's little treatise on How Plants Grow will also be useful to thebeginner who wishes to approach botany from its most attractiveside--that of the development of the creature from the seed to seed. 

There is another kind of training which every beginner in the art ofobserving Nature should obtain, and which many naturalists of reputewould do well to give themselves--namely, an education in what we maycall the art of distance and geographical forms. With the primitivesavage the capacity to remember and to picture to the eye the shape ofa country which he knows is native and instinctive. Accustomed torange the woods, and to trust to his recollection to guide him throughthe wilderness to his home, the primitive man develops an importantart which among civilized people is generally dormant. In fact, in ourwell-trodden ways people may go for many generations without everbeing called upon to use this natural sense of geography. The easiestway to cultivate the geographic sense is by practising the art ofmaking sketch maps. This the student, however untrained, can readilydo by taking first his own dwelling house, on which he should practiseuntil he can readily from memory make a tolerably correct andproportional plan of all its rooms. Then on a smaller scale he shouldbegin to make also from recollection a map showing the distribution ofthe roads, streams, and hills with which his daily life makes himfamiliar. From time to time this work from memory should be comparedwith the facts. At first the record will be found to be very poor, butwith a few months of occasional endeavour the observer will find thathis mind takes account of geographic features in a way it did notbefore, and, moreover, that his mind becomes enriched withimpressions of the country which are clear and distinct, in place ofthe shadowy recollections which he at first possessed. 

When the student has attained the point where, after walking or ridingover a country, he can readily recall its physical features of thesimpler sort, he will find it profitable to undertake the method ofmapping with contour lines--that is, by pencilling in indications toshow the exact shape of the elevations and depressions. The principleof contour lines is that each of them represents where water wouldcome against the slope if the area were sunk step by step below thesea level--in other words, each contour line marks the intersection ofa horizontal plane with the elevation of the country. Practice on thissomewhat difficult task will soon give the student some idea as to thecomplication of the surface of a region, and afford him the basis fora better understanding of what geography means than all the reading hecan do will effect. It is most desirable that training such as hasbeen described should be a part of our ordinary school education. 

Very few people have clear ideas of distances. Even the men whosetrade requires some such knowledge are often without that which alittle training could give them. Without some capacity in thisdirection, the student is always at a disadvantage in his contact withNature. He can not make a record of what he sees as long as theelement of horizontal and vertical distance is not clearly in mind. Toattain this end the student should begin by pacing some length of roadwhere the distances are well known. In this way he will learn thelength of his step, which with a grown man generally ranges betweentwo and a half and three feet. Learning the average length of hisstride by frequent counting, it is easy to repeat the trial until onecan almost unconsciously keep the count as he walks. Properly tosecure the training of this sort the observer should first attentivelylook across the distance which is to be determined. He should noticehow houses, fences, people, and trees appear at that distance. He willquickly perceive that each hundred feet of additional intervalsomewhat changes their aspect. In training soldiers to measure withthe eye the distances which they have to know in order effectively touse the modern weapons of war, a common device is to take a squad ofmen, or sometimes a company, under the command of an officer, whohalts one man at each hundred yards until the detachment is strung outwith that interval as far as the eye can see them. The men then walkto and fro so that the troops who are watching them may note theeffects of increased distance on their appearance, whether standing orin motion. At three thousand yards a man appears as a mere dot, whichis not readily distinguishable. Schoolboys may find this experimentamusing and instructive. 

After the student has gained, as he readily may, some sense of thedivisions of distance within the range of ordinary vision, he shouldtry to form some notion of greater intervals, as of ten, a hundred, and perhaps a thousand miles. The task becomes more difficult as thelength of the line increases, but most persons can with a littleaddress manage to bring before their eyes a tolerably clear image of ahundred miles of distance by looking from some elevation whichcommands a great landscape. It is doubtful, however, whether thebest-trained man can get any clear notion of a thousand miles--thatis, can present it to himself in imagination as he may readily do withshorter intervals. 

The most difficult part of the general education which the student hasto give himself is begun when he undertakes to picture long intervalsof time. Space we have opportunities to measure, and we come in a wayto appreciate it, but the longest lived of men experiences at most acentury of life, and this is too small a measure to give any notion asto the duration of such great events as are involved in the history ofthe earth, where the periods are to be reckoned by the millions ofyears. The only way in which we can get any aid in picturing toourselves great lapses of time is by expressing them in units ofdistance. Let a student walk away on a straight road for the distanceof a mile; let him call each step a year; when he has won the firstmilestone, he may consider that he has gone backward in time to theperiod of Christ's birth. Two miles more will take him to the stationwhich will represent the age when the oldest pyramids were built. Heis still, however, in the later days of man's history on this planet. To attain on the scale the time when man began, he might well have towalk fifty miles away, while a journey which would thus by successivesteps describe the years of the earth's history since life appearedupon its surface would probably require him to circle the earth atleast four times. We may accept it as impossible for any one to dealwith such vast durations save with figures which are never reallycomprehended. It is well, however, to enlarge our view as to the ageof the earth by such efforts as have just been indicated. 

When we go beyond the earth into the realm of the stars all effortstoward understanding the ranges of space or the durations of time arequite beyond the efforts of man. Even the distance of about twohundred and forty thousand miles which separates us from the moon cannot be grasped by even the greater minds. No human intelligence, however cultivated, can conceive the distance of about ninety-fivemillion miles which separates us from the sun. In the celestial realmwe can only deal with relations of space and time in a general andcomparative way. We can state the distances if we please in millionsof miles, or we can reckon the ampler spaces by using the intervalwhich separates the earth from the sun as we do a foot rule in ourordinary work, but the depths of the starry spaces can only be soundedby the winged imagination. 

Although the student has been advised to begin his studies of Natureon the field whereon he dwells, making that study the basis of hismost valuable communications with Nature, it is desirable that heshould at the same time gain some idea as to the range and scope ofour knowledge concerning the visible universe. As an aid toward thisend the following chapters of this book will give a very brief surveyof some of the most important truths concerning the heavens and theearth which have rewarded the studies of scientific men. Of remoterthings, such as the bodies in the stellar spaces, the account will bebrief, for that which is known and important to the general studentcan be briefly told. So, too, of the earlier ages of the earth'shistory, although a vast deal is known, the greater part of theknowledge is of interest and value mainly to geologists who cultivatethat field. That which is most striking and most important to the massof mankind is to be found in the existing state of our earth, theconditions which make it a fit abode for our kind, and replete withlessons which he may study with his own eyes without having to travelthe difficult paths of the higher sciences. 

Although physiography necessarily takes some account of the thingswhich have been, even in the remote past, and this for the reason thateverything in this day of the world depends on the events of earlierdays, the accent of its teaching is on the immediate, visible, as wemay say, living world, which is a part of the life of all itsinhabitants. 

 CHAPTER III. 

 THE STELLAR REALM. 

Even before men came to take any careful account of the Natureimmediately about them they began to conjecture and in a way toinquire concerning the stars and the other heavenly bodies. It isdifficult for us to imagine how hard it was for students to gain anyadequate idea of what those lights in the sky really are. At first menimagined the celestial bodies to be, as they seemed, small objects notvery far away. Among the Greeks the view grew up that the heavens wereformed of crystal spheres in which the lights were placed, much aslanterns may be hung upon a ceiling. These spheres were conceived tobe one above the other; the planets were on the lower of them, and thefixed stars on the higher, the several crystal roofs revolving aboutthe earth. So long as the earth was supposed to be a flat andlimitless expanse, forming the centre of the universe, it wasimpossible for the students of the heavens to attain any more rationalview as to their plan. 

The fact that the earth was globular in form was understood by theGreek men of science. They may, indeed, have derived the opinion fromthe Egyptian philosophers. The discovery rested upon the readilyobserved fact that on a given day the shadow of objects of a certainheight was longer in high latitude than in low. Within the tropics, when the sun was vertical, there would be no shadow, while as farnorth as Athens it would be of considerable length. The conclusionthat the earth was a sphere appears to have been the first largediscovery made by our race. It was, indeed, one of the most importantintellectual acquisitions of man. 

Understanding the globular form of the earth, the next and mostnatural step was to learn that the earth was not the centre of theplanetary system, much less of the universe, but that that centre wasthe sun, around which the earth and the other planets revolved. TheGreeks appear to have had some idea that this was the case, and theirspirit of inquiry would probably have led them to the whole truth butfor the overthrow of their thought by the Roman conquest and thespread of Christianity. It was therefore not until after the revivalof learning that astronomers won their way to our modern understandingconcerning the relation of the planets to the sun. With Galileo thisopinion was affirmed. Although for a time the Church, resting itsopposition on the interpretation of certain passages of Scripture, resisted this view, and even punished the men who held it, itsteadfastly made its way, and for more than two centuries has been thefoundation of all the great discoveries in the stellar realm. Yet longafter the fact that the sun was the centre of the solar system waswell established no one understood why the planets should move intheir ceaseless, orderly procession around the central mass. To Newtonwe owe the studies on the law of gravitation which brought us to ourpresent large conception as to the origin of this order. Starting withthe view that bodies attracted each other in proportion to theirweight, and in diminishing proportion as they are removed from eachother, Newton proceeded by most laborious studies to criticise thisview, and in the end definitely proved it by finding that the motionsof the moon about the earth, as well as the paths of the planets, exactly agreed with the supposition. 

The last great path-breaking discovery which has helped us in ourunderstanding of the stars was made by Fraunhofer and otherphysicists, who showed us that substances when in a heated, gaseous, or vaporous state produced, in a way which it is not easy to explainin a work such as this, certain dark lines in the spectrum, or streakof divided light which we may make by means of a glass prism, or, asin the rainbow, by drops of water. Carefully studying these verynumerous lines, those naturalists found that they could with singularaccuracy determine what substances there were in the flame which gavethe light. So accurate is this determination that it has been made toserve in certain arts where there is no better means of ascertainingthe conditions of a flaming substance except by the lines which itslight exhibits under this kind of analysis. Thus, in the manufactureof iron by what is called the Bessemer process, it has been found veryconvenient to judge as to the state of the molten metal by such ananalysis of the flame which comes forth from it. 

[Illustration: _Seal Rocks near San Francisco, California, showingslight effect of waves where there is no beach. _]

No sooner was the spectroscope invented than astronomers hastened byits aid to explore the chemical constitution of the sun. These studieshave made it plain that the light of our solar centre comes forth froman atmosphere composed of highly heated substances, all of which areknown among the materials forming the earth. Although for variousreasons we have not been able to recognise in the sun all the elementswhich are found in our sphere, it is certain that in general the twobodies are alike in composition. An extension of the same method ofinquiry to the fixed stars was gradually though with difficultyattained, and we now know that many of the elements common to the sunand earth exist in those distant spheres. Still further, this methodof inquiry has shown us, in a way which it is not worth while here todescribe, that among these remoter suns there are many aggregations ofmatter which are not consolidated as are the spheres of our own solarsystem, but remain in the gaseous state, receiving the name of nebulæ. 

Along with the growth of observational astronomy which has taken placesince the discoveries of Galileo, there has been developed a viewconcerning the physical history of the stellar world, known as thenebular hypothesis, which, though not yet fully proved, is believed bymost astronomers and physicists to give us a tolerably correct notionas to the way in which the heavenly spheres were formed from anearlier condition of matter. This majestic conception was firstadvanced, in modern times at least, by the German philosopher ImmanuelKant. It was developed by the French astronomer Laplace, and is oftenknown by his name. The essence of this view rests upon the factpreviously noted that in the realm of the fixed stars there are manyfaintly shining aggregations of matter which are evidently not solidafter the manner of the bodies in our solar system, but are in thestate where their substances are in the condition of dustlikeparticles, as are the bits of carbon in flame or the elements whichcompose the atmosphere. The view held by Laplace was to the effectthat not only our own solar system, but the centres of all the othersimilar systems, the fixed stars, were originally in this gaseousstate, the material being disseminated throughout all parts of theheavenly realm, or at least in that portion of the universe of whichwe are permitted to know something. In this ancient state of matter wehave to suppose that the particles of it were more separated from eachother than are the atoms of the atmospheric gases in the most perfectvacuum which we can produce with the air-pump. Still we have tosuppose that each of these particles attract the other in thegravitative way, as in the present state of the universe theyinevitably do. 

Under the influence of the gravitative attraction the materials ofthis realm of vapour inevitably tended to fall in toward the centre. If the process had been perfectly simple, the result would have beenthe formation of one vast mass, including all the matter which was inthe original body. In some way, no one has yet been able to make areasonable suggestion of just how, there were developed in theprocess of concentration a great many separate centres of aggregation, each of which became the beginning of a solar system. The student mayform some idea of how readily local centres may be produced inmaterials disseminated in the vaporous state by watching how fog orthe thin, even misty clouds of the sunrise often gather into theseparate shapes which make what we term a "mackerel" sky. It isdifficult to imagine what makes centres of attraction, but we readilyperceive by this instance how they might have occurred. 

When the materials of each solar system were thus set apart from theoriginal mass of star dust or vapour, they began an independentdevelopment which led step by step, in the case of our own solarsystem at least, and presumably also in the case of the other suns, the fixed stars, to the formation of planets and their moons orsatellites, all moving around the central sun. At this stage of theexplanation the nebular hypothesis is more difficult to conceive thanin the parts of it which have already been described, for we have nowto understand how the planets and satellites had their matterseparated from each other and from the solar centre, and why they cameto revolve around that central body. These problems are bestunderstood by noting some familiar instances connected with themovement of fluids and gases toward a centre. First let us take thecase of a basin in which the water is allowed to flow out through ahole in its centre. When we lift the stopper the fluid for a momentfalls straight down through the opening. Very quickly, however, allthe particles of the water start to move toward the centre, and almostat once the mass begins to whirl round with such speed that, althoughit is working toward the middle, it is by its movement pushed awayfrom the centre and forms a conical depression. As often as we try theexperiment, the effect is always the same. We thus see that there issome principle which makes particles of fluid that tend toward acentre fail directly to attain it, but win their way thereto in adevious, spinning movement. 

Although the fact is not so readily made visible to the eye, the sameprinciple is illustrated in whirling storms, in which, as we shallhereafter note with more detail, the air next the surface of the earthis moving in toward a kind of chimney by which it escapes to the upperregions of the atmosphere. A study of cyclones and tornadoes, or evenof the little air-whirls which in hot weather lift the dust of ourstreets, shows that the particles of the atmosphere in rushing intoward the centre of upward movement take on the same whirling motionas do the molecules of water in the basin--in fact, the two actionsare perfectly comparable in all essential regards, except that thefluid is moving downward, while the air flows upward. Briefly stated, the reason for the movement of fluid and gas in the whirling way is asfollows: If every particle on its way to the centre moved on aperfectly straight line toward the point of escape, the flow would bedirectly converging, and the paths followed would resemble the spokesof a wheel. But when by chance one of the particles sways ever solittle to one side of the direct way, a slight lateral motion wouldnecessarily be established. This movement would be due to the factthat the particle which pursued the curved line would press againstthe particles on the out-curved side of its path--or, in other words, shove them a little in that direction--to the extent that theydeparted from the direct line they would in turn communicate theshoving to the next beyond. When two particles are thus shoving on oneside of their paths, the action which makes for revolution is doubled, and, as we readily see, the whole mass may in this way become quicklyaffected, the particles driven out of their path, moving in a curvetoward the centre. We also see that the action is accumulative: themore curved the path of each particle, the more effectively it shoves;and so, in the case of the basin, we see the whirling rapidlydeveloped before our eyes. 

In falling in toward the centre the particles of star dust or vapourwould no more have been able one and all to pursue a perfectlystraight line than the particles of water in the basin. If a manshould spend his lifetime in filling and emptying such a vessel, it issafe to say that he would never fail to observe the whirling movement. As the particles of matter in the nebular mass which was to become asolar system are inconceivably greater than those of water in thebasin, or those of air in the atmospheric whirl, the chance of thewhirling taking place in the heavenly bodies is so great that we mayassume that it would inevitably occur. 

As the vapours in the olden day tended in toward the centre of oursolar system, and the mass revolved, there is reason to believe thatringlike separations took place in it. Whirling in the mannerindicated, the mass of vapour or dust would flatten into a disk or abody of circular shape, with much the greater diameter in the plane ofits whirling. As the process of concentration went on, this disk issupposed to have divided into ringlike masses, some approach to whichwe can discern in the existing nebulæ, which here and there among thefarther fixed stars appear to be undergoing such stages of developmenttoward solar systems. It is reasonably supposed that after these ringshad been developed they would break to pieces, the matter in themgathering into a sphere, which in time was to become a planet. Theoutermost of these rings led to the formation of the planet farthestfrom the sun, and was probably the first to separate from the parentmass. Then in succession rings were formed inwardly, each leading inturn to the creation of another planet, the sun itself being theremnant, by far the greater part of the whole mass of matter, whichdid not separate in the manner described, but concentrated on itscentre. Each of these planetary aggregations of vapour tended todevelop, as it whirled upon its centre, rings of its own, which inturn formed, by breaking and concentrating, the satellites or moonswhich attend the earth, as they do all the planets which lie fartheraway from the sun than our sphere. 

[Illustration: Fig. 1. --Saturn, Jan. 26, 1889 (Antoniadi). ]

As if to prove that the planets and moons of the solar system wereformed somewhat in the manner in which we have described it, one ofthese spheres, Saturn, retains a ring, or rather a band which appearsto be divided obscurely into several rings which lie between its groupof satellites and the main sphere. How this ring has been preservedwhen all the others have disappeared, and what is the exactconstitution of the mass, is not yet well ascertained. It seems clear, however, that it can not be composed of solid matter. It is either inthe form of dust or of small spheres, which are free to move on eachother; otherwise, as computation shows, the strains due to theattraction which Saturn itself and its moons exercise upon it wouldserve to break it in pieces. Although this ring theory of theformation of the planets and satellites is not completely proved, theoccurrence of such a structure as that which girdles Saturn affordspresumptive evidence that it is true. Taken in connection with what weknow of the nebulæ, the proof of Laplace's nebular hypothesis mayfairly be regarded as complete. 

It should be said that some of the fixed stars are not isolated sunslike our own, but are composed of two great spheres revolving aboutone another; hence they are termed double stars. The motions of thesebodies are very peculiar, and their conditions show us that it is notwell to suppose that the solar system in which we dwell is the onlytype of order which prevails in the celestial families; there may, indeed, be other variations as yet undetected. Still, thesedifferences throw no doubt on the essential truth of the theory as tothe process of development of the celestial systems. Though there ismuch room for debate as to the details of the work there, the generaltruth of the theory is accepted by nearly all the students of theproblem. 

A peculiar advantage of the nebular hypothesis is that it serves toaccount for the energy which appears as light and heat in the sun andthe fixed stars, as well as that which still abides in the mass of ourearth, and doubtless also in the other large planets. When the matterof which these spheres were composed was disseminated through therealms of space, it is supposed to have had no positive temperature, and to have been dark, realizing the conception which appears in thefirst chapter of Genesis, "without form, and void. " With each stage ofthe falling in toward the solar centres what is called the "energy ofposition" of this original matter became converted into light andheat. To understand how this took place, the reader should considercertain simple yet noble generalizations of physics. We readilyrecognise the fact that when a hammer falls often on an anvil it heatsitself and the metal on which it strikes. Those who have been able toobserve the descent of meteoric stones from the heavens have remarkedthat when they came to the earth they were, on their surfaces atleast, exceedingly hot. Any one may observe shining meteors now andthen flashing in the sky. These are known commonly to be very smallbits of matter, probably not larger than grains of sand, which, rushing into our atmosphere, are so heated by the friction which theyencounter that they burn to a gas or vapour before they attain theearth. As we know that these particles come from the starry spaces, where the temperature is somewhere near 500° below 0° Fahr. , it isevident that the light and heat are not brought with them into theatmosphere; it can only be explained by the fact that when they enterthe air they are moving at an average speed of about twenty miles asecond, and that the energy which this motion represents is by theresistance which the body encounters converted into heat. This factwill help us to understand how, as the original star dust fell intoward the centre of attraction, it was able to convert what we havetermed the energy of position into temperature. We see clearly thatevery such particle of dust or larger bit of matter which falls uponthe earth brings about the development of heat, even though it doesnot actually strike upon the solid mass of our sphere. The conceptionof what took place in the consolidation of the originally disseminatedmaterials of the sun and planets can be somewhat helped by a simpleexperiment. If we fit a piston closely into a cylinder, and thensuddenly drive it down with a heavy blow, the compressed air is soheated that it may be made to communicate fire. If the piston shouldbe slowly moved, the same amount of heat would be generated, or, as wemay better say, liberated by the compression, though the effect wouldnot be so striking. A host of experiments show that when a given massof matter is brought to occupy a less space the effect is inpractically all cases to increase the temperature. The energy whichkept the particles apart is, when they are driven together, convertedinto heat. These two classes of actions are somewhat different intheir nature; in the case of the meteors, or the equivalent star dust, the coming together of the particles is due to gravitation. In theexperiment with the cylinder above described, the compression is dueto mechanical energy, a force of another nature. 

There is reason for believing that all our planets, as well as the sunitself, and also the myriad other orbs of space, have all passedthrough the stages of a transition in which a continuallyconcentrating vapour, drawn together by gravitation, becameprogressively hotter and more dense until it assumed the condition ofa fluid. This fluid gradually parted with its heat to the cold spacesof the heavens, and became more and more concentrated and of a lowertemperature until in the end, as in the case of our earth and of otherplanets, it ceased to glow on the outside, though it remainedintensely heated in the inner parts. It is easy to see that the rateof this cooling would be in some proportion to the size of the sphere. Thus the earth, which is relatively small, has become relatively cold, while the sun itself, because of its vastly greater mass, stillretains an exceedingly high temperature. The reason for this canreadily be conceived by making a comparison of the rate of coolingwhich occurs in many of our ordinary experiences. Thus a vial of hotwater will quickly come down to the temperature of the air, while alarge jug filled with the fluid at the same temperature will retainits heat many times as long. The reason for this rests upon the simpleprinciple that the contents of a sphere increase with its enlargementmore rapidly than the surface through which the cooling takes place. 

The modern studies on the physical history of the sun and othercelestial bodies show that their original store of heat is constantlyflowing away into the empty realms of space. The rate at which thisform of energy goes away from the sun is vast beyond the powers of theimagination to conceive; thus, in the case of our earth, which viewedfrom the sun would appear no more than a small star, the amount ofheat which falls upon it from the great centre is enough each day tomelt, if it all could be put to such work, about eight thousand cubicmiles of ice. Yet the earth receives only 1/2, 170, 000, 000 part of thesolar radiation. The greater part of this solar heat--in fact, we maysay nearly all of it--slips by the few and relatively small planetsand disappears in the great void. 

The destiny of all the celestial spheres seems in time to be thatthey shall become cooled down to a temperature far below anythingwhich is now experienced on this earth. Even the sun, though its heatwill doubtless endure for millions of years to come, must in time, sofar as we can see, become dark and cold. So far as we know, we canperceive no certain method by which the life of the slowly decayingsuns can be restored. It has, however, been suggested that in manycases a planetary system which has attained the lifeless and lightlessstage may by collision with some other association of spheres be bythe blow restored to its previous state of vapour, the joint mass ofthe colliding systems once again to resume the process ofconcentration through which it had gone before. Now and then starshave been seen to flash suddenly into great brilliancy in a way whichsuggests that possibly their heat had been refreshed by a collisionwith some great mass which had fallen into them from the celestialspaces. There is room for much speculation in this field, but nocertainty appears to be attainable. 

The ancients believed that light and heat were emanations which weregiven off from the bodies that yielded them substantially as odoursare given forth by many substances. Since the days of Newton inquiryhas forced us to the conviction that these effects of temperature areproduced by vibrations having the general character of waves, whichare sent through the spaces with great celerity. When a ray of lightdeparts from the sun or other luminous body, it does not convey anypart of the mass; it transmits only motion. A conception of the actioncan perhaps best be formed by suspending a number of balls of ivory, stone, or other hard substance each by a cord, the series so arrangedthat they touch each other. Then striking a blow against one end ofthe line, we observe that the ball at the farther end of the line isset in motion, swinging a little away from the place it occupiedbefore. The movement of the intermediate balls may be so slight as toescape attention. We thus perceive that energy can be transmittedfrom one to another of these little spheres. Close observation showsus that under the impulse which the blow gives each separate body ismade to sway within itself much in the manner of a bell when it isrung, and that the movement is transmitted to the object with which itis in contact. In passing from the sun to the earth, the light andheat traverse a space which we know to be substantially destitute ofany such materials as make up the mass of the earth or the sun. Judgedby the standards which we can apply, this space must be essentiallyempty. Yet because motions go through it, we have to believe that itis occupied by something which has certain of the properties ofmatter. It has, indeed, one of the most important properties of allsubstances, in that it can vibrate. This practically unknown thing iscalled ether. 

The first important observational work done by the ancients led themto perceive that there was a very characteristic difference betweenthe planets and the fixed stars. They noted the fact that the planetswandered in a ceaseless way across the heavens, while the fixed starsshowed little trace of changing position in relation to one another. For a long time it was believed that these, as well as the remoterfixed stars, revolved about the earth. This error, though great, isperfectly comprehensible, for the evident appearance of the movementis substantially what would be brought about if they really coursedaround our sphere. It was only when the true nature of the earth andits relations to the sun were understood that men could correct thisfirst view. It was not, indeed, until relatively modern times that thesolar system came to be perceived as something independent and widelydetached from the fixed stars system; that the spaces which separatethe members of our own solar family, inconceivably great as they are, are but trifling as compared with the intervals which part us from thenearer fixed stars. At this stage of our knowledge men came to thenoble suggestion that each of the fixed stars was itself a sun, eachof the myriad probably attended by planetary bodies such as existabout our own luminary. 

It will be well for the student to take an imaginary journey from thesun forth into space, along the plane in which extends that vastaggregation of stars which we term the Milky Way. Let him suppose thathis journey could be made with something like the speed of light, or, say, at the rate of about two hundred thousand miles a second. It isfit that the imagination, which is free to go through all things, should essay such excursions. On the fancied outgoing, the observerwould pass the interval between the sun and the earth in about eightminutes. It would require some hours before he attained to the outerlimit of the solar system. On his direct way he would pass the orbitsof the several planets. Some would have their courses on one side orthe other of his path; we should say above or below, but for the factthat we leave these terms behind in the celestial realm. On the marginof the solar system the sun would appear shrunken to the state whereit was hardly greater than the more brilliant of the other fixedstars. The onward path would then lead through a void which it wouldrequire years to traverse. Gradually the sun which happened to liemost directly in his path would grow larger; with nearer approach, itwould disclose its planets. Supposing that the way led through thissolar system, there would doubtless be revealed planets and satellitesin their order somewhat resembling those of our own solar family, yetthere would doubtless be many surprises in the view. Arriving near thefirst sun to be visited, though the heavens would have changed theirshape, all the existing constellations having altered with the changein the point of view, there would still be one familiar element inthat the new-found planets would be near by, and the nearest fixedstars far away in the firmament. 

With the speed of light a stellar voyage could be taken along the pathof the Milky Way, which would endure for thousands of years. Throughall the course the journeyer would perceive the same vast girdle ofstars, faint because they were far away, which gives the dim light ofour galaxy. At no point is it probable that he would find the separatesuns much more aggregated or greatly farther apart than they are inthat part of the Milky Way which our sun now occupies. Looking forthon either side of the "galactic plane, " there would be the samescattering of stars which we now behold when we gaze at right anglesto the way we are supposing the spirit to traverse. 

As the form of the Milky Way is irregular, the mass, indeed, havingcertain curious divisions and branches, it well might be that thesupposed path would occasionally pass on one or the other side of thevast star layer. In such positions the eye would look forth into anempty firmament, except that there might be in the far away, tens ofthousands of years perhaps at the rate that light travels away fromthe observer, other galaxies or Milky Ways essentially like that whichhe was traversing. At some point the journeyer would attain the marginof our star stratum, whence again he would look forth into theunpeopled heavens, though even there he might discern other remotestar groups separated from his own by great void intervals. 

 * * * * *

The revelations of the telescope show us certain features in theconstitution and movements of the fixed stars which now demand ourattention. In the first place, it is plain that not all of thesebodies are in the same physical condition. Though the greater part ofthese distant luminous masses are evidently in the state ofaggregation displayed by our own sun, many of them retain more or lessof that vaporous, it may be dustlike, character which we suppose tohave been the ancient state of all the matter in the universe. Some ofthese masses appear as faint, almost indistinguishable clouds, whicheven to the greatest telescope and the best-trained vision show nodistinct features of structure. In other cases the nebulousappearance is hardly more than a mist about a tolerably distinctcentral star. Yet again, and most beautifully in the great nebula ofthe constellation of Orion, the cloudy mass, though hardly visible tothe naked eye, shows a division into many separate parts, the wholeappearing as if in process of concentration about many distinctcentres. 

The nebulas are reasonably believed by many astronomers to be examplesof the ancient condition of the physical universe, masses of matterwhich for some reason as yet unknown have not progressed in theirconsolidation to the point where they have taken on thecharacteristics of suns and their attendant planets. 

Many of the fixed stars, the incomplete list of which now amounts toseveral hundred, are curiously variable in the amount of light whichthey send out to the earth. Sometimes these variations are apparentlyirregular, but in the greater number of cases they have fixed periods, the star waxing and waning at intervals varying from a few months to afew years. Although some of the sudden flashings forth of stars fromapparent small size to near the greatest brilliancy may be due tocatastrophes such as might be brought about by the sudden falling inof masses of matter upon the luminous spheres, it is more likely thatthe changes which we observe are due to the fact that two sunsrevolving around a common centre are in different stages ofextinction. It may well be that one of these orbs, presumably thesmaller, has so far lost temperature that it has ceased to glow. If inits revolution it regularly comes between the earth and its luminouscompanion, the effect would be to give about such a change in theamount of light as we observe. 

The supposition that a bright sun and a relatively dark sun mightrevolve around a common centre of gravity may at first sight seemimprobable. The fact is, however, that imperfect as our observationson the stars really are, we know many instances in which this kind ofrevolution of one star about another takes place. In some cases thesestars are of the same brilliancy, but in others one of the lights ismuch brighter than the other. From this condition to the state whereone of the stars is so nearly dark as to be invisible, the transitionis but slight. In a word, the evidence goes to show that while we seeonly the luminous orbs of space, the dark bodies which people theheavens are perhaps as numerous as those which send us light, andtherefore appear as stars. 

Besides the greater spheres of space, there is a vast host of lesserbodies, the meteorites and comets, which appear to be in part membersof our solar system, and perhaps of other similar systems, and in partwanderers in the vast realm which intervenes between the solarsystems. Of these we will first consider the meteors, of which we knowby far the most; though even of them, as we shall see, our knowledgeis limited. 

From time to time on any starry night, and particularly in certainperiods of the year, we may behold, at the distance of fifty or moremiles above the surface of the earth, what are commonly called"shooting stars. " The most of these flashing meteors are evidentlyvery small, probably not larger than tiny sand grains, possibly nogreater than the fragments which would be termed dust. They enter theair at a speed of about thirty miles a second. They are so small thatthey burn to vapour in the very great heat arising from their frictionon the air, and do not attain the surface of the earth. These are sonumerous that, on the average, some hundreds of thousands probablystrike the earth's atmosphere each day. From time to time largerbodies fall--bodies which are of sufficient bulk not to be burned upin the air, but which descend to the ground. These may be from thesmallest size which may be observed to masses of many hundred poundsin weight. These are far less numerous than the dust meteorites; it isprobable, however, that several hundred fragments each year attain theearth's surface. They come from various directions of space, andthere is as yet no means of determining whether they were formed insome manner within our planetary system or whether they wander to usfrom remoter realms. We know that they are in part composed ofmetallic iron commingled with nickel and carbon (sometimes as verysmall diamonds) in a way rarely if ever found on the surface of oursphere, and having a structure substantially unknown in its deposits. In part they are composed of materials which somewhat resemble certainlavas. It is possible that these fragments of iron and stone whichconstitute the meteorites have been thrown into the planetary spacesby the volcanic eruption of our own and other planets. If hurled forthwith a sufficient energy, the fragments would escape from the controlof the attraction of the sphere whence they came, and would becomeindependent wanderers in space, moving around the sun in varied orbitsuntil they were again drawn in by some of the greater planets. 

As they come to us these meteorites often break up in the atmosphere, the bits being scattered sometimes over a wide area of country. Thus, in the case of the Cocke County meteorite of Tennessee, one of theiron species, the fragments, perhaps thousands in number, which camefrom the explosion of the body were scattered over an area of somethousand square miles. When they reach the surface in their naturalform, these meteors always have a curious wasted and indentedappearance, which makes it seem likely that they have been subject tofrequent collisions in their journeys after they were formed by someviolent rending action. 

In some apparent kinship with the meteorites may be classed thecomets. The peculiarity of these bodies is that they appear in mostcases to be more or less completely vaporous. Rushing down from thedepths of the heavens, these bodies commonly appear as faintlyshining, cloudlike masses. As they move in toward the sun long trailsof vapour stream back from the somewhat consolidated head. Swingingaround that centre, they journey again into the outer realm. As theyretreat, their tail-like streamers appear to gather again upon theircentres, and when they fade from view they are again consolidated. Insome cases it has been suspected that a part at least of the cometarymass was solid. The evidence goes to show, however, that the matter isin a dustlike or vaporous condition, and that the weight of thesebodies is relatively very small. 

[Illustration: Fig. 2. --The Great Comet of 1811, one of the manyvaried forms of these bodies. ]

Owing to their strange appearance, comets were to the ancients omensof calamity. Sometimes they were conceived as flaming swords; theirforms, indeed, lend themselves to this imagining. They were thought topresage war, famine, and the death of kings. Again, in more moderntimes, when they were not regarded as portents of calamity, it wasfeared that these wanderers moving vagariously through our solarsystem might by chance come in contact with the earth with disastrousresults. Such collisions are not impossible, for the reason that theplanets would tend to draw these errant bodies toward them if theycame near their spheres; yet the chance of such collisions happeningto the earth is so small that they may be disregarded. 

 MOTIONS OF THE SPHERES. 

Although little is known of the motions which occur among thecelestial bodies beyond the sphere of our solar family, that which hasbeen ascertained is of great importance, and serves to make it likelythat all the suns in space are upon swift journeys which in theirspeed equal, if they do not exceed, the rate of motion among theplanetary spheres, which may, in general, be reckoned at about twentymiles a second. Our whole solar system is journeying away from certainstars, and in the direction of others which are situated in theopposite part of the heavens. The proof of this fact is found in theobservations which show that on one side of us the stars areapparently coming closer together, while on the other side they aregoing farther apart. The phenomenon, in a word, is one of perspective, and may be made real to the understanding by noting what takes placewhen we travel down a street along which there are lights. We readilynote that these lights appear to close in behind us, and widen theirintervals in the direction in which we journey. By such evidenceastronomers have become convinced that our sphere, along with the sunwhich controls it, is each second a score of miles away from the pointwhere it was before. 

There is yet other and most curious evidence which serves to show thatcertain of the stars are journeying toward our part of the heavens atgreat speed, while others are moving away from us by their own propermotion. These indications are derived from the study of the lines inthe light which the spectrum reveals to us when critically examined. The position of these cross lines is, as we know, affected by themotion of the body whence the light comes, and by close analysis ofthe facts it has been pretty well determined that the distortion intheir positions is due to very swift motions of the several stars. Itis not yet certain whether these movements of our sun and of othersolar bodies are in straight lines or in great circles. 

It should be noted that, although the evidence from the spectroscopeserves to show that the matter in the stars is akin to that of our ownearth, there is reason to believe that those great spheres differ muchfrom each other in magnitude. 

We have now set forth some of the important facts exhibited by thestellar universe. The body of details concerning that realm is vast, and the conclusions drawn from it important; only a part, however, ofthe matter with which it deals is of a nature to be apprehended by thestudent who does not approach it in a somewhat professional way. Weshall therefore now turn to a description of the portion of the starryworld which is found in the limits of our solar system. There theinfluences of the several spheres upon our planet are matters of vitalimportance; they in a way affect, if they do not control, all theoperations which go on upon the surface of the earth. 

 THE SOLAR SYSTEM. 

We have seen that the matter in the visible universe everywhere tendsto gather into vast associations which appear to us as stars, and thatthese orbs are engaged in ceaseless motion in journeys through space. In only one of these aggregations--that which makes our own solarsystem--are the bodies sufficiently near to our eyes for us, even withthe resources of our telescopes and other instruments, to divinesomething of the details which they exhibit. In studying what we mayconcerning the family of the sun, the planets, and their satellites, we may reasonably be assured that we are tracing a history which withmany differences is in general repeated in the development of eachstar in the firmament. Therefore the inquiry is one of vast range andimport. 

Following, as we may reasonably do, the nebular hypothesis--a viewwhich, though not wholly proved, is eminently probable--we may regardour solar system as having begun when the matter of which it iscomposed, then in a finely divided, cloudy state, was separated fromthe similar material which went to make the neighbouring fixed stars. The period when our solar system began its individual life was remotebeyond the possibility of conception. Naturalists are pretty wellagreed that living beings began to exist upon the earth at least ahundred million years ago; but the beginnings of our solar system mustbe placed at a date very many times as remote from the present day. [1]

[Footnote 1: Some astronomers, particularly the distinguished ProfessorNewcomb, hold that the sun can not have been supplying heat as atpresent for more than about ten million years, and that all geologicaltime must be thus limited. The geologist believes that this reckoning isfar too short. ]

According to the nebular theory, the original vapour of the solarsystem began to fall in toward its centre and to whirl about thatpoint at a time long before the mass had shrunk to the present limitsof the solar system as defined by the path of the outermost planets. At successive stages of the concentration, rings after the manner ofthose of Saturn separated from the disklike mass, each breaking up andconsolidating into a body of nebulous matter which followed in thesame path, generally forming rings which became by the same processthe moons or satellites of the sphere. In this way the sun producedeight planets which are known, and possibly others of small size onthe outer verge of the system which have eluded discovery. Accordingto this view, the planetary masses were born in succession, thefarthest away being the oldest. It is, however, held by an ableauthority that the mass of the solar system would first form a ratherflat disk, the several rings forming and breaking into planets atabout the same time. The conditions in Saturn, where the inner ringremains parted, favours the view just stated. 

Before making a brief statement of the several planets, the asteroids, and the satellites, it will be well to consider in a general way themotions of these bodies about their centres and about the sun. Themost characteristic and invariable of these movements is that by whicheach of the planetary spheres, as well as the satellites, describes anorbit around the gravitative centre which has the most influence uponit--the sun. To conceive the nature of this movement, it will be wellto imagine a single planet revolving around the sun, each of thesebodies being perfect spheres, and the two the only members of thesolar system. In this condition the attraction of the two bodies wouldcause them to circle around a common centre of gravity, which, if theplanet were not larger or the sun smaller than is the case in oursolar system, would lie within the mass of the sun. In proportion asthe two bodies might approach each other in size, the centre ofgravity would come the nearer to the middle point in a line connectingthe two spheres. In this condition of a sun with a single planet, whatever were the relative size of sun and planet, the orbits whichthey traverse would be circular. In this state of affairs it should benoted that each of the two bodies would have its plane of rotationpermanently in the same position. Even if the spheres were more orless flattened about the poles of their axes, as is the case with allthe planets which we have been able carefully to measure, as well aswith the sun, provided the axes of rotation were precisely parallel toeach other, the mutual attraction of the masses would cause nodisturbance of the spheres. The same would be the case if the polaraxis of one sphere stood precisely at right angles to that of theother. If, however, the spheres were somewhat flattened at the poles, and the axes inclined to each other, then the pull of one mass on theother would cause the polar axes to keep up a constant movement whichis called nutation, or nodding. 

The reason why this nodding movement of the polar axes would occurwhen these lines were inclined to each other is not difficult to seeif we remember that the attraction of masses upon each other isinversely as the square of the distance; each sphere, pulling on theequatorial bulging of the other, pulls most effectively on the part ofit which is nearest, and tends to draw it down toward its centre. Theresult is that the axes of the attracted spheres are given a wobblingmovement, such as we may note in the spinning top, though in the toythe cause of the motion is not that which we are considering. 

If, now, in that excellent field for the experiment we are essaying, the mind's eye, we add a second planet outside of the single spherewhich we have so far supposed to journey about the sun, or ratherabout the common centre of gravity, we perceive at once that we haveintroduced an element which leads to a complication of muchimportance. The new sphere would, of course, pull upon the others inthe measure of its gravitative value--i. E. , its weight. The centre ofgravity of the system would now be determined not by two distinctbodies, but by three. If we conceive the second planet to journeyaround the sun at such a rate that a straight line always connectedthe centres of the three orbs, then the only effect on theirgravitative centre would be to draw the first-mentioned planet alittle farther away from the centre of the sun; but in our own solarsystem, and probably in all others, this supposition is inadmissible, because the planets have longer journeys to go and also move slower, the farther they are from the sun. Thus Mercury completes the circleof its year in eighty-eight of our days, while the outermost planetrequires sixty thousand days (more than one hundred and sixty-fouryears) for the same task. The result is not only that the centre ofgravity of the system is somewhat displaced--itself a matter of nogreat account--but also that the orbit of the original planet ceasesto be circled and becomes elliptical, and this for the evident reasonthat the sphere will be drawn somewhat away from the sun when thesecond planet happens to lie in the part of its orbit immediatelyoutside of its position, in which case the pull is away from the solarcentre; while, on the other hand, when the new planet was on the otherside of the sun, its pull would serve to intensify the attractionwhich drew the first sphere toward the centre of gravity. As thepulling action of the three bodies upon each other, as well as upontheir equatorial protuberances, would vary with every change in theirrelative position, however slight, the variations in the form of theirorbits, even if the spheres were but three in number, would be veryimportant. The consequences of these perturbations will appear in thesequel. 

In our solar system, though there are but eight great planets, thegroup of asteroids, and perhaps a score of satellites, the variety oforbital and axial movement which is developed taxes the computinggenius of the ablest astronomer. The path which our earth followsaround the sun, though it may in general and for convenience bedescribed as a variable ellipse, is, in fact, a line of suchcomplication that if we should essay a diagram of it on the scale ofthis page it would not be possible to represent any considerable partof its deviations. These, in fact, would elude depiction, even if thedraughtsman had a sheet for his drawing as large as the orbit itself, for every particle of matter in space, even if it be lodged beyond thelimits of the farthest stars revealed to us by the telescope, exercises a certain attraction, which, however small, is effective onthe mass of the earth. Science has to render its conclusions ingeneral terms, and we can safely take them as such; but in this, as inother instances, it is well to qualify our acceptance of thestatements by the memory that all things are infinitely morecomplicated than we can possibly conceive or represent them to be. 

We have next to consider the rotations of the planetary spheres upontheir axes, together with the similar movement, or lack of it, in thecase of their satellites. This rotation, according to the nebularhypothesis, may be explained by the movements which would set up inthe share of matter which was at first a ring of the solar nebula, andwhich afterward gathered into the planetary aggregation. The way of itmay be briefly set forth as follows: Such a ring doubtless had adiameter of some million miles; we readily perceive that the particlesof matter in the outer part of the belt would have a swifter movementaround the sun than those on the inside. When by some disturbance, aspossibly by the passage of a great meteoric body of a considerablegravitative power, this ring was broken in two, the particlescomposing it on either side would, because of their mutual attraction, tend to draw away from the breach, widening that gap until the matterof the broken ring was aggregated into a sphere of the star dust orvapour. When the nebulous matter originally in the ring becameaggregated into a spherical form, it would, on account of thedifferent rates at which the particles were moving when they cametogether, be the surer to fall in toward the centre, not in straightlines, but in curves--in other words, the mass would necessarily takeon a movement of rotation essentially like that which we havedescribed in setting forth the nebular hypothesis. 

In the stages of concentration the planetary nebulæ might well repeatthose through which the greater solar mass proceeded. If the volume ofthe material were great, subordinate rings would be formed, which whenthey broke and concentrated would constitute secondary planets orsatellites, such as our moon. For some reason as yet unknown the outerplanets--in fact, all those in the solar system except the two inner, Venus and Mercury and the asteroids--formed such attendants. All thesesatellite-forming rings have broken and concentrated except the innerof Saturn, which remains as an intellectual treasure of the solarsystem to show the history of its development. 

To the student who is not seeking the fulness of knowledge whichastronomy has to offer, but desires only to acquaint himself with themore critical and important of the heavenly phenomena which help toexplain the earth, these features of planetary movement should proveespecially interesting for the reason that they shape the history ofthe spheres. As we shall hereafter see, the machinery of the earth'ssurface, all the life which it bears, its winds and rains--everything, indeed, save the actions which go on in the depths of the sphere--isdetermined by the heat and light which come from the sun. Theconditions under which this vivifying tide is received have theirorigin in the planetary motion. If our earth's path around the centreof the system was a perfect circle, and if its polar axis lay at rightangles to the plane of its journey, the share of light and heat whichwould fall upon any one point on the sphere would be perfectlyuniform. There would be no variations in the length of day or night;no changes in the seasons; the winds everywhere would blow withexceeding steadiness--in fact, the present atmospheric confusion wouldbe reduced to something like order. From age to age, except so far asthe sun itself might vary in the amount of energy which it radiated, or lands rose up into the air or sunk down toward the sea level, theclimate of each region would be perfectly stable. In the existingconditions the influences bring about unending variety. First of all, the inclined position of the polar axis causes the sun apparently tomove across the heavens, so that it comes in an overhead position onceor twice in the year in quite half the area of the lands and seas. This apparent swaying to and fro of the sun, due to the inclination ofthe axis of rotation, also affects the width of the climatal belts oneither side of the equator, so that all parts of the earth receive aconsiderable share of the sun's influence. If the axis of the earth'srotation were at right angles to the plane of its orbit, there wouldbe a narrow belt of high temperature about the equator, north andsouth of which the heat would grade off until at about the parallelsof fifty degrees we should find a cold so considerable and uniformthat life would probably fade away; and from those parallels to thepoles the conditions would be those of permanent frost, and of dayswhich would darken into the enduring night or twilight in the realmof the far north and south. Thus the wide habitability of the earth isan effect arising from the inclination of its polar axis. 

[Illustration: Fig. 3. --Inclination of Planetary Orbits (fromChambers). ]

As the most valuable impression which the student can receive from hisstudy of Nature is that sense of the order which has made possible alllife, including his own, it will be well for him to imagine, as he mayreadily do, what would be the effect arising from changes in relationsof earth and sun. Bringing the earth's axis in imagination into aposition at right angles to the plane of the orbit, he will see thatthe effect would be to intensify the equatorial heat, and to rob thehigh latitudes of the share which they now have. On moving the axisgradually to positions where it approaches the plane of the orbit, hewill note that each stage of the change widens the tropic belt. Bringing the polar axis down to the plane of the orbit, one hemispherewould receive unbroken sunshine, the other remaining in perpetualdarkness and cold. In this condition, in place of an equatorial linewe should have an equatorial point at the pole nearest the sun; thencethe temperatures would grade away to the present equator, beyond whichhalf the earth would be in more refrigerating condition than are thepoles at the present day. In considering the movements of our planet, we shall see that no great changes in the position of the polar axiscan have taken place. On this account the suggested alterations of theaxis should not be taken as other than imaginary changes. 

It is easy to see that with a circular orbit and with an inclined axiswinter and summer would normally come always at the same point in theorbit, and that these seasons would be of perfectly even length. But, as we have before noted, the earth's path around the sun is in itsform greatly affected by the attractions which are exercised by theneighbouring planets, principally by those great spheres which lie inthe realm without its orbit, Jupiter and Saturn. When these attractingbodies, as is the case from time to time, though at long intervals, are brought together somewhere near to that part of the solar systemin which the earth is moving around the sun, they draw our planettoward them, and so make its path very elliptical. When, however, theyare so distributed that their pulling actions neutralize each other, the orbit returns more nearly to a circular form. The range in itseccentricity which can be brought about by these alterations is verygreat. When the path is most nearly circular, the difference in themajor and minor axis may amount to as little as about five hundredthousand miles, or about one one hundred and eighty-sixth of itsaverage diameter. When the variation is greatest the difference inthese measurements may be as much as near thirteen million miles, orabout one seventh of the mean width of the orbit. 

The first and most evident effect arising from these changes of theorbit comes from the difference in the amount of heat which the earthmay receive according as it is nearer or farther from the sun. As inthe case of other fires, the nearer a body is to it the larger theshare of light and heat which it will receive. In an orbit madeelliptical by the planetary attraction the sun necessarily occupiesone of the foci of the ellipse. The result is, of course, that theside of the earth which is toward the sun, while it is thus broughtthe nearer to the luminary, receives more energy in the form of lightand heat than come to any part which is exposed when the spheres arefarther away from each other in the other part of the orbit. Computations clearly show that the total amount of heat and theattendant light which the earth receives in a year is not affected bythese changes in the form of its path. While it is true that itreceives heat more rapidly in the half of the ellipse which is nearestthe source of the inundation, it obtains less while it is fartheraway, and these two variations just balance each other. 

Although the alterations in the eccentricity of its orbit do not varythe annual supply of heat which the earth receives, they are capableof changing the character of the seasons, and this in the way which wewill now endeavour to set forth, though we must do it at the cost ofconsiderable attention on the part of the reader, for the facts aresomewhat complicated. In the first place, we must note that theellipticity of the earth's orbit is not developed on fixed lines, butis endlessly varied, as we can readily imagine it would be for thereason that its form depends upon the wandering of the outer planetaryspheres which pull the earth about. The longer axis of the ellipse isitself in constant motion in the direction in which the earth travels. This movement is slow, and at an irregular rate. It is easy to seethat the effect of this action, which is called the revolution of theapsides, or, as the word means, the movement of the poles of theellipse, is to bring the earth, when a given hemisphere is turnedtoward the sun, sometimes in the part of the orbit which is nearestthe source of light and heat, and sometimes farther away. It may thuswell come about that at one time the summer season of a hemispherearrives when it is nearest the sun, so that the season, though hot, will be very short, while at another time the same season will arrivewhen the earth is farthest from the sun, and receives much less heat, which would tend to make a long and relatively cool summer. The reasonfor the difference in length of the seasons is to be found in therelative swiftness of the earth's revolution when it is nearest thesun, and the slowness when it is farther away. 

There is a further complication arising from that curious phenomenoncalled the precession of the equinoxes, which has to be taken intoaccount before we can sufficiently comprehend the effect of thevarying eccentricity of the orbit on the earth's seasons. Tounderstand this feature of precession we should first note that itmeans that each year the change from the winter to the summer--or, aswe phrase it, the passage of the equinoctial line--occurs a littlesooner than the year before. The cause of this is to be found in theattraction which the heavenly bodies, practically altogether the moon, exercises on the equatorial protuberance of the earth. We know thatthe diameter of our sphere at the equator is, on the average, something more than twenty-six miles greater than it is through thepoles. We know, furthermore, that the position of the moon in relationto the earth is such that it causes the attraction on one half of thisprotuberance to be greater than it is upon the other. We readilyperceive that this action will cause the polar axis to make a certainrevolution, or, what comes to the same thing, that the plane of theequator will constantly be altering its position. Now, as theequinoctial points in the orbit depend for their position upon theattitude of the equatorial plane, we can conceive that the effect is achange in position of the place in that orbit where summer and winterbegin. The actual result is to bring the seasonal points backward, step by step, through the orbit in a regular measure until intwenty-two thousand five hundred years they return to the place wherethey were before. This cycle of change was of old called the AnnusMagnus, or great year. 

If the earth's orbit were an ellipse, the major axis of which remainedin the same position, we could readily reckon all the effects whicharise from the variations of the great year. But this ellipse is everchanging in form, and in the measure of its departure from a circlethe effects on the seasons distributed over a great period of time areexceedingly irregular. Now and then, at intervals of hundreds ofthousands or millions of years, the orbit becomes very elliptical;then again for long periods it may in form approach a circle. When inthe state of extreme ellipticity, the precession of the equinoxes willcause the hemispheres in turn each to have their winter and summeralternately near and far from the sun. It is easily seen that when thesummer season comes to a hemisphere in the part of the orbit which isthen nearest the sun the period will be very hot. When the summercame farthest from the sun that part of the year would have thetemperature mitigated by its removal to a greater distance from thesource of heat. A corresponding effect would be produced in the winterseason. As long as the orbit remained eccentric the tendency would beto give alternately intense seasons to each hemisphere through periodsof about twelve thousand years, the other hemisphere having at thesame time a relatively slight variation in the summer and winter. 

At first sight it may seem to the reader that these studies we havejust been making in matters concerning the shape of the orbit and theattendant circumstances which regulate the seasons were of no verygreat consequence; but, in the opinion of some students of climate, weare to look to these processes for an explanation of certain climatalchanges on the earth, including the Glacial periods, accidents whichhave had the utmost importance in the history of man, as well as ofall the other life of the planet. 

It is now time to give some account as to what is known concerning thegeneral conditions of the solar bodies--the planets and satellites ofour own celestial group. For our purpose we need attend only to thegeneral physical state of these orbs so far as it is known to us bythe studies of astronomers. The nearest planet to the sun is Mercury. This little sphere, less than half the diameter of our earth, is soclose to the sun that even when most favourably placed for observationit is visible for but a few minutes before sunrise and after sunset. Although it may without much difficulty be found by the ordinary eye, very few people have ever seen it. To the telescope when it is in the_full moon_ state it appears as a brilliant disk; it is held by mostastronomers that the surface which we see is made up altogether ofclouds, but this, as most else that has been stated concerning thisplanet, is doubtful. The sphere is so near to the sun that if it werepossessed of water it would inevitably bear an atmosphere full ofvapour. Under any conceivable conditions of a planet placed asMercury is, provided it had an atmosphere to retain the heat, itstemperature would necessarily be very high. Life as we know it couldnot well exist upon such a sphere. 

Next beyond Mercury is Venus, a sphere only a little less in diameterthan the earth. Of this sphere we know more than we do of Mercury, forthe reason that it is farther from the sun and so appears in thedarkened sky. Most astronomers hold that the surface of this planetapparently is almost completely and continually hidden from us by whatappears to be a dense cloud envelope, through which from time to timecertain spots appear of a dark colour. These, it is claimed, retaintheir place in a permanent way; it is, indeed, by observing them thatthe rotation period of the planet has, according to some observers, been determined. It therefore seems likely that these spots are thesummits of mountains, which, like many of our own earth, rise abovethe cloud level. 

Recent observations on Venus made by Mr. Percival Lowell appear toshow that the previous determinations of the rotation of that planet, as well as regards its cloud wrap, are in error. According to theseobservations, the sphere moves about the sun, always keeping the sameside turned toward the solar centre, just as the moon does in itsmotion around the earth. Moreover, Mr. Lowell has failed to discoverany traces of clouds upon the surface of the planet. As yet theseresults have not been verified by the work of other astronomers;resting, however, as they do on studies made with an excellenttelescope and in the very translucent and steady air of the FlagstaffStation, they are more likely to be correct than those obtained byother students. If it be true that Venus does not turn upon its axis, such is likely to be the case also with the planet Mercury. 

Next in the series of the planets is our own earth. As the details ofthis planet are to occupy us during nearly all the remainder of thiswork, we shall for the present pass it by. 

Beyond the earth we pass first to the planet Mars, a sphere which hasalready revealed to us much concerning its peculiarities of form andphysical state, and which is likely in the future to give moreinformation than we shall obtain from any other of our companions inspace, except perhaps the moon. Mars is not only nearer to us than anyother planet, but it is so placed that it receives the light of thesun under favourable conditions for our vision. Moreover, its skyappears to be generally almost cloudless, so that when in its orbitalcourse the sphere is nearest our earth it is under favourableconditions for telescopic observation. At such times there is revealedto the astronomer a surface which is covered with an amazing number ofshadings and markings which as yet have been incompletely interpreted. The faint nature of these indications has led to very contradictorystatements as to their form; no two maps which have been drawn agreeexcept in their generalities. There is reason to believe that Mars hasan atmosphere; this is shown by the fact that in the appropriateseason the region about either pole is covered by a white coating, presumably snow. This covering extends rather less far toward theplanet's equator than does the snow sheet on our continents. Takinginto account the colour of the coating, and the fact that itdisappears when the summer season comes to the hemisphere in which itwas formed, we are, in fact, forced to believe that the deposit isfrozen water, though it has been suggested that it may be frozencarbonic acid. Taken in connection with what we have shortly to noteconcerning the apparent seas of this sphere, the presumption isoverwhelmingly to the effect that Mars has seasons not unlike our own. 

The existence of snow on any sphere may safely be taken as evidencethat there is an atmosphere. In the case of Mars, this supposition isborne out by the appearance of its surface. The ruddy light which itsends back to us, and the appearance on the margin of the sphere, which is somewhat dim, appears to indicate that its atmosphere isdense. In fact, the existence of an atmosphere much denser than thatof our own earth appears to be demanded by the fact that thetemperatures are such as to permit the coming and going of snow. It iswell known that the temperature of any point on the earth, otherthings being equal, is proportionate to the depth of atmosphere aboveits surface. If Mars had no more air over its surface than has anequal area of the earth, it would remain at a temperature so low thatsuch seasonal changes as we have observed could not take place. Theplanet receives one third less heat than an equal area of the earth, and its likeness to our own temperature, if such exists, is doubtlessbrought about by the greater density of its atmosphere, that serves toretain the heat which comes upon its surface. The manner in which thisis effected will be set forth in the study of the earth's atmosphere. 

[Illustration: Fig. 4. --Mars, August 27, 1892 (Guiot), the white patchis the supposed Polar Snow Cap. ]

As is shown by the maps of Mars, the surface is occupied by shadingswhich seem to indicate the existence of water and lands. Thoseportions of the area which are taken to be land are very much dividedby what appear to be narrow seas. The general geographic conditionsdiffer much from those of our own sphere in that the parts of theplanet about the water level are not grouped in great continents, andthere are no large oceans. The only likeness to the conditions of ourearth which we can perceive is in a general pointing of the somewhattriangular masses of what appears to be land toward one pole. As awhole, the conditions of the Martial lands and seas as regards theirform, at least, is more like that of Europe than that of any otherpart of the earth's surface. Europe in the early Tertiary times had aconfiguration even more like that of Mars than it exhibits at present, for in that period the land was very much more divided than it now is. 

If the lands of Mars are framed as are those of our own earth, thereshould be ridges of mountains constituting what we may term thebackbones of the continent. As yet such have not been discerned, whichmay be due to the fact that they have not been carefully looked for. The only peculiar physical features which have as yet been discernedon the lands of Mars are certain long, straight, rather narrowcrevicelike openings, which have received the name of "canals. " Thesefeatures are very indistinct, and are just on the limit of visibility. As yet they have been carefully observed by but few students, so thattheir features are not yet well recorded; as far as we know them, these fissures have no likeness in the existing conditions of ourearth. It is difficult to understand how they are formed or preservedon a surface which is evidently subjected to rainfalls. 

It will require much more efficient telescopes than we now have beforeit will be possible to begin any satisfactory study on the geographyof this marvellous planet. We can not hope as yet to obtain anyindications as to the details of its structure; we can not see closelyenough to determine whether rivers exist, or whether there is acoating which we may interpret as vegetation, changing its hues in thedifferent seasons of the year. An advance in our instruments ofresearch during the coming century, if made with the same speed asduring the last, will perhaps enable us to interpret the nature ofthis neighbour, and thereby to extend the conception of planetaryhistories which we derive from our own earth. 

[Illustration: Fig. 5. --Comparative Sizes of the Planets (Chambers). ]

Beyond Mars we find one of the most singular features of our solarsystem in a group of small planetary bodies, the number of which nowknown amounts to some two hundred, and the total may be far greater. These bodies are evidently all small; it is doubtful if the largest isthree hundred and the smaller more than twenty miles in diameter. Sofar as it has been determined by the effect of their aggregate mass inattracting the other spheres, they would, if put together, make asphere far less in diameter than our earth, perhaps not more than fivehundred miles through. The forms of these asteroids is as yet unknown;we therefore can not determine whether their shapes are spheroidal, asare those of the other planets, or whether they are angular bits likethe meteorites. We are thus not in a position to conjecture whethertheir independence began when the nebulous matter of the ring to whichthey belonged was in process of consolidation, or whether, after theaggregation of the sphere was accomplished, and the matter solidified, the mass was broken into bits in some way which we can not yetconceive. It has been conjectured that such a solid sphere might havebeen driven asunder by a collision with some wandering celestial body;but all we can conceive of such actions leads us to suppose that ablow of this nature would tend to melt or convert materials subjectedto it into the state of vapour, rather than to drive them asunder inthe manner of an explosion. 

The four planets which lie beyond the asteroids give us relativelylittle information concerning their physical condition, though theyafford a wide field for the philosophic imagination. From this pointof view the reader is advised to consult the writings of the late R. A. Proctor, who has brought to the task of interpreting the planetaryconditions the skill of a well-trained astronomer and a remarkableconstructive imagination. 

The planet Jupiter, by far the largest of the children of the sun, appears to be still in a state where its internal heat has not so farescaped that the surface has cooled down in the manner of our earth. What appear to be good observations show that the equatorial part ofits area, at least, still glows from its own heat. The sphere iscloud-wrapped, but it is doubtful whether the envelope be of wateryvapour; it is, indeed, quite possible that besides such vapour it maycontain some part of the many substances which occupy the atmosphereof the sun. If the Jovian sphere were no larger than the earth, itwould, on account of its greater age, long ago have parted with itsheat; but on account of its great size it has been able, notwithstanding its antiquity, to retain a measure of temperaturewhich has long since passed away from our earth. 

In the case of Saturn, the cloud bands are somewhat less visible thanon Jupiter, but there is reason to suppose in this, as in thelast-named planet, that we do not behold the more solid surface of thesphere, but see only a cloud wrap, which is probably due rather to theheat of the sphere itself than to that which comes to it from the sun. At the distance of Saturn from the centre of the solar system a givenarea of surface receives less than one ninetieth of the sun's heat ascompared with the earth; therefore we can not conceive that anydensity of the atmosphere whatever would suffice to hold in enoughtemperature to produce ordinary clouds. Moreover, from time to timebright spots appear on the surface of the planet, which must be due tosome form of eruptions from its interior. 

Beyond Saturn the two planets Uranus and Neptune, which occupy theouter part of the solar system, are so remote that even our besttelescopes discern little more than their presence, and the fact thatthey have attendant moons. 

From the point of view of astronomical science, the outermost planetNeptune, of peculiar interest for the reason that it was, as we maysay, discovered by computation. Astronomers had for many yearsremarked the fact that the next inner planetary sphere exhibitedpeculiarities in its orbit which could only be accounted for on thesupposition that it was subjected to the attraction of anotherwandering body which had escaped observation. By skilful computationthe place in the heavens in which this disturbing element lay was soaccurately determined that when the telescope was turned to the givenfield a brief study revealed the planet. Nothing else in the historyof the science of astronomy, unless it be the computation of eclipses, so clearly and popularly shows the accuracy of the methods by whichthe work of that science may be done. 

As we shall see hereafter, in the chapters which are devoted toterrestrial phenomena, the physical condition of the sun determinesthe course of all the more important events which take place on thesurface of the earth. It is therefore fit that in this preliminarystudy of the celestial bodies, which is especially designed to makethe earth more interpretable to us, we should give a somewhat specialattention to what is known under the title of "Solar Physics. "

The reader has already been told that the sun is one of many millionsimilar bodies which exist in space, and, furthermore, that theseaggregations of matter have been developed from an original nebulouscondition. The facts indicate that the natural history of the sun, aswell as that of its attendant spheres, exhibits three momentousstages: First, that of vapour; second, that of igneous fluidity;third, that in which the sphere is so far congealed that it becomesdark. Neither of these states is sharply separated from the other; amass may be partly nebulous and partly fluid; even when it has beenconverted into fluid, or possibly into the solid state, it may stillretain on the exterior some share of its original vaporous condition. In our sun the concentration has long since passed beyond the limitsof the nebulous state; the last of the successively developed ringshas broken, and has formed itself into the smallest of the planets, which by its distance from the sun seems to indicate that the processof division by rings long ago attained in our solar system its end, the remainder of its nebulous material concentrating on its centrewithout sign of any remaining tendency to produce these planet-makingcircles. 

 THE CONSTITUTION OF THE SUN. 

Before the use of the telescope in astronomical work, which was begunby the illustrious Galileo in 1608, astronomers were unable toapproach the problem of the structure of the sun. They could discernno more than can be seen by any one who looks at the great spherethrough a bit of smoked glass, as we know this reveals a disklike bodyof very uniform appearance. The only variation in this simple aspectoccurs at the time of a total eclipse, when for a minute or two themoon hides the whole body of the sun. On such occasions even theunaided eye can see that there is about the sphere a broad, ratherbright field, of an aspect like a very thin cloud or fog, which risesin streamer like projections at points to a quarter of a million milesor more above the surface of the sphere. The appearance of thisshining field, which is called the corona, reminds one of the aurorawhich glows in the region about either pole of the earth. 

One of the first results of the invention of the telescope was therevelation of the curious dark objects on the sun's disk, known by thename of spots from the time of their discovery, or, at least, from thetime when it was clearly perceived that they were not planets, butreally on the solar body. The interest in the constitution of thesphere has increased during the last fifty years. This interest hasrapidly grown until at the present time a vast body of learning hasbeen gathered for the solution of the many problems concerning thecentre of our system. As yet there is great divergence in the views ofastronomers as to the interpretation of their observations, butcertain points of great general interest have been tolerably welldetermined. These may be briefly set forth by an account of what wouldmeet the eye if an observer were able to pass from the surface of theearth to the central part of the sun. 

[Illustration: _Lava stream, in Hawaiian Islands, flowing into thesea. Note the "ropy" character of the half-frozen rock on the sides ofthe nearest rivulet of the lava. _]

In passing from the earth to a point about a quarter of a millionmiles from the sun's surface--a distance about that of the moon fromour sphere--the observer would traverse the uniformly empty spaces ofthe heavens, where, but for the rare chance of a passing meteorite orcomet, there would be nothing that we term matter. Arriving at a pointsome two or three hundred thousand miles from the body of the sun, hewould enter the realm of the corona; here he would find scatteredparticles of matter, the bits so far apart that there would perhaps benot more than one or two in the cubic mile; yet, as they would glowintensely in the central light, they would be sufficient to give theillumination which is visible in an eclipse. These particles are mostlikely driven up from the sun by some electrical action, and areconstantly in motion, much as are the streamers of the aurora. 

Below the corona and sharply separated from it the observer findsanother body of very dense vapour, which is termed the chromosphere, and which has been regarded as the atmosphere of the sun. This layeris probably several thousand miles thick. From the manner in which itmoves, in the way the air of our own planet does in great storms, itis not easy to believe that it is a fluid, yet its sharply definedupper surface leads us to suppose that it can not well be a mere massof vapour. The spectroscope shows us that this chromosphere containsin the state of vapour a number of metallic substances, such as ironand magnesium. To an observer who could behold this envelope of thesun from the distance at which we see the moon, the spectacle would bemore magnificent than the imagination, guided by the sight of all therelatively trifling fractures of our earth, can possibly conceive. From the surface of the fiery sea vast uprushes of heated matter riseto the height of two or three hundred thousand miles, and then fallback upon its surface. These jets of heated matter have the aspect offlames, but they would not be such in fact, for the materials are notburning, but merely kept at a high temperature by the heat of thegreat sphere beneath. They spring up with such energy that they attimes move with a speed of one hundred and fifty miles a second, or ata rate which is attained by no other matter in the visible universe, except that strange, wandering star known to astronomers as"Grombridge, 1830, " which is traversing the firmament with a speed ofnot less than two hundred miles a second. 

Below the chromosphere is the photosphere, the lower envelope of thesun, if it be not indeed the body of the sphere itself; from thiscomes the light and heat of the mass. This, too, can not well be afirm-set mass, for the reason that the spots appear to form in andmove over it. It may be regarded as an extremely dense mass of gas, soweighed down by the vast attraction of the great sphere below it thatit is in effect a fluid. The near-at-hand observer would doubtlessfind this photosphere, as it appears in the telescope, to be sharplyseparated from the thinner and more vaporous envelopes--thechromosphere and the corona--which are, indeed, so thin that they areinvisible even with the telescope, except when the full blaze of thesun is cut off in a total eclipse. The fact that the photosphere, except when broken by the so-called spots, lies like a great smoothsea, with no parts which lie above the general line, shows that it hasa very different structure from the envelope which lies upon it. Ifthey were both vaporous, there would be a gradation between them. 

On the surface of the photosphere, almost altogether within thirtydegrees of the equator of the sun, a field corresponding approximatelyto the tropical belt of the earth, there appear from time to time thecurious disturbances which are termed spots. These appear to beuprushes of matter in the gaseous state, the upward movement beingupon the margins of the field and a downward motion taking place inthe middle of the irregular opening, which is darkened in its centralpart, thus giving it, when seen by an ordinary telescope, the aspectof a black patch on the glowing surface. These spots, which are fromsome hundred to some thousand miles in diameter, may endure formonths before they fade away. It is clear that they are most abundantat intervals of about eleven years, the last period of abundance beingin 1893. The next to come may thus be expected in 1904. In the timesof least spotting more than half the days of a year may pass withoutthe surface of the photosphere being broken, while in periods ofplenty no day in the year is likely to fail to show them. 

[Illustration: Fig. 6. --Ordinary Sun-spot, June 22, 1885. ]

It is doubtful if the closest seeing would reveal the cause of thesolar spots. The studies of the physicists who have devoted the mostskill to the matter show little more than that they are tumults in thephotosphere, attended by an uprush of vapours, in which iron and othermetals exist; but whether these movements are due to outbreaks fromthe deeper parts of the sun or to some action like the whirling stormsof the earth's atmosphere is uncertain. It is also uncertain whateffect these convulsions of the sun have on the amount of the heat andlight which is poured forth from the orb. The common opinion that thesun-spot years are the hottest is not yet fully verified. 

Below the photosphere lies the vast unknown mass of the unseen solarrealm. It was at one time supposed that the dark colour of the spotswas due to the fact that the photosphere was broken through in thosespaces, and that we looked down through them upon the surface of theslightly illuminated central part of the sphere. This view isuntenable, and in its place we have to assume that for the eighthundred and sixty thousand miles of its diameter the sun is composedof matter such as is found in our earth, but throughout in a state ofheat which vastly exceeds that known on or in our planet. Owing to itsheat, this matter is possibly not in either the solid or the fluidstate, but in that of very compressed gases, which are kept frombecoming solid or even fluid by the very high temperature which existsin them. This view is apparently supported by the fact that, while thepressure upon its matter is twenty-seven times greater in the sun thanit is in the earth, the weight of the whole mass is less than weshould expect under these conditions. 

As for the temperature of the sun, we only know that it is hot enoughto turn the metals into gases in the manner in which this is done in astrong electric arc, but no satisfactory method of reckoning the scaleof this heat has been devised. The probabilities are to the effectthat the heat is to be counted by the tens of thousands of degreesFahrenheit, and it may amount to hundreds of thousands; it has, indeed, been reckoned as high as a million degrees. This vastdischarge is not due to any kind of burning action--i. E. , to thecombustion of substances, as in a fire. It must be produced by thegradual falling in of the materials, due to the gravitation of themass toward its centre, each particle converting its energy ofposition into heat, as does the meteorite when it comes into the air. 

It is well to close this very imperfect account of the learning whichrelates to the sun with a brief tabular statement showing the relativemasses of the several bodies of the solar system. It should beunderstood that by mass is meant not the bulk of the object, but theactual amount of matter in it as determined by the gravitativeattraction which it exercises on other celestial bodies. In this testthe sun is taken as the measure, and its mass is for conveniencereckoned at 1, 000, 000, 000. 

 TABLE OF RELATIVE MASSES OF SUN AND PLANETS. [2] +------------------------------------------------------------+ | The sun 1, 000, 000, 000 | | Mercury 200 | | Venus 2, 353 | | Earth 3, 060 | | Mars 339 | | Asteroids ? | | Saturn 285, 580 | | Jupiter 954, 305 | | Uranus 44, 250 | | Neptune 51, 600 | | Combined mass of the four inner planets 5, 952 | | Combined mass of all the planets 1, 341, 687 | +------------------------------------------------------------+

[Footnote 2: See Newcomb's Popular Astronomy, p. 234. Harper Brothers, New York. ]

It thus appears that the mass of all the planets is about one sevenhundredth that of the sun. 

Those who wish to make a close study of celestial geography will dowell to procure the interesting set of diagrams prepared by the lateJames Freeman Clarke, in which transparencies placed in a convenientlantern show the grouping of the important stars in eachconstellation. The advantage of this arrangement is that the littlemaps can be consulted at night and in the open air in a veryconvenient manner. After the student has learned the position of adozen of the constellations visible in the northern hemisphere, he canrapidly advance his knowledge in the admirable method invented by Dr. Clarke. 

Having learned the constellations, the student may well proceed tofind the several planets, and to trace them in their apparent pathacross the fixed stars. It will be well for him here to gain if he canthe conception that their apparent movement is compounded of theirmotion around the sun and that of our own sphere; that it would bevery different if our earth stood still in the heavens. At this stagehe may well begin to take in mind the evidence which the planetarymotion supplies that the earth really moves round the sun, and notthe sun and planets round the earth. This discovery was one of thegreat feats of the human mind; it baffled the wits of the best men forthousands of years. Therefore the inquirer who works over the evidenceis treading one of the famous paths by which his race climbed thesteeps of science. 

The student must not expect to find the evidence that the sun is thecentre of the solar system very easy to interpret; and yet any youthof moderate curiosity, and that interest in the world about him whichis the foundation of scientific insight, can see through the matter. He will best begin his inquiries by getting a clear notion of the factthat the moon goes round the earth. This is the simplest case ofmovements of this nature which he can see in the solar system. Notingthat the moon occupies a different place at a given hour in thetwenty-four, but is evidently at all times at about the same distancefrom the earth, he readily perceives that it circles about our sphere. This the people knew of old, but they made of it an evidence that thesun also went around our sphere. Here, then, is the critical point. Why does the sun not behave in the same manner as the moon? At thisstage of his inquiry the student best notes what takes place in themotions of the planets between the earth and the sun. He observes thatthose so-called inferior planets Mercury and Venus are never very faraway from the central body; that they appear to rise up from it, andthen to go back to it, and that they have phases like the moon. Nowand then Venus may be observed as a black spot crossing the disk ofthe sun. A little consideration will show that on the theory thatbodies revolve round each other in the solar system these movements ofthe inner planets can only be explained on the supposition that theyat least travel around the great central fire. Now, taking up theouter planets, we observe that they occasionally appear very bright, and that they are then at a place in the heavens where we see thatthey are far from the solar centre. Gradually they move down towardthe sunset and disappear from view. Here, too, the movement, thoughless clearly so, is best reconcilable with the idea that these bodiestravel in orbits, such as those which are traversed by the innerplanets. The wonder is that with these simple facts before them, andwith ample time to think the matter over, the early astronomers didnot learn the great truth about the solar system--namely, that the sunis the centre about which the planets circled. Their difficulty laymainly in the fact that they did not conceive the earth as a sphere, and even after they attained that conception they believed that ourglobe was vastly larger than the planets, or even than the sun. Thismisconception kept even the thoughtful Greeks, who knew that the earthwas spherical in form, from a clear notion as to the structure of oursystem. It was not, indeed, until mathematical astronomy attained aconsiderable advance, and men began to measure the distances in thesolar system, and until the Newtonian theory of gravitation wasdeveloped, that the planetary orbits and the relation of the variousbodies in the solar system to each other could be perfectly discerned. 

Care has been taken in the above statements to give the studentindices which may assist him in working out for himself the evidencewhich may properly lead a person, even without mathematicalconsiderations of a formal kind, to construct a theory as to therelation of the planets to the sun. It is not likely that he can gothrough all the steps of this argument at once, but it will be mostuseful to him to ponder upon the problem, and gradually win his way toa full understanding of it. With that purpose in mind, he should avoidreading what astronomers have to say on the matter until he issatisfied that he has done as much as he can with the matter on hisown account. He should, however, state his observations, and as far aspossible draw the results in his note-book in a diagrammatic form. Heshould endeavour to see if the facts are reconcilable with any othersupposition than that the earth and the other planets move around thesun. When he has done his task, he will have passed over one of themost difficult roads which his predecessors had to traverse on theirway to an understanding of the heavens. Even if he fail he will havehelped himself to some large understandings. 

The student will find it useful to make a map of the heavens, orrather make several representing their condition at different times inthe year. On this plot he should put down only the stars whose placesand names he has learned, but he should plot the position of theplanets at different times. In this way, though at first his effortswill be very awkward, he will soon come to know the general geographyof the heavens. 

Although the possession or at least the use of a small astronomicaltelescope is a great advantage to a student after he has made acertain advance in his work, such an instrument is not at allnecessary, or, indeed, desirable at the outset of his studies. Anordinary opera-glass, however, will help him in picking out the starsin the constellations, in identifying the planets, and in getting abetter idea as to the form of the moon's surface--a matter which willbe treated in this work in connection with the structure of the earth. 

 CHAPTER IV. 

 THE EARTH. 

In beginning the study of the earth it is important that the studentshould at once form the habit of keeping in mind the spherical form ofthe planet. Many persons, while they may blindly accept the fact thatthe earth is a sphere, do not think of it as having that form. Perhapsthe simplest way of securing the correct image of the shape is toimagine how the earth would appear as seen from the moon. In its fullcondition the moon is apt to appear as a disk. When it is new, andalso when in its waning stages it is visible in the daytime, thespherical form is very apparent. Imagining himself on the surface ofthe moon, the student can well perceive how the earth would appear asa vast body in the heavens; its eight thousand miles of diameter, about four times that of the satellite, would give an area sixteentimes the size which the moon presents to us. On this scale thecontinents and oceans would appear very much more plain than do therelatively slight irregularities on the lunar surface. 

With the terrestrial globe in hand, the student can readily constructan image which will represent, at least in outline, the appearancewhich the sphere he inhabits would present when seen from a distanceof about a quarter of a million miles away. The continent ofEurope-Asia would of itself appear larger than all the lunar surfacewhich is visible to us. Every continent and all the greater islandswould be clearly indicated. The snow covering which in the winter ofthe northern hemisphere wraps so much of the land would be seen tocome and go in the changes of the seasons; even the permanent iceabout either pole, and the greater regions of glaciers, such as thoseof the Alps and the Himalayas, would appear as brilliant patches ofwhite amid fields of darker hue. Even the changes in the aspect of thevegetation which at one season clothes the wide land with a greenmantle, and at another assumes the dun hue of winter, would be, to theunaided eye, very distinct. It is probable that all the greater riverswould be traceable as lines of light across the relatively darksurface of the continents. By such exercises of the constructiveimagination--indeed, in no other way--the student can acquire thehabit of considering the earth as a vast whole. From time to time ashe studies the earth from near by he should endeavour to assemble thephenomena in the general way which we have indicated. 

The reader has doubtless already learned that the earth is a slightlyflattened sphere, having an average diameter of about eight thousandmiles, the average section at the equator being about twenty-six milesgreater than that from pole to pole. In a body of such largeproportions this difference in measurement appears not important; itis, however, most significant, for it throws light upon the history ofthe earth's mass. Computation shows that the measure of flattening atthe poles is just what would occur if the earth were or had been atthe time when it assumed its present form in a fluid condition. Wereadily conceive that a soft body revolving in space, while all itsparticles by gravitation tended to the centre, would in turningaround, as our earth does upon its axis, tend to bulge out in thoseparts which were remote from the line upon which the turning tookplace. Thus the flattening of our sphere at the poles corroborates theopinion that its mass was once molten--in a word, that its ancienthistory was such as the nebular theory suggests. 

Although we have for convenience termed the earth a flattenedspheroid, it is only such in a very general sense. It has an infinitenumber of minor irregularities which it is the province of thegeographer to trace and that of the geologist to account for. In thefirst place, its surface is occupied by a great array of ridges andhollows. The larger of these, the oceans and continents, first deserveour attention. The difference in altitude of the earth's surface fromthe height of the continents to the deepest part of the sea isprobably between ten and eleven miles, thus amounting to about twofifths of the polar flattening before noted. The average differencebetween the ocean floor and the summits of the neighbouring continentsis probably rather less than four miles. It happens, most fortunatelyfor the history of the earth, that the water upon its surface fillsits great concavities on the average to about four fifths of theirtotal depth, leaving only about one fifth of the relief projectingabove the ocean level. We have termed this arrangement fortunate, forit insures that rainfall visits almost all the land areas, and therebymakes those realms fit for the uses of life. If the ocean had onlyhalf its existing area, the lands would be so wide that only theirfringes would be fertile. If it were one fifth greater than it is, thedry areas would be reduced to a few scattered islands. 

From all points of view the most important feature of the earth'ssurface arises from its division into land and water areas, and thisfor the reason that the physical and vital work of our sphere isinevitably determined by this distribution. The shape of the seas andlands is fixed by the positions at which the upper level of the greatwater comes against the ridges which fret the earth's surface. Theseelevations are so disposed that about two thirds of the hard mass isat the present time covered with water, and only one third exposed tothe atmosphere. This proportion is inconstant. Owing to the endlessup-and-down goings of the earth's surface, the place of the shorelines varies from year to year, and in the geological ages greatrevolutions in the forms and relative area of water and land arebrought about. 

Noting the greater divisions of land and water as they are shown on aglobe, we readily perceive that those parts of the continental ridgeswhich rise above the sea level are mainly accumulated in the northernhemisphere--in fact, far more than half the dry realm is in that partof the world. We furthermore perceive that all the continents more orless distinctly point to the southward; they are, in a word, triangles, with their bases to the northward, and their apices, usually rather acute, directed to the southward. This form is verywell indicated in three of the great lands, North and South Americaand Africa; it is more indistinctly shown in Asia and in Australia. Asyet we do not clearly understand the reason why the continents aretriangular, why they point toward the south pole, or why they aremainly accumulated in the northern hemisphere. As stated in thechapter on astronomy, some trace of the triangular form appears in theland masses of the planet Mars. There, too, these triangles appear topoint toward one pole. 

Besides the greater lands, the seas are fretted by a host of smallerdry areas, termed islands. These, as inquiry has shown, are of twovery diverse natures. Near the continents, practically never more thana thousand miles from their shores, we find isles, often of greatsize, such as Madagascar, which in their structure are essentiallylike the continents--that is, they are built in part or in whole ofnon-volcanic rocks, sandstones, limestones, etc. In most cases theseislands, to which we may apply the term continental, have at some timebeen connected with the neighbouring mainland, and afterward separatedfrom it by a depression of the surface which permitted the sea to flowover the lowlands. Geologists have traced many cases where in the pastelevations which are now parts of a continent were once islands nextits shore. In the deeper seas far removed from the margins of thecontinents the islands are made up of volcanic ejections of lava, pumice, and dust, which has been thrown up from craters and fallenaround their margin or are formed of coral and other organic remains. 

Next after this general statement as to the division of sea and landwe should note the peculiarities which the earth's surface exhibitswhere it is bathed by the air, and where it is covered by the water. Beginning with the best-known region, that of the dry land, we observethat the surface is normally made up of continuous slopes of varyingdeclivity, which lead down from the high points to the sea. Here andthere, though rarely, these slopes centre in a basin which is occupiedby a lake or a dead sea. On the deeper ocean floors, so far as we mayjudge with the defective information which the plumb line gives us, there is no such continuity in the downward sloping of the surface, the area being cast into numerous basins, each of great extent. 

When we examine in some detail the shape of the land surface, wereadily perceive that the continuous down slopes are due to thecutting action of rivers. In the basin of a stream the waters act towear away the original heights, filling them into the hollows, untilthe whole area has a continuous down grade to the point where thewaters discharge into the ocean or perhaps into a lake. On the bottomof the sea, except near the margin of the continent, where the floormay in recent geological times have been elevated into the air, andthus exposed to river action, there is no such agent working toproduce continuous down grades. 

Looking upon a map of a continent which shows the differences inaltitude of the land, we readily perceive that the area is ratherclearly divided into two kinds of surface, mountains and plains, eachkind being sharply distinguished from the other by many importantpeculiarities. Mountains are characteristically made up of distinct, more or less parallel ridges and valleys, which are grouped in veryelongated belts, which, in the case of the American Cordilleras, extend from the Arctic to the Antarctic Circle. Only in rare instancesdo we find mountains occupying an area which is not very distinctlyelongated, and in such cases the elevations are usually of no greatheight. Plains, on the other hand, commonly occupy the larger part ofthe continent, and are distributed around the flanks of the mountainsystems. There is no rule as to their shape; they normally grade awayfrom the bases of the mountains toward the sea, and are oftenprolonged below the level of the water for a considerable distancebeyond the shore, forming what is commonly known as the continentalshelf or belt of shallows along the coast line. We will now considersome details concerning the form and structure of mountains. 

In almost any mountain region a glance over the surface of the countrywill give the reader a clew to the principal factor which hasdetermined the existence of these elevations. Wherever the bed rocksare revealed he will recognise the fact that they have been muchdisturbed. Almost everywhere the strata are turned at high angles;often their slopes are steeper than those of house roofs, and notinfrequently they stand in attitudes where they appear vertical. Underthe surface of plains bedded rocks generally retain the nearlyhorizontal position in which all such deposits are most likely to befound. If the observer will attentively study the details of positionof these tilted rocks of mountainous districts, he will in most casesbe able to perceive that the beds have been flexed or folded in themanner indicated by the diagram. Sometimes, though rarely, the tops ofthese foldings or arches have been preserved, so that the nature ofthe movement can be clearly discerned. More commonly the upper partsof the upward-arching strata have been cut off by the action of thedecay-bringing forces--frost, flowing water, or creeping ice inglaciers--so that only the downward pointing folds which were formedin the mountain-making are well preserved, and these are almostinvariably hidden within the earth. 

[Illustration: Fig. 7. --Section of mountains. Rockbridge and Bathcounties, Va. (from Dana). The numbers indicate the severalformations. ]

By walking across any considerable mountain chain, as, for instance, that of the Alleghanies, it is generally possible to trace a number ofthese parallel up-and-down folds of the strata, so that we readilyperceive that the original beds had been packed together into a muchless space than they at first occupied. In some cases we could provethat the shortening of the line has amounted to a hundred miles ormore--in other words, points on the plain lands on either side of themountain range which now exists may have been brought a hundred milesor so nearer together than they were before the elevations wereproduced. The reader can make for himself a convenient diagram showingwhat occurred by pressing a number of leaves of this book so that thesheets of paper are thrown into ridges and furrows. By this experimenthe also will see that the easiest way to account for such foldings aswe observe in mountains is by the supposition that some force residingin the earth tends to shove the beds into a smaller space than theyoriginally occupied. Not only are the rocks composing the mountainsmuch folded, but they are often broken through after the manner ofmasonry which has been subjected to earthquake shocks, or of ice whichhas been strained by the expansion that affects it as it becomeswarmed before it is melted. In fact, many of our small lakes in NewEngland and in other countries of a long winter show in a miniatureway during times of thawing ice folds which much resemble mountainarches. 

At first geologists were disposed to attribute all the phenomena ofmountain-folding to the progressive cooling of the earth. Althoughthis sphere has already lost a large part of the heat with which itwas in the beginning endowed, it is still very hot in its deeperparts, as is shown by the phenomena of volcanoes. This internal heat, which to the present day at the depth of a hundred miles below thesurface is probably greater than that of molten iron, is constantlyflowing away into space; probably enough of it goes away on theaverage each day to melt a hundred cubic miles or more of ice, or, inmore scientific phrase, the amount of heat rendered latent by meltingthat volume of frozen water. J. R. Meyer, an eminent physicist, estimated the quantity of heat so escaping each day of the year to besufficient to melt two hundred and forty cubic miles of ice. Theeffect of this loss of heat is constantly to shrink the volume of theearth; it has, indeed, been estimated that the sphere on this accountcontracts on the average to the amount of some inches each thousandyears. For the reason that almost all this heat goes from the depthsof the earth, the cool outer portion losing no considerable part ofit, the contraction that is brought about affects the interiorportions of the sphere alone. The inner mass constantly shrinking asit loses heat, the outer, cold part is by its weight forced to settledown, and can only accomplish this result by wrinkling. An analogousaction may be seen where an apple or a potato becomes dried; in thiscase the hard outer rind is forced to wrinkle, because, losing nowater, it does not diminish in its extent, and can only accommodateitself to the interior by a wrinkling process. In one case it is waterwhich escapes, in the other heat; but in both contraction of the partwhich suffers the loss leads to the folding of the outside of thespheroid. 

Although this loss of heat on the part of the earth accounts in somemeasure for the development of mountains, it is not of itselfsufficient to explain the phenomena, and this for the reason thatmountains appear in no case to develop on the floors of the wide sea. The average depth of the ocean is only fifteen thousand feet, whilethere are hundreds, if not thousands, of mountain crests which exceedthat height above the sea. Therefore if mountains grew on the seafloor as they do upon the land, there should be thousands of peaksrising above the plain of the waters, while, in fact, all of theislands except those near the shores of continents are of volcanicorigin--that is, are lands of totally different nature. 

Whenever a considerable mountain chain is formed, although the actualfolding of the beds is limited to the usually narrow field occupied bythese disturbances, the elevation takes place over a wide belt ofcountry on one or both sides of the range. Thus if we approach theRocky Mountains from the Mississippi Valley, we begin to mount up aninclined plane from the time we pass westward from the MississippiRiver. The beds of rock as well as the surface rises gradually untilat the foot of the mountain; though the rocks are still withoutfoldings, they are at a height of four or five thousand feet above thesea. It seems probable--indeed, we may say almost certain--that whenthe crust is broken, as it is in mountain-building, by extensive foldsand faults, the matter which lies a few score miles below the crustcreeps in toward those fractures, and so lifts up the country on whichthey lie. When we examine the forms of any of our continents, we findthat these elevated portions of the earth's crust appear to be made upof mountains and the table-lands which fringe those elevations. Thereis not, as some of our writers suppose, two different kinds ofelevation in our great lands--the continents and the mountains whichthey bear--but one process of elevation by which the foldings and themassive uplifts which constitute the table-lands are simultaneouslyand by one process formed. 

Looking upon continents as the result of mountain growth, we may saythat here and there on the earth's crust these dislocations haveoccurred in such association and of such magnitude that great areashave been uplifted above the plain of the sea. In general, we findthese groups of elevations so arranged that they produce thetriangular form which is characteristic of the great lands. It will beobserved, for instance, that the form of North America is in generaldetermined by the position of the Appalachian and Cordilleran systemson its eastern and western margins, though there are a number ofsmaller chains, such as the Laurentians in Canada and the ice-coveredmountains of Greenland, which have a measure of influence in fixingits shore lines. 

[Illustration: _Waterfall near Gadsden, Alabama. The upper shelf ofrock is a hard sandstone, the lower beds are soft shale. Theconditions are those of most waterfalls, such as Niagara. _]

The history of plains, as well as that of mountains, will have furtherlight thrown upon it when in the next chapter we come to consider theeffect of rain water on the land. We may here note the fact that thelevel surfaces which are above the seashores are divisible into twomain groups--those which have been recently lifted above the sealevel, composed of materials laid down in the shallows next the shore, and which have not yet shared in mountain-building disturbances, andthose which have been slightly tilted in the manner before indicatedin the case of the plains which border the Rocky Mountains on theeast. The great southern plain of eastern and southern United States, extending from near New York to Mexico, is a good specimen of thelevel lands common on all the continents which have recently emergedfrom the sea. The table-lands on either side of the MississippiValley, sloping from the Alleghanies and the Cordilleras, representthe more ancient type of plain which has already shared in theelevation which mountain-building brings about. In rarer cases plainsof small area are formed where mountains formerly existed by thecomplete moving down of the original ridges. 

There is a common opinion that the continents are liable in the courseof the geologic ages to very great changes of position; that what isnow sea may give place to new great lands, and that those alreadyexisting may utterly disappear. This opinion was indeed generally heldby geologists not more than thirty years ago. Further study of theproblem has shown us that while parts of each continent may at anytime be depressed beneath the sea, the whole of its surface rarely ifever goes below the water level. Thus, in the case of North America, we can readily note very great changes in its form since the landbegan to rise above the water. But always, from that ancient day toour own, some portion of the area has been above the level of the sea, thus providing an ark of refuge for the land life when it wasdisturbed by inundations. The strongest evidence in favour of theopinion that the existing continents have endured for many millionyears is found in the fact that each of the great lands preserves manydistinct groups of animals and plants which have descended fromancient forms dwelling upon the same territory. If at any time therelatively small continent of Australia had gone beneath the sea, allof the curious pouched animals akin to the opossum and kangaroo whichabound in that country--creatures belonging in the ancient life of theworld--would have been overwhelmed. 

We have already noted the fact that the uplifting of mountains and ofthe table-lands about them, which appears to have been the basis ofcontinental growth, has been due to strains in the rocks sufficientlystrong to disturb the beds. At each stage of the mountain-buildingmovement these compressive strains have had to contend with the verygreat weight of the rocks which they had to move. These lands are notto be regarded as firm set or rigid arches, but as highly elasticstructures, the shapes of which may be determined by any actions whichput on or take off burden. We see a proof of this fact from numerousobservations which geologists are now engaged in making. Thus duringthe last ice epoch, when almost all the northern part of thiscontinent, as well as the northern part of Europe, was covered by anice sheet several thousand feet thick, the lands sank down under theirload, and to an extent roughly proportional to the depth of the icycovering. While the northern regions were thus tilted down by theweight which was upon them, the southern section of this land, theregion about the Gulf of Mexico, was elevated much above its presentlevel; it seems likely, indeed, that the peninsula of Florida rose tothe height of several hundred feet above its present shore line. Afterthe ice passed away the movements were reversed, the northern regionrising and the southern sinking down. These movements are attested bythe position of the old shore lines formed during the later stages ofthe Glacial epoch. Thus around Lake Ontario, as well as the otherGreat Lakes, the beaches which mark the higher positions of thoseinland seas during the closing stages of the ice time, and which, ofcourse, were when formed horizontal, now rise to the northward at therate of from two to five feet for each mile of distance. Recentstudies by Mr. G. K. Gilbert show that this movement is still inprogress. 

Other evidence going to show the extent to which the movements of theearth's crust are affected by the weight of materials are found in thefact that wherever along the shores thick deposits of sediments areaccumulated the tendency of the region where they lie is gradually tosink downward, so that strata having an aggregate thickness of tenthousand feet or more may be accumulated in a sea which was alwaysshallow. The ocean floor, in general, is the part of the earth'ssurface where strata are constantly being laid down. In the greatreservoir of the waters the _débris_ washed from the land, the dustfrom volcanoes, and that from the stellar spaces, along with the vastaccumulation of organic remains, almost everywhere lead to thesteadfast accumulation of sedimentary deposits. On the other hand, therealms of the surface above the ocean level are constantly being wornaway by the action of the rivers and glaciers, of the waves which beatagainst the shores, and of the winds which blow over desert regions. The result is that the lands are wearing down at the geologicallyrapid average rate of somewhere about one foot in five thousand years. All this heavy matter goes to the sea bottoms. Probably to this causewe owe in part the fact that in the wrinklings of the crust due to thecontraction of the interior the lands exhibit a prevailing tendency touprise, while the ocean floors sink down. In this way the continentsare maintained above the level of the sea despite the powerful forceswhich are constantly wearing their substance away, while the seasremain deep, although they are continually being burdened withimported materials. 

[Illustration: Fig. 8. --Diagram showing the effect of the position ofthe fulcrum point in the movement of the land masses. In diagrams Iand II, the lines _a b_ represent the land before the movement, and_a' b'_ its position after the movement; _s_, _s_, the position of theshore line; _p_, _p_, the pivotal points; _l_, _s_, the sea line. Indiagram III, the curved line designates a shore; the line _a b_, connecting the pivotal points _p_, _p_, is partly under the land andpartly under the sea. ]

It is easy to see that if the sea floors tend to sink downward, whilethe continental lands uprise, the movements which take place may becompared with those which occur in a lever about a fulcrum point. Inthis case the sea end of the bar is descending and the land endascending. Now, it is evident that the fulcrum point may fall to theseaward or to the landward of the shore; only by chance and here andthere would it lie exactly at the coast line. By reference to thediagram (Fig. 8), it will be seen that, while the point of rotation isjust at the shore, a considerable movement may take place withoutaltering the position of the coast line. Where the point of nomovement is inland of the coast, the sea will gain on the continent;where, however, the point is to seaward, beneath the water, the landwill gain on the ocean. In this way we can, in part at least, accountfor the endless changes in the attitude of the land along the coastalbelt without having to suppose that the continents cease to rise orthe sea floors to sink downward. It is evident that the bar or sectionof the rocks from the interior of the land to the bottoms of the seasis not rigid; it is also probable that the matter in the depths of theearth, which moves with the motions of this bar, would change theposition of the fulcrum point from time to time. Thus it may well comeabout that our coast lines are swaying up and down in ceaselessvariation. 

In very recent geological times, probably since the beginning of thelast Glacial period, the region about the Dismal Swamp in Virginia hasswayed up and down through four alternating movements to the extent offrom fifty to one hundred feet. The coast of New Jersey is now sinkingat the rate of about two feet in a hundred years. The coast of NewEngland, though recently elevated to the extent of a hundred feet ormore, at a yet later time sank down, so that at some score of pointsbetween New York and Eastport, Me. , we find the remains of forestswith the roots of their trees still standing below high-tide mark inpositions where the trees could not have grown. Along all the marinecoasts of the world which have been carefully studied from this pointof view there are similar evidences of slight or great modern changesin the level of the lands. At some points, particularly on the coastof Alaska and along the coast of Peru, these uplifts of the land haveamounted to a thousand feet or more. In the peninsular district ofScandinavia the swayings, sometimes up and sometimes down, which arenow going on have considerably changed the position of the shore linessince the beginning of the historical period. 

There are other causes which serve to modify the shapes and sizes ofthe continents which may best be considered in the sequel; for thepresent we may pass from this subject with the statement that ourgreat lands are relatively permanent features; their forms change fromage to age, but they have remained for millions of years habitable tothe hosts of animals and plants which have adapted their life to theconditions which these fields afford them. 

 CHAPTER V. 

 THE ATMOSPHERE. 

The firm-set portion of the earth, composed of materials which becamesolid when the heat so far disappeared from the sphere that rockymatter could pass from its previous fluid condition to the solid orfrozen state, is wrapped about by two great envelopes, the atmosphereand the waters. Of these we shall first consider the lighter and moreuniversal air; in taking account of its peculiarities we shall have tomake some mention of the water with which it is greatly involved;afterward we shall consider the structure and functions of that fluid. 

Atmospheric envelopes appear to be common features about the celestialspheres. In the sun there is, as we have noted, a very deep envelopeof this sort which is in part composed of the elements which form ourown air; but, owing to the high temperature of the sphere, these arecommingled with many substances which in our earth--at least in itsouter parts--have entered in the solid state. Some of the planets, sofar as we can discern their conditions, seem also to have gaseouswraps; this is certainly the case with the planet Mars, and even thelittle we know of the other like spheres justifies the suppositionthat Jupiter and Saturn, at least, have a like constitution. We mayregard an atmosphere, in a word, as representing a normal andlong-continued state in the development of the heavenly orbs. In onlyone of these considerable bodies of the solar system, the moon, do wefind tolerably clear evidence that there is no atmosphere. 

The atmosphere of the earth is composed mainly of very volatileelements, known as nitrogen and argon. This is commingled with oxygen, also a volatile element. Into this mass a number of other substancesenter in varying but always relatively very small proportions. Ofthese the most considerable are watery vapour and carbon dioxide; theformer of these rarely amounts to one per cent of the weight of theair, considering the atmosphere as a whole, and the latter is nevermore than a small fraction of one per cent in amount. As a whole, theair envelope of the earth should be regarded as a mass of nitrogen andargon, which only rarely, under the influence of conditions whichexist in the soil, enters into combinations with other elements bywhich it assumes a solid form. The oxygen, though a permanent elementin the atmosphere, tends constantly to enter into combinations whichfix it temporarily or permanently in the earth, in which it forms, indeed, in its combined state about one half the weight of all themineral substances we know. The carbon dioxide, or carbonic-acid gas, as it is commonly termed, is a most important substance, as it affordsplants all that part of their bodies which disappear on burning. It isconstantly returned to the atmosphere by the decay of organic matter, as well as by volcanic action. 

In addition to the above-noted materials composing the air, all ofwhich are imperatively necessary to the wonderful work accomplished bythat envelope, we find a host of other substances which areaccidentally, variably, and always in small quantities contained inthis realm. Thus near the seashores, and indeed for a considerabledistance into the continent, we find the air contains a certain amountof salt so finely divided that it floats in the atmosphere. So, too, we find the air, even on the mountain tops amid eternal snows, chargedwith small particles of dust, which, though not evident to theunassisted eye, become at once visible when we permit a slender ray oflight to enter a dark chamber. 

It is commonly asserted that the atmosphere does not effectivelyextend above the height of forty-five miles; we know that it isdensest on the surface of the earth, the most so in those depressionswhich lie below the level of the sea. This is proved to us by theweight which the air imposes upon the mercury at the open end of abarometric tube. If we could deepen these cavities to the extent of athousand miles, the pressure would become so great that if the pitwere kept free from the heat of the earth the gaseous materials wouldbecome liquefied. Upward from the earth's surface at the sea level theatoms and molecules of the air become farther apart until, at theheight of somewhere between forty and fifty miles, the quantity ofthem contained in the ether is so small that we can trace littleeffect from them on the rays of light which at lower levels aresomewhat bent by their action. At yet higher levels, however, meteorsappear to inflame by friction against the particles of air, and evenat the height of eighty miles very faint clouds have at times beendiscerned, which are possibly composed of volcanic dust floating inthe very rarefied medium, such as must exist at this great elevation. 

The air not only exists in the region where we distinctly recogniseit; it also occupies the waters and the under earth. In the waters itoccurs as a mechanical mixture which is brought about as the rainforms and falls in the air, as the streams flow to the sea, and as thewaves roll over the deep and beat against the shores. In the realm ofthe waters, as well as on the land, the air is necessary for themaintenance of all animal forms; but for its presence such life wouldvanish from the earth. 

Owing to certain peculiarities in its constitution, the atmosphere ofour earth, and that doubtless of myriad other spheres, serves as amedium of communication between different regions. It is, as we know, in ceaseless motion at rates which may vary from the speed in thegreatest tempests, which may move at the rate of somewhere a hundredand fifty miles an hour, to the very slow movements which occur incaverns, where the transfer is sometimes effected at an almostmicroscopic rate in the space of a day. The motion of the atmosphereis brought about by the action of heat here and there, and in atrifling way, by the heat from the interior of the earth escapingthrough hot springs or volcanoes, but almost altogether by the heat ofthe sun. If we can imagine the earth cut off from the solar radiation, the air would cease to move. We often note how the variable winds fallaway in the nighttime. Those who in seeking for the North Pole havespent winters in the long-continued dark of that region have notedthat the winds almost cease to blow, the air being disturbed only whena storm originated in the sunlit realm forced its way into thecircumpolar darkness. 

The sun's heat does not directly disturb the atmosphere; if we couldtake the solid sphere of the world away, leaving the air, the rayswould go straight through, and there would be no winds produced. Thisis due to the fact that the air permits the direct rays of heat, suchas come from the sun, to pass through it with very slight resistance. In an aërial globe such as we have imagined, the rays impinging uponits surface would be slightly thrown out of their path as they are inpassing through a lens, but they would journey on in space without inany considerable measure warming the mass. Coming, however, upon thesolid earth, the heat rays warm the materials on which they arearrested, bringing them to a higher temperature than the air. Thenthese heated materials radiate the energy into the air; it happens, however, that this radiant heat can not journey back into space aseasily as it came in; therefore the particles of air next the surfaceacquire a relatively high temperature. Thus a thermometer next theground may rise to over a hundred degrees Fahrenheit, while at thesame time the fleecy clouds which we may observe floating at theheight of five or six miles above the surface are composed of frozenwater. 

The effect of the heated air which acquires its temperature byradiation from the earth's surface is to produce the winds. This itbrings about in a very simple manner, though the details of theprocess have a certain complication. The best illustration of the modein which the winds are produced is obtained by watching what takesplace about an ordinary fire at the bottom of a chimney. As soon asthe fire is lit, we observe that the air about it, so far as it isheated, tends upward, drawing the smoke with it. If the air in thechimney be cold, it may not draw well at first; but in a few minutesthe draught is established, or, in other words, the heated lower airbreaks its way up the shaft, gradually pushing the cooler matter outat the top. In still air we may observe the column from the flueextending about the chimney-top, sometimes to the height of a hundredfeet or more before it is broken to pieces. It is well here to notethe fact that the energy of the draught in a chimney is, with a givenheat of fire and amount of air which is permitted to enter the shaft, directly proportionate to the height; thus in very tall flues, betweentwo and three hundred feet high, which are sometimes constructed, theuprush is at the speed of a gale. 

Whenever the air next the surface is so far heated that it mayovercome the inertia of the cooler air above, it forces its way upthrough it in the general manner indicated in the chimney flue. Whensuch a place of uprush is established, the hot air next the surfaceflows in all directions toward the shaft, joining the expedition tothe heights of the atmosphere. Owing to the conditions of the earth'ssurface, which we shall now proceed to trace, these ascents of heatedair belong in two distinct classes--those which move upward throughmore or less cylindrical chimneys in the atmosphere, shafts which areimpermanent, which vary in diameter from a few feet to fifty orperhaps a hundred miles, and which move over the surface of the earth;and another which consists of a broad, beltlike shaft in theequatorial regions, which in a way girdles the earth, remains inabout the same place, continually endures, and has a width of hundredsof miles. Of these two classes of uprushes we shall first consider thegreatest, which occurs in the central portions of the tropical realm. 

Under the equator, owing to the fact that the sun for a considerablebelt of land and sea maintains the earth at a high temperature, thereis a general updraught which began many million years ago, probablybefore the origin of life, in the age when our atmosphere assumed itspresent conditions. Into this region the cooler air from the north andsouth necessarily flows, in part pressed in by the weight of the coldair which overlies it, but aided in its motion by the fact that theparticles which ascend leave place for others to occupy. Over thesurfaces of the land within the tropical region this draught towardwhat we may term the equatorial chimney is perturbed by theirregularities of the surface and many local accidents. But on thesea, where the conditions are uniform, the air moving toward the pointof ascent is marked in the trade winds, which blow with a steadfastsweep down toward the equator. Many slight actions, such as themovement of the hot and cold currents of the sea, the local airmovements from the lands or from detached islands, somewhat perturbthe trade winds, but they remain among the most permanent features inthis changeable world. It is doubtful if anything on this sphereexcept the atoms and molecules of matter have varied as little as thetrade winds in the centre of the wide ocean. So steadfast and uniformare they that it is said that the helm and sails of a ship may be setnear the west coast of South America and be left unchanged for avoyage which will carry the navigator in their belt across the widthof the Pacific. 

Rising up from the earth in the tropical belt, the air attains theheight of several thousand feet; it then begins to curve off towardthe north and south, and at the height of somewhere about three tofive miles above the surface is again moving horizontally towardeither pole; attaining a distance on that journey, it graduallysettles down to the surface of the earth, and ceases to move towardhigher latitudes. If the earth did not revolve upon its axis thecourse of these winds along the surface toward the equator, and in theupper air back toward the poles, would be made in what we may call asquare manner--that is, the particles of air would move toward thepoint where they begin to rise upward in due north and south lines, according as they came from the southern or northern hemisphere, andthe upper currents or counter trades would retrace their paths alsoparallel with the meridians or longitude lines. But because the earthrevolves from west to east, the course of the trade winds is obliqueto the equator, those in the northern hemisphere blowing fromnortheast to southwest, those in the southern from southeast tonorthwest. The way in which the motion of the earth affects thedirection of these currents is not difficult to understand. It is asfollows:

Let us conceive a particle of air situated immediately over theearth's polar axis. Such an atom would by the rotation of the sphereaccomplish no motion except, indeed, that it might turn round on itsown centre. It would acquire no velocity whatever by virtue of theearth's movement. Then let us imagine the particle moving toward theequator with the speed of an ordinary wind. At every step of itsjourney toward lower latitudes it would come into regions having agreater movement than those which it had just left. Owing to itsinertia, it would thus tend continually to lag behind the particles ofmatter about it. It would thus fall off to the westward, and, in placeof moving due south, would in the northern hemisphere drift to thesouthwest, and in the southern hemisphere toward the northwest. A goodillustration of this action may be obtained from an ordinaryturn-table such as is used about railway stations to reverse theposition of a locomotive. If the observer will stand in the centre ofsuch a table while it is being turned round he will perceive that hisbody is not swayed to the right or left. If he will then try to walktoward the periphery of the rotating disk, he will readily note thatit is very difficult, if not impossible, to walk along the radius ofthe circle; he naturally falls behind in the movement, so that hispath is a curved line exactly such as is followed by the winds whichmove toward the equator in the trades. If now he rests a moment on theperiphery of the table, so that his body acquires the velocity of thedisk at that point, and then endeavours to walk toward the centre, hewill find that again he can not go directly; his path deviates in theopposite direction--in other words, the body continually going to aplace having a less rate of movement by virtue of the rotation of theearth, on account of its momentum is ever moving faster than thesurface over which it passes. This experiment can readily be tried onany small rotating disk, such as a potter's wheel, or by rolling amarble or a shot from the centre to the circumference and from thecircumference to the centre. A little reflection will show theinquirer how these illustrations clearly account for the obliquethough opposite sets of the trade winds in the upper and lower partsof the air. 

The dominating effect of the tropical heat in controlling themovements of the air currents extends, on the ocean surface, ingeneral about as far north and south as the parallels of fortydegrees, considerably exceeding the limits of the tropics, those lineswhere the sun, because of the inclination of the earth's axis, at sometime of the year comes just overhead. Between these belts of tradewinds there is a strip or belt under the region where the atmosphereis rising from the earth, in which the winds are irregular and havelittle energy. This region of the "doldrums" or frequent calms is oneof much trouble to sailing ships on their voyages from one hemisphereto another. In passing through it their sails are filled only by theairs of local storms, or winds which make their way into that part ofthe sea from the neighbouring continents. Beyond the trade-wind belt, toward the poles, the movements of the atmosphere are dependent inpart on the counter trades which descend to the surface of the earthin latitudes higher than that in which the surface or trade windsflow. Thus along our Atlantic coast, and even in the body of thecontinent, at times when the air is not controlled by some localstorm, the counter trade blows with considerable regularity. 

The effect of the trade and counter-trade movements of the air on thedistribution of temperature over the earth's surface is momentous. Inpart their influence is due to the direct heat-carrying power of theatmosphere; in larger measure it is brought about by the movement ofthe ocean waters which they induce. Atmospheric air, when deprived ofthe water which it ordinarily contains, has very littleheat-containing capacity. Practically nearly all the power ofconveying heat which it possesses is due to the vapour of water whichit contains. By virtue of this moisture the winds do a good deal totransfer heat from the tropical or superheated portion of the earth'ssurface to the circumpolar or underheated realms. At first, therelatively cool air which journeys toward the equator along thesurface of the sea constantly gains in heat, and in that process takesup more and more water, for precisely the same reason that causesanything to dry more rapidly in air which has been warmed next a fire. The result is that before it begins to ascend in the tropicalupdraught, being much moisture-laden, the atmosphere stores a gooddeal of heat. As it rises, rarefies, and cools, the moisture descendsin the torrential rains which ordinarily fall when the sun is nearlyvertical in the tropical belt. 

Here comes in a very interesting principle which is of importance inunderstanding the nature of great storms, either the continuous stormof the tropics or the local and irregular whirlings which occur invarious parts of the earth. When the moisture-laden air starts on itsupward journey from the earth it has, by virtue of the watery vapourwhich it contains, a store of energy which becomes applied topromoting the updraught. As it rises, the moisture in the air gatherstogether or condenses, and in so doing parts with the heat whichcaused it to evaporate from the ocean surface. For a given weight ofwater, the amount of heat required to effect the evaporation is verygreat; this we may roughly judge by observing what a continuous fireis required to send a pint of water into the state of steam. Thisenergy, when it is released by the condensation of water into rain orsnow, becomes again heat, and tends somewhat, as does the fire in thechimney, to accelerate the upward passage of the air. The result isthat the water which ascends in the equatorial updraught becomes whatwe may term fuel to promote this important element in the earth'saërial circulation. Trades and counter trades would doubtless existbut for the efficiency of this updraught, which is caused by thecondensation of watery vapour, but the movement would be much lessthan it is. 

 WHIRLING STORMS. 

In the region near the equator, or near the line of highesttemperature, which for various reasons does not exactly follow theequator, there is, as we have noticed, a somewhat continuous uprushingcurrent where the air passes upward through an ascending chimney, which in a way girdles the sea-covered part of the earth. In thisregion the movements of the air are to a great extent under thecontrol of the great continuous updraught. As we go to the north andsouth we enter realms where the air at the surface of the earth is, bythe heat which it acquires from contact with that surface, more orless impelled upward; but there being no permanent updraught for itsescape, it from time to time breaks through the roof of cold air whichoverlies it and makes a temporary channel of passage. Going polarwardfrom the equator, we first encounter these local and temporaryupcastings of the air near the margin of the tropical belt. In thesedistricts, at least over the warmer seas, during the time of the yearwhen it is midsummer, and in the regions where the trade winds are notstrong enough to sweep the warm and moisture-laden air down to theequatorial belt, the upward tending strain of the atmosphere next theearth often becomes so strong that the overlying air is displaced, forming a channel through which the air swiftly passes. As themoisture condenses in the way before noted, the energy set free servesto accelerate the updraught, and a hurricane is begun. At first themovement is small and of no great speed, but as the amount of airtending upward is likely to be great, as is also the amount ofmoisture which it contains, the aërial chimney is rapidly enlarged, and the speed of the rising air increased. The atmosphere next thesurface of the sea flows in toward the channel of escape; its passageis marked by winds which are blowing toward the centre. On theperiphery of the movement the particles move slowly, but as they wintheir way toward the centre they travel with accelerating velocity. Onthe principle which determines the whirling movement of the waterescaping through a hole in the bottom of a basin, the particles of theair do not move on straight lines toward the centre, but journey inspiral paths, at first along the surface, and then ascending. 

We have noted the fact that in a basin of water the direction of thewhirling is what we may term accidental--that is, dependent onconditions so slight that they elude our observation--but inhurricanes a certain fact determines in an arbitrary way the directionin which the spin shall take place. As soon as such a movement of theair attains any considerable diameter, although in its beginning itmay have spun in a direction brought about by local accidents, it willbe affected by the diverse rates of travel, by virtue of the earth'srotation, of the air on its equatorial and polar sides. On theequatorial side this air is moving more rapidly than it is on thepolar side. By observing the water passing from a basin thisprinciple, with a few experiments, can be made plain. The result is tocause these great whirlwinds of the hurricanes of higher latitudes towhirl round from right to left in the northern hemisphere and in thereverse way in the southern. The general system of the air currentsstill further affects these, as other whirling storms, by drivingtheir centres or chimneys over the surface of the earth. The principleon which this is done may be readily understood by observing how theair shaft above a chimney, through which we may observe the smoke torise during a time of calm, is drawn off to one side by the slightcurrent which exists even when we feel no wind; it may also bediscerned in the little dust whirls which form in the streets on asummer day when the air is not much disturbed. While they spin theymove on in the direction of the air drift. In this way a hurricaneoriginating in the Gulf of Mexico may gradually journey under theinfluence of the counter trades across the Antilles, or over southernFlorida, and thence pursue a devious northerly course, generally nearthe Atlantic coast and in the path of the Gulf Stream, until it hastravelled a thousand miles or more toward the North Atlantic. Thefarther it goes northward the less effectively it is fed with warm andmoisture-laden air, the feebler its movement becomes, until at lengthit is broken up by the variable winds which it encounters. 

A very interesting and, from the point of view of the navigator, important peculiarity of these whirls is that at their centre there isa calm, similar in origin and nature to the calm under the equatorbetween the trade-wind belts. Both these areas are in the field wherethe air is ascending, and therefore at the surface of the earth doesnot affect the sails of ships, though if men ever come to use flyingmachines and sail through the tropics at a good height above the seait will be sensible enough. The difference between the doldrum of theequator and that of the hurricane, besides their relative areas, isthat one is a belt and the other a disk. If the seafarer happens tosail on a path which leads him through the hurricane centre, he willfirst discern, as from the untroubled air and sea he approaches theperiphery of the storm, the horizon toward the disturbance beset bytroubled clouds, all moving in one direction. Entering beneath thispall, he finds a steadily increasing wind, which in twenty miles ofsailing may, and in a hundred miles surely will, compel him to take inall but his storm sails, and is likely to bring his ship into graveperil. The most furious winds the mariner knows are those which heencounters as he approaches the still centre. These trials are madethe more appalling by the fact that in the furious part of the whirlthe rain, condensing from the ascending air, falls in torrents, andthe electricity generated in the condensation gives rise to vividlightning. If the storm-beset ship can maintain her way, in a score ortwo of miles of journey toward the centre, generally very quickly, itpasses into the calm disk, where the winds, blowing upward, cease tobe felt. In this area the ship is not out of danger, for the waves, rolling in from the disturbed areas on either side, make a torment ofcross seas, where it is hard to control the movements of a sailingvessel because the impulse of the winds is lost. Passing through thisdisk of calm, the ship re-encounters in reverse order the furiousportion of the whirl, afterward the lessening winds, until it escapesagain into the airs which are not involved in the great torment. 

In the old days, before Dove's studies of storms had shown the laws ofhurricane movement, unhappy shipmasters were likely to be caught andretained in hurricanes, and to battle with them for weeks until theirvessels were beaten to pieces. Now the "Sailing Directions, " which arethe mariner's guide, enable him, from the direction of the winds andthe known laws of motion of the storm centre, to sail out of thedanger, so that in most cases he may escape calamity. It is otherwisewith the people who dwell upon the land over which these atmosphericconvulsions sweep. Fortunately, where these great whirlwinds trespasson the continent, they quickly die out, because of the relative lackof moisture which serves to stimulate the uprush which creates them. Thus in their more violent forms hurricanes are only felt near thesea, and generally on islands and peninsulas. There the hurricanewinds, by the swiftness of their movement, which often attains a speedof a hundred miles or more, apply a great deal of energy to allobstacles in their path. The pressure thus produced is only lessdestructive than that which is brought about by the tornadoes, whichare next to be described. 

There is another effect from hurricanes which is even more destructiveto life than that caused by the direct action of the wind. In thesewhirlings great differences in atmospheric pressure are brought aboutin contiguous areas of sea. The result is a sudden elevation in thelevel of one part of the water. These disturbances, where the shorelands are low and thickly peopled, as is the case along the westerncoast of the Bay of Bengal, may produce inundations which are terriblydestructive to life and property. They are known also in southernFlorida and along the islands of the Caribbean, but in that region arenot so often damaging to mankind. 

Fortunately, hurricanes are limited to a very small part of thetropical district. They occur only in those regions, on the easternfaces of tropical lands, where the general westerly set of the windsfavours the accumulation of great bodies of very warm, moist air nextthe surface of the sea. The western portion of the Gulf of Mexico andthe Caribbean, the Bay of Bengal, and the southeastern portion of Asiaare especially liable to their visitations. They sometimes develop, though with less fury, in other parts of the tropics. On the westerncoast of South America and Africa, where the oceans are visited by thedry land winds, and where the waters are cooled by currents settingin from high latitudes, they are unknown. 

Only less in order of magnitude than the hurricanes are the circularstorms known as cyclones. These occur on the continents, especiallywhere they afford broad plains little interrupted by mountain ranges. They are particularly well exhibited in that part of North Americanorth of Mexico and south of Hudson Bay. Like the hurricanes, theyappear to be due to the inrush of relatively warm air entering anupdraught which had been formed in the overlying, cooler portions ofthe atmosphere. They are, however, much less energetic, and often ofgreater size than the hurricane whirl. The lack of energy is probablydue to the comparative dryness of the air. The greater width of theascending column may perhaps be accounted for by the fact that, originating at a considerable height above the sea, they have a lessthickness of air to break through, and so the upward setting column isreadily made broad. 

The cyclones of North America appear generally to originate in theregion of the Rocky Mountains, though it is probable that in someinstances, perhaps in many, the upward set of the air which begins thestorm originates in the ocean along the Pacific coast. They gatherenergy as they descend the great sloping plain leading eastward fromthe Rocky Mountains to the central portion of the great continentalvalley. Thence they move on across the country to the Atlantic coast. Not infrequently they continue on over the ocean to the Europeancontinent. The eastward passage of the storm centre is due to theprevailing eastward movement of the air in its upper part throughoutthat portion of the northern hemisphere. Commonly they inclinesomewhat to the northward of east in their journey. In all cases thewinds appear to blow spirally into the common storm centre. There isthe same doldrum area or calm field in the centre of the storm that wenote between the trade winds and in the middle of a hurricane disk, though this area is less defined than in the other instances, and theforward motion of the storm at a considerable speed is in most casescharacteristic of the disturbance. On the front of one of these stormsin North America the winds commonly begin in the northeast, thencethey veer by the east to the southwest. At this stage in the movementthe storm centre has passed by, the rainfall commonly ceases, andcold, dry winds setting to the northwestward set in. This is caused bythe fact that the ascending air, having attained a height above theearth, settles down behind the storm, forming an anticyclone or massof dry air, which presses against the retreating side of the greatwhirlwind. 

In front of the storm the warm and generally moist relatively warmair, pressing in toward the point of uprise and overlaid by the uppercold air, is brought into a condition where it tends to form smallsubordinate shafts up through which it whirls on the same principle, but with far greater intensity than the main ascending column. Thereason for the violence of this movement is that the difference intemperature of the air next the surface and that at the height of afew thousand feet is great. As might be expected, these localspinnings are most apt to occur in the season when the air next theearth is relatively warm, and they are aptest to take place in thehalf of the advancing front lying between the east and south, for thereason that there the highest temperatures and the greatest humidityare likely to coexist. In that part of the field, during the time whenthe storm is advancing from the Rocky Mountains to the Atlantic, adozen or more of these spinning uprushes may be produced, though fewof them are likely to be of large size or of great intensity. 

The secondary storms of cyclones, such as are above noted, receive thename of tornadoes. They are frequent and terrible visitations of thecountry from northern Texas, Florida, and Alabama to about the line ofthe Great Lakes; they are rarely developed in the region west ofcentral Kansas, and only occasionally do they exhibit much energy inthe region east of the plain-lands of the Ohio Valley. Although knownin other lands, they nowhere, so far as our observations go, exhibitthe paroxysmal intensity which they show in the central portion of theNorth American continent. There the air which they affect acquires aspeed of movement and a fury of action unknown in any otheratmospheric disturbances, even in those of the hurricanes. 

The observer who has a chance to note from an advantageous positionthe development of a tornado observes that in a tolerably still air, or at least an air unaffected by violent winds--generally in what istermed a "sultry" state of the atmosphere--the storm clouds in thedistance begin to form a kind of funnel-shaped dependence, whichgradually extends until it appears to touch the earth. As the cloudsare low, this downward-growing column probably in no case is observedfor the height of more than three or four thousand feet. As the funneldescends, the clouds above and about it may be seen to take on awhirling movement around the centre, and under favourablecircumstances an uprush of vapours may be noted in the centre of theswaying shaft. As the whirl comes nearer, the roar of the disturbance, which at a distance is often compared to the sound made by a threshingmachine or to that of distant musketry, increases in loudness until itbecomes overwhelming. When a storm such as this strikes a building, itis not only likely to be razed by the force of the wind, but it may beexploded, as by the action of gunpowder fired within its walls, through the sudden expansion of the air which it contains. In thecentre of the column, although it rarely has a diameter of more than afew hundred feet, the uprush is so swift that it makes a partialvacuum. The air, striving to get into the space which it is eager tooccupy, is whirling about at such a rate that the centrifugal motionwhich it thus acquires restrains its entrance. In this way there maybe, as the column rapidly moves by, a difference of pressureamounting probably to what the mercury of a barometer would indicateby four or five inches of fall. Unless the structure is small and itswalls strong, its roof and sides are apt to be blown apart by thisdifference of pressure and the consequent expansion of the containedair. In some cases where wooden buildings have withstood this curiousaction the outer clapboards have been blown off by the expansion ofthe small amount of air contained in the interspaces between thatcovering and the lath and plaster within (see Fig. 9). 

[Illustration: Fig. 9. --Showing effect of expansion of air containedin a hollow wall during the passage of the storm. ]

The blow of the air due to its rotative whirling has in several casesproved sufficient to throw a heavy locomotive from the track of awell-constructed railway. In all cases where it is intense it willoverturn the strongest trees. The ascending wind in the centre of thecolumn may sometimes lift the bodies of men and of animals, as well asthe branches and trunks of trees and the timber of houses, to theheight of hundreds of feet above the surface. One of the most strikingexhibitions of the upsucking action in a tornado is afforded by theeffect which it produces when it crosses a small sheet of water. Incertain cases where, in the Northwestern States of this country, thepath of the storm lay over the pool, the whole of the water from abasin acres in extent has been entirely carried away, leaving thesurface, as described by an observer, apparently dry enough to plough. 

Fortunately for the interests of man, as well as those of the lowerorganic life, the paths of these storms, or at least the portion oftheir track where the violence of the air movement makes them verydestructive, often does not exceed five hundred feet in width, and israrely as great as half a mile in diameter. In most cases the lengthof the journey of an individual tornado does not exceed thirty miles. It rarely if ever amounts to twice that distance. 

In every regard except their small size and their violence thesetornadoes closely resemble hurricanes. There is the same broad disk ofair next the surface spirally revolving toward the ascending centre, where its motion is rapidly changed from a horizontal to a verticaldirection. The energy of the uprush in both cases is increased by theenergy set free through the condensation of the water, which tendsfurther to heat and thus to expand the air. The smaller size of thetornado may be accounted for by the fact that we have in theiroriginating conditions a relatively thin layer of warm, moist air nextthe earth and a relatively very cold layer immediately overlying it. Thus the tension which serves to start the movement is intense, thoughthe masses involved are not very great. The short life of a tornadomay be explained by the fact that, though it apparently tends to growin width and energy, the central spout is small, and is apt to bebroken by the movements of the atmosphere, which in the front of acyclone are in all cases irregular. 

On the warmer seas, but often beyond the limits of the tropics, another class of spinning storms, known as waterspouts, may often beobserved. In general appearance these air whirls resemble tornadoes, except that they are in all cases smaller than that group ofwhirlings. As in the tornadoes, the waterspout begins with a funnel, which descends from the sky to the surface of the sea. Up the tubevapours may be seen ascending at great speed, the whole appearing likea gigantic pillar of swiftly revolving smoke. When the whirl reachesthe water, it is said that the fluid leaps up into the tube in theform of dense spray, an assertion which, in view of the fact of theaction of a tornado on a lake as before described, may well bebelieved. Like the tornadoes and dust whirls, the life of a waterspoutappears to be brief. They rarely endure for more than a few minutes, or journey over the sea for more than two or three miles before thecolumn appears to be broken by some swaying of the atmosphere. Asthese peculiar storms are likely to damage ships, the old-fashionedsailors were accustomed to fire at them with cannon. It has beenclaimed that a shot would break the tube and end the littleconvulsion. This, in view of the fact that they appear to be easilybroken up by relatively trifling air currents, may readily bebelieved. The danger which these disturbances bring to ships isprobably not very serious. 

The special atmospheric conditions which bring about the formation ofwaterspouts are not well known; they doubtless include, however, warm, moist air next the surface of the sea and cold air above. Just whythese storms never attain greater size or endurance is not yet known. These disturbances have been seen for centuries, but as yet they havenot been, in the scientific sense, observed. Their picturesquenessattracts all beholders; it is interesting to note the fact thatperhaps the earliest description of their phenomena--one which takesaccount in the scientific spirit of all the features which theypresent--was written by the poet Camoëns in the Lusiad, in which hestrangely mingles fancy and observation in his account of the greatvoyage of Vasco da Gama. The poet even notes that the water whichfalls when the spout is broken is not salt, but fresh--a point whichclearly proves that not much of the water which the tube contains isderived from the sea. It is, in fact, watery vapour drawn from the airnext the surface of the ocean, and condensed in its ascent through thetube. In this and other descriptions of Nature Camoëns shows more ofthe scientific spirit than any other poet of his time. He was in thisregard the first of modern writers to combine a spiritual admirationfor Nature with some sense of its scientific meaning. 

In treating of the atmosphere, meteorologists base their studieslargely on changes in the weight of that medium, which they determineby barometric observations. In fact, the science of the air had itsbeginning in Pascal's admirable observation on the changes in theheight of a column of mercury contained in a bent tube as he ascendedthe volcanic peak known as Puy de Dome, in central France. As beforenoted, it is to the disturbances in the weight of the air, broughtabout mainly by variations in temperature, that we owe all itscurrents, and it is upon these winds that the features we term climatein largest measure depend. Every movement of the winds is not onlybrought about by changes in the relative weight of the air at certainpoints, but the winds themselves, owing to the momentum which the airattains by them, serve to bring about alterations in the quantity ofair over different parts of the earth, which are marked mostdistinctly by barometric variations. These changes are exceedinglycomplicated; a full account of them would demand the space of thisvolume. A few of the facts, however, should be presented here. In thefirst place, we note that each day there is normally a range in thepressure which causes the barometer to be at the lowest at about fouro'clock in the morning and four o'clock in the afternoon, and highestat about ten o'clock in those divisions of the day. This change issupposed to be due to the fact that the motes of dust in theatmosphere in the night, becoming cooled, condense the water vapourupon their surfaces, thus diminishing the volume of the air. When thesun rises the water evaporated by the heat returns from these littlestorehouses into the body of the atmosphere. Again in the evening thecondensation sets in; at the same time the air tends to drift in fromthe region to the westward, where the sun is still high, toward thefield where the barometer has been thus lowered; the current graduallyattains a certain volume, and so brings about the rise of thebarometer about ten o'clock at night. 

In the winter time, particularly on the well-detached continent ofNorth America, we find a prevailing high barometer in the interior ofthe country and a corresponding low state of pressure on the AtlanticOcean. In the summer season these conditions are on the wholereversed. 

Under the tropics, in the doldrum belt, there is a zone of lowbarometer connected to the ascending currents which take place alongthat line. This is a continuous manifestation of the same action whichgives a large area of a disklike form in the centre or eye of thehurricane and in the middle portion of the tornado's whirl. Ingeneral, it may be said that the weight of the air is greatest in theregions from which it is blowing toward the points of upward escape, and least in and about those places where the superincumbent air isrising through a temporary or permanent line of escape. In otherwords, ascending air means generally a relatively low barometer, whiledescending air is accompanied by greater pressure in the field uponwhich it falls. 

In almost every part of the earth which is affected by a particularphysiography we find that the movements of the atmosphere next thesurface are qualified by the condition which it encounters. In fact, if a person were possessed of all the knowledge which could beobtained concerning winds, he could probably determine as by a map theplace where he might chance to find himself, provided he could extendhis observations over a term of years. In other words, the regimen ofthe winds--at least those of a superficial nature--is almost ascharacteristic of the field over which they go as is a map of thecountry. Of these special winds a number of the more important havebeen noted, only a few of which we can advert to. First among thesemay well come the land and sea breezes which are remarked about allislands which are not continuously swept by permanent winds. One ofthe most characteristic instances of these alternate winds is perhapsthat afforded on the island of Jamaica. 

The island of Jamaica is so situated within the basin of the Caribbeanthat it does not feel the full influence of the trades. It has a rangeof high mountains through its middle part. In the daytime the surfaceof the land, which has the sun overhead twice each year, and is alwaysexposed to nearly vertical radiation, becomes intensely hot, so thatan upcurrent is formed. The formation of this current is favoured bythe mountains, which apply a part of the heat at the height of about amile above the surface of the sea. This action is parallel to that wenotice when, in order to create a draught in the air of a chimney, weput a torch some distance up above the fireplace, thus diminishing theheight of the column of air which has to be set in motion. It isfurther shown by the fact that when miners sought to make an upcurrentin a shaft, in order to lead pure air into the workings through otheropenings, they found after much experience that it was better to havethe fire near the top of the shaft rather than at the bottom. 

The ascending current being induced up the mountain sides of Jamaica, the air is forced in from the sea to the relatively free space. Beforenoon the current, aided in its speed by a certain amount of thecondensation of the watery vapour before described, attains theproportions of a strong wind. As the sun begins to sink, the earth'ssurface pours forth its heat; the radiation being assisted by theextended surfaces of the plants, cooling rapidly takes place. Meanwhile the sea, because of the great heat-storing power of water, is very little cooled, the ascent of the air ceases, the temporarychimney with its updraught is replaced by a downward current, and thewinds blow from the land until the sun comes again to reverse thecurrent. In many cases these movements of the daily winds flowing intoand from islands induce a certain precipitation of moisture in theform of rain. Generally, however, their effect is merely to amelioratethe heat by bringing alternately currents from the relatively cool seaand from the upper atmosphere to lessen the otherwise excessivetemperature of the fields which they traverse. 

Although characteristic sea and land winds are limited to regionswhere the sun's heat is great, they are traceable even in highlatitudes during the periods of long-continued calm attended withclear skies. Thus on the island of Martha's Vineyard, inMassachusetts, the writer has noted, when the atmosphere was in such astate, distinct night and day, or sea and land, breezes coming intheir regular alternation. During the night when these alternate windsprevail the central portion of the island, at the distance of threemiles from the sea, is remarkably cold, the low temperature being dueto the descending air current. To the same physical cause may beattributed the frequent insets of the sea winds toward midday alongthe continental shores of various countries. Thus along the coast ofNew England in the summer season a clear, still, hot day is certain tolead to the creation of an ingoing tide of air, which reaches somemiles into the interior. This stream from the sea enters as a thinwedge, it often being possible to note next the shore when themovement begins a difference of ten degrees of temperature between thesurface of the ground to which the point of the wedge has attained, and a position twenty feet higher in the air. This is a beautifulexample to show at once how the relative weight of the atmosphere, even when the differences are slight, may bring about motion, and alsohow masses of the atmosphere may move by or through the rest of themedium in a way which we do not readily conceive from our observationson the transparent mass. Very few people have any idea how general isthe truth that the air, even in continuous winds, tends to move inmore or less individualized masses. This, however, is made veryevident by watching the gusts of a storm or the wandering patches ofwind which disturb the surface of an otherwise smooth sea. 

[Illustration: _South shore, Martha's Vineyard, Massachusetts, showinga characteristic sand beach with long slope and low dunes. Note thethree lines of breakers and the splash flows cutting little bays inthe sand. _]

Among the notable local winds are those which from their likeness tothe Föhn of the Swiss valleys receive that name. Föhns are producedwhere a body of air blowing against the slope of a continuous mountainrange is lifted to a considerable height, and, on passing over thecrest, falls again to a low position. In its ascent the air is cooled, rarefied, and to a great extent deprived of its moisture. Indescending it is recondensed, and by the process by which its atomsare brought together its latent heat is made sensible. There being butlittle watery vapour in the mass, this heat is not much called for bythat heat-storing fluid, and so the air is warmed. So far Föhn windshave only been remarked as conspicuous features in Switzerland and onthe eastern face of the Rocky Mountains. In the region about the headwaters of the Missouri and to the northward their influence in whatare called the Chinook winds is distinctly to ameliorate the severewinter climate of the country. 

In almost all great desert regions, particularly in the typicalSahara, we find a variety of storm belonging to the whirlwind group, which, owing to the nature of the country, take on specialcharacteristics. These desert storms take up from the verdurelessearth great quantities of sand and other fine _débris_, which often soclouds the air as to bring the darkness of night at midday. Theirwhirlings appear in size to be greater than those which producetornadoes or waterspouts, but less than hurricanes or cyclones. Little, however, is known about them. They have not been wellobserved by meteorologists. In some ways they are important, for thereason that they serve to carry the desert sand into regionspreviously verdure-clad, and thus to extend the bounds of the desolatefields in which they originate. Where they blow off to the seaward, they convey large quantities of dust into the ocean, and thus serve towear down the surface of the land in regions where there are no riversto effect that action in the normal way. 

Notwithstanding its swift motion when impelled by differences inweight, the movements of the air have had but little direct andimmediate influence on the surface of the earth. The greater part ofthe work which it does, as we shall see hereafter, is done through thewaters which it impels and bears about. Yet where winds blow oververdureless surfaces the effect of the sand which they sweep beforethem is often considerable. In regions of arid mountains the windsoften drive trains of sand through the valleys, where the sharpparticles cut the rocks almost as effectively as torrents of waterwould, distributing the wearing over the width of the valley. The dustthus blown, from a desert region may, when it attains a countrycovered with vegetation, gradually accumulate on its surface, formingvery thick deposits. Thus in northwestern China there is a wide areawhere dust accumulations blown from the arid districts of central Asiahave gradually heaped up in the course of ages to the depth ofthousands of feet, and this although much of the _débris_ iscontinually being borne away by the action of the rain waters as theyjourney toward the sea. Such dust accumulations occur in other partsof the world, particularly in the districts about the upperMississippi and in the valleys of the Rocky Mountains, but nowhere arethey so conspicuous as in the region first mentioned. 

Where prevailing winds from the sea, from great lakes, and even fromconsiderable rivers, blow against sandy shores or cliffs of the samenature, large quantities of sand and dust are often driven inlandfrom the coast line. In most cases these wind-borne materials take onthe form of dunes, or heaps of sand, varying from a few feet toseveral hundred feet in height. It is characteristic of these hills ofblown sand that they move across the face of the country. Underfavourable conditions they may journey scores of miles from the shore. The marching of a dune is effected through the rolling up of the sandon the windward side of the elevation, when it is impelled by thecurrent of air to the crest where it falls into the lee or shelterwhich the hill makes to the wind. In this way in the course of a daythe centre of the dune, if the wind be blowing furiously, may advancea measurable distance from the place it occupied before. By fits andstarts this ongoing may be indefinitely continued. A notable andpicturesque instance of the march of a great dune may be had from thecase in which one of them overwhelmed in the last century the villageof Eccles in southeastern England. The advancing sand gradually creptinto the hamlet, and in the course of a decade dispossessed the peopleby burying their houses. In time the summit of the church spiredisappeared from view, and for many years thereafter all trace of thehamlet was lost. Of late years, however, the onward march of the sandshas disclosed the church spire, and in the course of another centurythe place may be revealed on its original site, unchanged except thatthe marching hill will be on its other side. 

In the region about the head of the Bay of Biscay the quantity ofthese marching sands is so great that at one time they jeopardized theagriculture of a large district. The French Government has nowsucceeded, by carefully planting the surface of the country withgrasses and other herbs which will grow in such places, in checkingthe movement of the wind-blown materials. By so doing they have merelyhastened the process by which Nature arrests the march of dunes. Asthese heaps creep away from the sea, they generally come into regionswhere a greater variety of plants flourish; moreover, their sandgrains become decayed, so that they afford a better soil. Graduallythe mat of vegetation binds them down, and in time covers them over sothat only the expert eye can recognise their true nature. Only indesert regions can the march of these heaps be maintained for greatdistances. 

Characteristic dunes occur from point to point all along the Atlanticcoast from the State of Maine to the northern coast of Florida. Theyalso occur along the coasts of our Great Lakes, being particularlywell developed at the southern end of Lake Michigan, where they form, perhaps, the most notable accumulations within the limits of theUnited States. 

When blown sands invade a forest and the deposit is rapidlyaccumulated, the trees are often buried in an undecayed condition. Inthis state, with certain chemical reactions which may take place inthe mass, the woody matter is apt to become replaced by silexdissolved from the sand, which penetrates the tissues of the plants. In this way salicified forests are produced, such as are found in theregion of the Rocky Mountains, where the trunks of the trees, now veryhard stone, so perfectly preserve their original structure that whencut and polished they may be used for decorative purposes. Conspicuousas is this work of the dunes, it is in a geological way much lessimportant than that accomplished by the finer dust which drifts fromone region of land to another or into the sea. Because of theirweight, the sand grains journey over the surface of the earth, except, indeed, where they are uplifted by whirl storms. They thus can nottravel very fast or far. Dust, however, rises into the air, andjourneys for indefinite distances. We thus see how slight differencesin the weight of substances may profoundly affect the conditions oftheir deportation. 

 THE SYSTEM OF WATERS. 

The envelope of air wraps the earth completely about, and, thoughvarying in thickness, is everywhere present over its surface. That ofthe waters is much less equally distributed. Because of its weight, itis mainly gathered in the depths of the earth, where it lies in theinterstices of the rocks and in the great realm of the seas. Only avery small portion of the fluid is in the atmosphere or on the land. Perhaps less than a ten thousandth part of the whole is at any onetime on this round from the seas through the air to the land and backto the great reservoir. 

The great water store of the earth is contained in two distinctrealms--in the oceans, where the fluid is concentrated in a quantitywhich fills something like nine tenths of the hollows formed by thecorrugations of the earth's surface; and in the rocks, where it isstored in a finely divided form, partly between the grains of thestony matter and partly in the substance of its crystals, where itexists in a combination, the precise nature of which is not wellknown, but is called water of crystallization. On the average, itseems likely that the materials of the earth, whether under the sea oron the land, have several per cent of their mass of the fluid. 

It is not yet known to what depth the water-bearing section of theearth extends; but, as we shall see more particularly hereafter whenwe come to consider volcanoes, the lavas which they send up to thesurface are full of contained water, which passes from them in theform of steam. The very high temperature of these volcanic ejectionsmakes it necessary for us to suppose that they come from a greatdepth. It is difficult to believe that they originate at less than ahundred miles below the earth's surface. If, then, the rocks containan average of even five per cent of water to the depth of one hundredmiles, the quantity of the fluid stored within the earth is greaterthan that which is contained in the reservoir of the ocean. Theoceans, on the average, are not more than three miles deep; spreadevenly over the surface of the whole earth, their depth would be lessthan two miles, while the water in the rocks, if it could be added tothe seas, would make the total depth seven miles or more. As we shallnote hereafter, the processes of formation of strata tend to imprisonwater in the beds, which in time is returned to the earth's surface bythe forces which operate within the crust. 

Although the water in the seas is, as we have seen, probably less thanone half of the store which the earth possesses, the part it plays inthe economy of the planet is in the highest measure important. Theunderground water operates solely to promote certain changes whichtake place in the mineral realm. Its effect, except in volcanicprocesses, are brought about but slowly, and are limited in theiraction. The movements of this buried water are exceedingly gradual;the forces which impel it about and which bring it to do its workoriginate in the earth. In the seas the fluid has an exceeding freedomof motion; it can obey the varied impulses which the solar energyimposes upon it. The rôle of these wonderful actions which we areabout to trace includes almost everything which goes on upon thesurface of the planet--that which relates to the development of animaland vegetable life, as well as to the vast geological changes whichthe earth is undergoing. 

If the surface of the earth were uniformly covered with water to thedepth of ten thousand feet or more, every particle of fluid would, ina measure, obey the attraction of the sun, of the moon, and, theoretically, also of all the other bodies in space, on the principlethat every particle of matter in the universe exercises a gravitativeeffect on every other. As it is, owing to the divided condition of thewater on the earth's surface, only that which is in the ocean andlarger seas exhibits any measurable influence from these distantattractions. In fact, only the tides produced by the moon and sun areof determinable magnitude, and of these the lunar is of greaterimportance, the reason being the near position of our satellite to ourown sphere. The solar tide is four tenths as great as the lunar. Thewater doubtless obeys in a slight way the attraction of the othercelestial bodies, but the motions thus imparted are too small to bediscerned; they are lost in the great variety of influences whichaffect all the matter on the earth. 

Although the tides are due to the attraction of the solar bodies, mainly to that of the moon, the mode in which the result is broughtabout is somewhat complicated. It may briefly and somewhatincompletely be stated as follows: Owing to the fact that theattracting power of the earth is about eighty times greater than thatof the moon, the centre of gravity of the two bodies lies within theearth. About this centre the spheres revolve, each in a way swingingaround the other. At this point there is no centrifugal motion arisingfrom the revolution of the pair of spheres, but on the side of theearth opposite the moon, some six thousand miles away, the centrifugalforce is considerable, becoming constantly greater as we pass awayfrom the turning point. At the same time the attraction of the moon onthe water becomes less. Thus the tide opposite the satellite isformed. On the side toward the moon the same centrifugal actionoperates, though less effectively than in the other case, for thereason that the turning point is nearer the surface; but this actionis re-enforced by the greater attraction of the moon, due to the factthat the water is much nearer that body. 

In the existing conditions of the earth, what we may call the normalrun of the tides is greatly interrupted. Only in the southern oceancan the waters obey the lunar and solar attraction in anything like anormal way. In that part of the earth two sets of tides arediscernible, the one and greater due to the moon, the other, muchsmaller, to the sun. As these tides travel round at different rates, the movements which they produce are sometimes added to each otherand sometimes subtracted--that is, at times they come together, whileagain the elevation of one falls in the hollow of the other. Onceagain supposing the earth to be all ocean covered, computation showsthat the tides in such a sea would be very broad waves, having, indeed, a diameter of half the earth's circumference. Those producedby the moon would have an altitude of about one foot, and those by thesun of about three inches. The geological effects of these swayingswould be very slight; the water would pass over the bottom to and frotwice each day, with a maximum journey of a hundred or two feet eachway from a fixed point. This movement would be so slow that it couldnot stir the fine sediment; its only influence would perhaps be tohelp feed the animals which were fixed upon the bottom by drawing thenurture-bringing water by their mouths. 

Although the divided condition of the ocean perturbs the action of thetides, so that except by chance their waves are rarely with theircentres where the attracting bodies tend to make them, the influenceof these divisions is greatly to increase the geological orchange-bringing influences arising from these movements. When from thesouthern ocean the tides start to the northward up the bays of theAtlantic, the Pacific, or the Indian Ocean, they have, as beforenoted, a height of perhaps less than two feet. As they pass up thenarrowing spaces the waves become compressed--that is, an equal volumeof moving water has less horizontal room for its passage, and isforced to rise higher. We see a tolerably good illustration of thesame principle when we observe a wind-made wave enter a small recessof the shore, the sides of which converge in the direction of themotion. With the diminished room, the wave gains in height. It thuscomes about that the tide throughout the Atlantic basin is much higherthan in the southern ocean. On the same principle, when the tide rollsin against the shores every embayment of a distinct kind, whose sidesconverge toward the head, packs up the tidal wave, often increasingits height in a remarkable way. When these bays are wide-mouthed andof elongate triangular form, with deep bottoms, the tides which ontheir outer parts have a height of ten or fifteen feet may attain analtitude of forty or fifty feet at the apex of the triangle. 

We have already noted the fact that the tide, such as runs in thesouthern ocean, exercises little or no influence upon the bottom ofthe sea over which it moves. As the height of the confined watersincreases, the range of their journey over the bottom as the wavecomes and goes rapidly increases. When they have an elevation of tenfeet they can probably stir the finer mud on the ocean floor, and inshallow water move yet heavier particles. In the embayments of theland, where a great body of water journeys like an alternating riverinto extensive basins, the tidal action becomes intense; the currentmay be able to sweep along large stones quite as effectively as amountain torrent. Thus near Eastport, Me. , where the tides have amaximum rise and fall of over twenty feet, the waters rush in placesso swiftly that at certain stages of the movement they are as muchtroubled as those at the rapids of the St. Lawrence. In such portionsof the shore the tides do important work in carving channels into thelands. 

Along the shores of the continents about the North Atlantic, where thetides act in a vigorous manner, we almost everywhere find anunderwater shelf extending from the shore with a declivity of onlyfive to ten feet to the mile toward the centre of the sea, until thedepth of about five hundred feet is attained; from this point thebottom descends more steeply into the ocean's depth. It is probablethat the larger part of the material composing these continentalshelves has been brought to its position by tidal action. Each timethe tidal wave sweeps in toward the shore it urges the finer particlesof sediment along with it. When it moves out it drags them on thereturn journey toward the depths of the sea. If this shelf wereperfectly horizontal, the two journeys of the sand and mud grainswould be of the same length; but as the movement takes place up anddown a slope, the bits will travel farther under the impulse whichleads them downward than under that which impels them up. The resultwill be that the particles will travel a little farther out from theshore each time it is swung to and fro in the alternating movement ofthe tide. 

The effect of tidal movement in nurturing marine life is very great. It aids the animals fixed on the bottoms of the deep seas to obtaintheir provision of food and their share of oxygen by drawing the waterby their bodies. All regions which are visited by strong tidescommonly have in the shallows near the shores a thick growth ofseaweed which furnishes an ample provision of food for the fishes andother forms of animal life. 

A peculiar effect arising from tidal action is believed by students ofthe phenomena to be found in the slowing of the earth's rotation onits axis. The tides rotate around the earth from east to west, orrather, we should say, the solid mass of the earth rubs against themas it spins from west to east. As they move over the bottom and asthey strike against the shores this push of the great waves tends in aslight measure to use up the original spinning impulse which causesthe earth's rotation. Computation shows that the amount of this actionshould be great enough gradually to lengthen the day, or the timeoccupied by the earth in making a complete revolution on the polaraxis. The effect ought to be great enough to be measurable byastronomers in the course of a thousand years. On the other hand, therecords of ancient eclipses appear pretty clearly to show that thelength of the day has not changed by as much as a second in the courseof three thousand years. This evidence does not require us to abandonthe supposition that the tides tend to diminish the earth's rate ofrotation. It is more likely that the effect of the reduction in theearth's diameter due to the loss of heat which is continually going oncounterbalances the influence of the tidal friction. As the diameterof a rotating body diminishes, the tendency is for the mass to spinmore rapidly; if it expands, to turn more slowly, provided in eachcase the amount of the impulse which leads to the turning remains thesame. This can be directly observed by whirling a small weightattached to a string in such a manner that the cord winds around thefinger with each revolution; it will be noted that as the lineshortens the revolution is more quickly accomplished. We can readilyconceive that the earth is made up of weights essentially like thatused in the experiment, each being drawn toward the centre by thegravitative stress, which is like that applied to the weight by thecord. 

The fact that the days remain of the same length through vast periodsof time is probably due to this balance between the effects of tidalaction and those arising from the loss of heat--in other words, wehave here one of those delicate arrangements in the way ofcounterpoise which serve to maintain the balanced conditions of theearth's surface amid the great conflicts of diverse energies which areat work in and upon the sphere. 

It should be understood that the effects of the attraction whichproduces tides are much more extensive than they are seen to be in themovements of the sea. So long as the solar and planetary spheresremain fluid, the whole of their masses partake of the movement. It isa consequence of this action, as the computations of Prof. GeorgeDarwin has shown, that the moon, once nearer the earth than it is atpresent, has by a curious action of the tidal force been pushed awayfrom the centre of our sphere, or rather the two bodies have repelledeach other. An American student of the problem, Mr. T. J. J. See, hasshown that the same action has served to give to the double stars theexceeding eccentricity of their orbits. 

Although these recent studies of tidal action in the celestial sphereare of the utmost importance to the theory of the universe, for theymay lead to changes in the nebular hypotheses, they are as yet tooincomplete and are, moreover, too mathematical to be presented in anelementary treatise such as this. 

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We now turn to another class of waves which are of even moreimportance than those of the tides--to the undulations which areproduced by the action of the wind on the surface of the water. Whilethe tide waves are limited to the open ocean, and to the seas and bayswhich afford them free entrance, wind waves are produced everywherewhere water is subjected to the friction of air which flows over it. While tidal waves come upon the shores but twice each day, the windwaves of ordinary size which roll in from the ocean deliver theirblows at intervals of from three to ten seconds. Although the tidalwaves sometimes, by a packing-up process, attain the height of fiftyfeet, their average altitude where they come in contact with the shoreprobably does not much exceed four feet; usually they come in gently. It is likely that in a general way the ocean surges which beat againstthe coast are of greater altitude. 

Wind waves are produced and perform their work in a manner which weshall now describe. When the air blows over any resisting surface, ittends, in a way which we can hardly afford here to describe, toproduce motions. If the particle is free to move under the impulsewhich it communicates, it bears it along; if it is linked together inthe manner of large masses, which the wind can not transport, it tendsto set it in motion in an alternating way. The sounds of our musicalinstruments which act by wind are due to these alternating vibrations, such as all air currents tend to produce. An Æolian harp illustratesthe action which we are considering. Moving over matter which has thequalities that we denote by the term fluid, the swayings which the airproduces are of a peculiar sort, though they much resemble those ofthe fiddle string. The surface of the liquid rises and falls in whatwe term waves, the size of which is determined by the measure offluidity, and by the energy of the wind. Thus, because fresh water isconsiderably lighter than salt, a given wind will produce in a givendistance for the run of the waves heavier surges in a lake than itwill in the sea. For this reason the surges in a great storm whichroll on the ocean shore, because of the wide water over which theyhave gathered their impetus, are in size very much greater than thoseof the largest lakes, which do not afford room for the development ofgreat undulations. 

To the eye, a wave in the water appears to indicate that the fluid isborne on before the wind. Examination, however, shows that the amountof motion in the direction in which the wind is blowing is veryslight. We may say, indeed, that the essential feature of a wave isfound in the transmission of impulse rather than in the movement ofthe fluid matter. A strip of carpet when shaken sends through itslength undulations which are almost exactly like water waves. If weimagine ourselves placed in a particle of water, moving in theswayings of a wave in the open and deep sea, we may conceive ourselvescarried around in an ellipse, in each revolution returning throughnearly the same orbit. Now and then, when the particle came to thesurface, it would experience the slight drift which the continualfriction of the wind imposes on the water. If the wave in which thejourney was made lay in the trade winds, where the long-continued, steadfast blowing had set the water in motion to great depths, theorbit traversed would be moving forward with some rapidity; where alsothe wind was strong enough to blow the tops of the waves over, formingwhite-caps, the advance of the particle very near the surface would bespeedy. Notwithstanding these corrections, waves are to be regardedeach as a store of energy, urging the water to sway much in the mannerof a carpet strip, and by the swaying conveying the energy in thedirection of the wave movement. 

The rate of movement of wind waves increases with their height. Slight undulations go forward at the rate of less than half a mile anhour. The greater surges of the deeps when swept by the strongestwinds move with the speed which, though not accurately determined, hasbeen estimated by the present writer as exceeding forty miles an hour. As these surges often have a length transverse to the wind of a mileor more, a width of about an eighth of a mile, and a height of fromthirty-five to forty-five feet, the amount of energy which theytransmit is very great. If it could be effectively applied to theshores in the manner in which the energy of exploding gunpowder isapplied by cannon shot, it is doubtful whether the lands could havemaintained their position against the assaults of the sea. But thereare reasons stated below why the ocean waves can use only a very smallpart of their energy in rending the rocks against which they strike onthe coast line. 

In the first place, we should note that wind waves have very littleinfluence on the bottom of the deep sea. If an observer could stand onthe sea floor at the depth of a mile below a point over which thegreatest waves were rolling, he could not with his unaided sensesdiscern that the water was troubled. He would, indeed, requireinstruments of some delicacy to find out that it moved at all. Makingthe same observations at the depth of a thousand feet, it is possiblethat he would note a slight swaying motion in the water, enoughsensibly to affect his body. At five hundred feet in depth themovement would probably be sufficient to disturb fine mud. At twohundred feet, the rasping of the surge on the bottom would doubtlessbe sufficient to push particles of coarse sand to and fro. At onehundred feet in depth, the passage of the surge would be strong enoughto urge considerable pebbles before it. Thence up the slope thedriving action would become more and more intense until we attainedthe point where the wave broke. It should furthermore be noted that, while the movement of the water on the floor of the deep sea as thewave passes overhead would be to and fro, with every advance in theshallowing and consequent increased friction on the bottom, theforward element in the movement would rapidly increase. Near the coastline the effect of the waves is continually to shove the detritus upthe slopes of the continental shelf. Here we should note the fact thaton this shelf the waves play a part exactly the opposite of thateffected by the tides. The tides, as we have noted, tend to drag theparticles down the slope, while the waves operate to roll them up thedeclivity. 

As the wave in advancing toward the shore ordinarily comes intocontinually shallowing water, the friction on the bottom isever-increasing, and serves to diminish the energy the surge contains, and therefore to reduce its proportions. If this action operatedalone, the subtraction which the friction makes would cause the surfwaves which roll in over a continental shelf to be very low, probablyin height less than half that which they now attain. In fact, however, there is an influence at work to increase the height of the waves atthe expense of its width. Noting that the friction rapidly increaseswith the shallowing, it is easy to see that this resistance isgreatest on the advancing front of the wave, and least on its seawardside. The result is that the front moves more slowly than the rear, sothat the wave is forced to gain in height; but for the fact that thetotal friction which the wave encounters takes away most of itsimpetus, we might have combers a hundred feet high rolling upon theshelving shores which almost everywhere face the seas. 

As the wave shortens its width and gains in relative height, thoughnot in actual elevation, another action is introduced which hasmomentous consequences. The water in the bottom of the wave is greatlyretarded in its ongoing by its friction over the sea floor, while theupper part of the surge is much less affected in this way. The resultis that at a certain point in the advance, the place of which isdetermined by the depth, the size, and the speed of the undulation, the front swiftly steepens until it is vertical, and the top shootsforward to a point where it is no longer supported by underlyingwater, when it plunges down in what is called the surf or breaker. Inthis part of the wave's work the application of the energy which ittransmits differs strikingly from the work previously done. Before thewave breaks, the only geological task which it accomplishes iseffected by forcing materials up the slope, in which movement they areslightly ground over each other until they come within the batteringzone of the shore, where they may be further divided by the action ofthe mill which is there in operation. 

When the wave breaks on the shore it operates in the following manner:First, the overturning of its crest sends a great mass of water, itmay be from the height of ten or more feet, down upon the shore. Thusfalling water has not only the force due to its drop from the summitof the wave, but it has a share of the impulse due to the velocitywith which the surge moved against the shore. It acts, in a word, likea hammer swung down by a strong arm, where the blow represents notonly the force with which the weight would fall of itself, but theimpelling power of the man's muscles. Any one who will expose his bodyto this blow of the surf will recognise how violent it is; he may, ifthe beach be pebbly, note how it drives the stones about; fragmentsthe size of a man's head may be hurled by the stroke to the distanceof twenty feet or more; those as large as the fist may be thrown clearbeyond the limits of the wave. So vigorous is this stroke that thesound of it may sometimes be heard ten miles inland from the coastwhere it is delivered. 

Moving forward up the slope of a gently inclined beach, the fragmentsof the wave are likely to be of sufficient volume to permit them toregather into a secondary surge, which, like the first, though muchsmaller, again rises into a wall, forming another breaker. Underfavourable conditions as many as four or five of these successivediminishing surf lines may be seen. The present writer has seen incertain cases as many as a dozen in the great procession, the lowestand innermost only a few inches high, the outer of all with a heightof perhaps twenty feet. 

Along with the direct bearing action of the surf goes a to-and-fromovement, due to the rushing up and down of the water on the beach. This swashing affects not only the broken part of the waves, but allthe water between the outer breaker and the shore. These swayings inthe surf belt often swing the _débris_ on the inner margin over arange of a hundred feet or more, the movement taking place with greatswiftness, affecting the pebbles to the depth of several inches, andgrinding the bits together in a violent way. Listening to the turmoilof a storm, we can on a pebbly beach distinctly hear the sound of thedownward stroke, a crashing tone, and the roar of the rolling stones. 

As waves are among the interesting things in the world, partly onaccount of their living quality and partly because of their immediateand often exceeding interest to man, we may here note one or twopeculiar features in their action. In the first place, as the readerwho has gained a sense of the changes in form of action may readilyperceive, the beating of waves on the shore converts the energy whichthey possess into heat. This probably warms the water during greatstorms, so that by the hand we may note the difference in temperaturenext the coast line and in the open waters. This relative warmth ofthe surf water is perhaps a matter of some importance in limiting thedevelopment of ice along the shore line; it may also favour theprotection of the coast life against the severe cold of the winterseason. 

The waves which successively come against the shore in any given time, particularly if a violent wind is blowing on to the coast, are usuallyof about the same size. When, however, in times of calm an old sea, asit is called, is rolling in, the surges may occasionally undergo verygreat variations in magnitude. Not infrequently these occasional wavesare great enough to overwhelm persons who are upon the rocks next theshore. These greater surges are probably to be accounted for by thefact that in the open sea waves produced by winds blowing in differentdirections may run on with their diverse courses and varied intervalsuntil they come near the shore. Running in together, it very wellhappens that two of the surges belonging to different sets may combinetheir forces, thus doubling the swell. The danger which theseconjoined waves bring is obviously greatest on cliff shores, where, onaccount of the depth of water, the waves do not break until theystrike the steep. 

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Having considered in a general way the action of waves as they roll into the shore, bearing with them the solar energy which was contributedto them by the winds, we shall now take up in some detail the workwhich goes on along the coast line--work which is mainly accomplishedby wave action. 

On most coast lines the observer readily notes that the shore isdivided into two different kinds of faces--those where the innermargin of the wave-swept belt comes against rocky steeps, and thosebordered by a strand altogether composed of materials which the surgeshave thrown up. These may be termed for convenience cliff shores andwall-beach shores. We shall begin our inquiry with cliff shores, forin those sections of the coast line the sea is doing its mostcharacteristic and important work of assaulting the land. If thestudent has an opportunity to approach a set of cliffs of hard rock intime of heavy storm, when the waves have somewhere their maximumheight, he should seek some headland which may offer him safe footholdwhence he can behold the movements which are taking place. If he is sofortunate as to have in view, as well may be the case, cliffs whichextend down into deep water, and others which are bordered by rudeand generally steeply sloping beaches covered with large stones, hemay perceive that the waves come in against the cliffs which plungeinto deep water without taking on the breaker form. In this case theundulation strikes but a moderate blow; the wave is not greatlybroken. The part next the rock may shoot up as a thin sheet to aconsiderable height; it is evident that while the ongoing wave appliesa good deal of pressure to the steep, it does not deliver its energyin the effective form of a blow as when the wave overturns, or in theconsequent rush of the water up a beach slope. It is easy to perceivethat firm-set rock cliffs, with no beaches at their bases, can almostindefinitely withstand the assaults. On the steep and stony beach, because of its relatively great declivity, the breaker or surf formsfar in, and even in its first plunge often attains the base of theprecipice. The blow of the overfalling as well as that of the inrushmoves about stones of great size; those three feet or more in diameterare often hurled by the action against the base of the steep, strikingblows, the sharp note of which can often be heard above the generalroar which the commotion produces. The needlelike crags forming islesstanding at a distance from the shore, such as are often found alonghard rock coasts, are singularly protected from the action ofeffective waves. The surges which strike against them are unarmed withstones, and the water at their bases is so deep that it does not swaywith the motion with sufficient energy to move them on the bottom. Where a cliff is in this condition, it may endure until an elevationof the coast line brings its base near the level of the sea, or untilthe process of decay has detached a sufficient quantity of stone toform a talus or inclined plane reaching near to the water level. 

As before noted, it is the presence of a sloping beach reaching toabout the base of the cliff which makes it possible for the waves tostrike at with a hammer instead of with a soft hand. Battering at thebase of the cliff, the surges cut a crease along the strip on whichthey strike, which gradually enters so far that the overhanging rockfalls of its own weight. The fragments thus delivered to the sea arein turn broken up and used as battering instruments until they areworn to pieces. We may note that in a few months of heavy weather thestones of such a fall have all been reduced to rudely spherical forms. Observations made on the eastern face of Cape Ann, Mass. , where theseas are only moderately heavy, show that the storms of a singlewinter reduce the fragments thrown into the sea from the granitequarries to spheroidal shapes, more than half of their weight commonlybeing removed in the form of sand and small pebbles which have beenworn from their surfaces. 

We can best perceive the effect of battering action which the seaapplies to the cliffs by noting the points where, owing to some chancefeatures in the structure in the rock, it has proved most effective. Where a joint or a dike, or perhaps a softer layer, if the rocks bebedded, causes the wear to go on more rapidly, the waves soon excavatea recess in which the pebbles are retained, except in stormy weather, in an unmoved condition. When the surges are heavy, these stones arekept in continuous motion, receding as the wave goes back, and rushingforward with its impulse until they strike against the firm-set rockat the end of the chasm. In this way they may drive in a cut havingthe length of a hundred feet or more from the face of the precipice. In most cases the roofs over these sea caves fall in, so that thestructure is known as a chasm. Occasionally these roofs remain, inwhich case, for the reason that the floor of the cutting inclinesupward, an opening is made to the surface at their upper end, formingwhat is called in New England a "spouting horn"; from the inland endof the tunnel the spray may be thrown far into the air. As long as thecave is closed at this inner end, and is not so high but that it maybe buried beneath a heavy wave, the inrushing water compresses theair in the rear parts of the opening. When the wave begins to retreatthis air blows out, sending a gust of spray before it, the actionresembling the discharge of a great gun from the face of afortification. It often happens that two chasms converging separate arock from the cliff. Then a lowering of the coast may bring the massto the state of a columnar island, such as abound in the Hebrides andalong various other shores. 

If a cliff shore retreats rapidly, it may be driven back into theshore, and its face assumes the curve of a small bay. With every stepin this change the bottom is sure to become shallower, so that thewaves lose more and more of their energy in friction over the bottom. Moreover, in entering a bay the friction which the waves encounter inrunning along the sides is greater than that which they meet incoming in upon a headland or a straight shore. The result is, with theinward retreat of the steep it enters on conditions which diminish theeffectiveness of the wave stroke. The embayment also is apt to holddetritus, and so forms in time a beach at the foot of the cliff, overwhich the waves rarely are able to mount with such energy as willenable them to strike the wall in an effective manner. With thissketch of the conditions of a cliff shore, we will now consider thefate of the broken-tip rock which the waves have produced on thatsection of the coast land. 

By observation of sea-beaten cliffs the student readily perceives thata great amount of rocky matter has been removed from most cliff-facedshores. Not uncommonly it can be shown that such sea faces haveretreated for several miles. The question now arises, What becomes ofthe matter which has been broken up by the wave action? In some partthe rock, when pulverized by the pounding to which it is subjected, has dissolved in the water. Probably ninety per cent of it, however, retains the visible state, and has a fate determined by the size ofthe fragments of which it is composed. If these be as fine as mud, sothat they may float in the water, they are readily borne away by thecurrents which are always created along a storm-swept shore, particularly by the undertow or bottom outcurrent--the "sea-puss, " asit is sometimes called--that sweeps along the bottom from every shore, against which the waves form a surf. If as coarse as sand grains, oreven very small pebbles, they are likely to be drawn out, rolling overthe bottom to an indefinite distance from the sea margin. The coarserstones, however, either remain at the foot of the cliff until they arebeaten to pieces, or are driven along the shore until they find someembayment into which they enter. The journey of such fragments may, when the wind strikes obliquely to the shore, continue for many miles;the waves, running with the wind, drive the fragments in oscillatingjourneys up and down the beach, sometimes at the rate of a mile ormore a day. The effect of this action can often be seen where a vesselloaded with brick or coal is wrecked on the coast. In a monthfragments of the materials may be stretched along for the distance ofmany miles on either side of the point where the cargo came ashore. Entering an embayment deep enough to restrain their further journey, the fragments of rock form a boulder beach, where the bits roll to andfro whenever they are struck by heavy surges. The greater portion ofthem remain in this mill until they are ground to the state of sandand mud. Now and then one of the fragments is tossed up beyond thereach of the waves, and is contributed to the wall of the beach. Invery heavy storms these pebbles which are thrown inland may amount inweight to many tons for each mile of shore. 

The study of a pebbly beach, drawn from crest to the deep wateroutside, will give an idea as to the history of its work. On eitherhorn of the crescent by which the pebbles are imported into the pocketwe find the largest fragments. If the shore of the bay be long, theinnermost part of the recess may show even only very small pebbles, orperhaps only fine sand, the coarser material having been worn out inthe journey. On the bottom of the bay, near low tide, we begin to findsome sand produced by the grinding action. Yet farther out, belowhigh-tide mark, there is commonly a layer of mud which represents thefiner products of the mill. 

Boulder beaches are so quick in answering to every slight change inthe conditions which affect them that they seem almost alive. If byany chance the supply of detritus is increased, they fill in betweenthe horns, diminish the incurve of the bay, and so cause its beach tobe more exposed to heavy waves. If, on the other hand, the supply ofgrist to the mill is diminished, the beach becomes more deeplyincurved, and the wave action is proportionately reduced. We may say, in general, that the curve of these beaches represents a balancebetween the consumption and supply of the pebbles which they grind up. The supply of pebbles brought along the shore by the waves is in manycases greatly added to by a curious action of seaweeds. If the bottomof the water off the coast is covered by these fragments, as is thecase along many coast lines within the old glaciated districts, thespores of algæ are prone to take root upon them. Fastening themselvesin those positions, and growing upward, the seaweeds may attainconsiderable size. Being provided with floats, the plant exercises acertain lifting power on the stone, and finally the tugging action ofthe waves on the fronds may detach the fragments from the bottom, making them free to journey toward the shore. Observing from near athand the straight wall of the wave in times of heavy storm, thepresent writer has seen in one view as many as a dozen of theseplant-borne stones, sometimes six inches in diameter, hanging in thewalls of water as it was about to topple over. As soon as they strikethe wave-beaten part of the shore these stones are apt to becomeseparated from the plants, though we can often notice the remains orprints of the attachments adhering to the surface of the rock. Wherethe pebbles off the shore are plenty, a rocky beach may be producedby this process of importation through the agency of seaweeds withoutany supply being brought by the waves along the coast line. 

Returning to sand beaches, we enter the most interesting field ofcontact between seas and lands. Probably nine tenths of all the coastlines of the open ocean are formed of arenaceous material. In general, sand consists of finely broken crystals of silica or quartz. Thesebits are commonly distinctly faceted; they rarely have a sphericalform. Not only do accumulations of sand border most of the shore line, but they protect the land against the assaults of the sea, and this inthe following curious manner: When shore waves beat pebbles againsteach other, they rapidly wear to bits; we can hear the sound of thewearing action as the wave goes to and fro. We can often see that thewater is discoloured by the mud or powdered rock. When, however, thewaves tumble on a sandy coast, they make but a muffled sound, andproduce no mud. In fact, the particles of sand do not touch each otherwhen they receive the blow. Between them there lies a thin film ofwater, drawn in by the attraction known as capillarity, which sucksthe fluid into a sponge or between plates of glass placed neartogether. The stroke of the waves slightly compresses this capillarywater, but the faces of the grains are kept apart as sheets of glassmay be observed to be restrained from contact when water is betweenthem. If the reader would convince himself as to the condition of thesand grains and the water which is between them, he may do so bypressing his foot on the wet beach which the wave has just left. Hewill observe that it whitens and sinks a little under the pressure, but returns in good part to its original form when the foot is lifted. In the experiment he has pushed a part of the contained water aside, but he has not brought the grains together; they do not make the soundwhich he will often hear when the sand is dry. The result is that thesand on the seashore may wear more in going the distance of a mile inthe dry sand dune than in travelling for hundreds along the wet shore. 

If the rock matter in the state of sand wore as rapidly under theheating of the waves as it does in the state of pebbles, thecontinents would doubtless be much smaller than they are. Those coastswhich have no other protection than is afforded by a low sand beachare often better guarded against the inroads of the sea than therock-girt parts of the continents. It is on account of this remarkableendurance of sand of the action of the waves that the stratified rockswhich make up the crust of the earth are so thick and are to such anextent composed of sand grains. 

The tendency of the _débris_-making influences along the coast line isto fill in the irregularities which normally exist there; to batteroff the headlands, close up the bays and harbours, and generally toreduce the shores to straight lines. Where the tide has access tothese inlets, it is constantly at work in dragging out the detrituswhich the waves make and thrust into the recesses. These two actionscontend with each other, and determine the conditions of the coastline, whether they afford ports for commerce or are sealed in by sandbars, as are many coast lines which are not tide-swept, as that ofnorthern Africa, which faces the Mediterranean, a nearly tideless sea. The same is the case with the fresh-water lakes; even the greater ofthem are often singularly destitute of shelters which can serve theuse of ships, and this because there are no tides to keep the bays andharbours open. 

 THE OCEAN CURRENTS. 

The system of ocean currents, though it exhibits much complication indetail, is in the main and primarily dependent on the action of theconstant air streams known as the trade winds. With the breath fromthe lips over a basin of water we can readily make an experiment whichshows in a general way the method in which the winds operate inproducing the circulation of the sea. Blowing upon the surface of thewater in the basin, we find that even this slight impulse at once setsthe upper part in motion, the movement being of two kinds--pulsatingmovements or waves are produced, and at the same time the friction ofthe air on the surface causes its upper part to slide over the under. With little floats we can shortly note that the stream which formspasses to the farther side of the vessel, there divides, and returnsto the point of beginning, forming a double circle, or rather twoellipses, the longer sides of which are parallel with the line of theair current. Watching more closely, aiding the sight by the particleswhich float at various distances below the surface, we note the factthat the motion which was at first imparted to the surface graduallyextends downward until it affects the water to the depth of someinches. 

In the trade-wind belt the ocean waters to the depth of some hundredsof feet acquire a continuous movement in the direction in which theyare impelled by those winds. This motion is most rapid at the surfaceand near the tropics. It diminishes downwardly in the water, and alsotoward the polar sides of the trade-wind districts. Thus the tradesproduce in the sea two broad, slow-moving, deep currents, flowing inthe northern hemisphere toward the southwest, and in the southernhemisphere toward the northwest. Coming down upon each otherobliquely, these broad streams meet about the middle of the tropicalbelt. Here, as before noted, the air of the trade winds leaves thesurface and rises upward. The waters being retained on their level, form a current which moves toward the west. If the earth within thetropics were covered by a universal sea, the result of this movementwould be the institution of a current which, flowing under theequator, would girdle the sphere. 

With a girdling equatorial current, because of the intense heat of thetropics and the extreme cold of the parallels beyond the fortiethdegree of latitude, the earth would be essentially uninhabitable toman, and hardly so to any forms of life. Its surface would be visitedby fierce winds induced by the very great differences of temperaturewhich would then prevail. Owing, however, to the barriers which thecontinents interpose to the motions of these windward-setting tropicalcurrents, all the water which they bear, when it strikes the opposingshores, is diverted to the right and left, as was the stream in theexperiment with the basin and the breath, the divided currents seekingways toward high latitudes, conveying their store of heat to thecircumpolar lands. So effective is this transfer of temperature that avery large part of the heat which enters the waters in the tropicalregion is taken out of that division of the earth's surface anddistributed over the realms of sea and land which lie beyond thelimits of the vertical sun. Thus the Gulf Stream, the northern branchof the Atlantic tropical current, by flowing into the North Atlantic, contributes to the temperature of the region within the Arctic Circlemore heat than actually comes to that district by the direct influxfrom the sun. 

The above statements as to the climatal effect of the ocean streamsshow us how important it is to obtain a sufficient conception as tothe way in which these currents now move and what we can of theirhistory during the geologic ages. This task can not yet be adequatelydone. The fields of the sea are yet too imperfectly explored to affordus all the facts required to make out the whole story. Only in thecase of our Gulf Stream can we form a full conception as to thejourney which the waters undergo and the consequence of their motion. In the case of this current, observations clearly show that it arisesfrom the junction near the equatorial line of the broad stream createdby the two trade-wind belts. Uniting at the equator, these produce awesterly setting current, having the width of some hundred miles and adepth of several hundred feet. Its velocity is somewhat greater than amile an hour. The centre of the current, because of the greaterstrength of the northern as compared with the southern trades, isconsiderably south of the equator. When this great slow-moving streamcomes against the coast of South America, it encounters the projectingshoulder of that land which terminates at Cape St. Roque. There itdivides, as does a current on the bows of an anchored ship, apart--rather more than one half--of the stream turning to thenorthward, the remainder passing toward the southern pole; thisnortherly portion becomes what is afterward known as the Gulf Stream, the history of which we shall now briefly follow. 

Flowing by the northwesterly coast of South America, the northernshare of the tropical current, being pressed in against the land bythe trade winds, is narrowed, and therefore acquires at once a swifterflow, the increased speed being due to conditions like those which addto the velocity of the water flowing through a hose when it comes tothe constriction of the nozzle. Attaining the line of the southeasternor Lesser Antilles, often known as the Windward Islands, a part ofthis current slips through the interspaces between these isles andenters the Gulf of Mexico. Another portion, failing to find sufficientroom through these passages, skirts the Antilles on their eastern andnorthern sides, passes by and among the Bahama Islands, there torejoin the part of the stream which entered the Caribbean. ThisCaribbean portion of the tide spreads widely in that broad sea, isconstricted again between Cuba and Yucatan, again expands in the Gulfof Mexico, and is finally poured forth through the Straits of Floridaas a stream having the width of forty or fifty miles, a depth of athousand feet or more, and a speed of from three to five miles anhour, exceeding in its rate of flow the average of the greatestrivers, and conveying more water than do all the land streams of theearth. In this part of its course the deep and swift stream from theGulf of Mexico, afterward to be named the Gulf Stream, receives thecontribution of slower moving and shallower currents which skirted theAntilles on their eastern verge. The conjoined waters then movenorthward, veering toward the east, at first as a swift river of thesea having a width of less than a hundred miles and of great depth;with each step toward the pole this stream widens, diminishingproportionately in depth; the speed of its current decreases as theoriginal impetus is lost, and the baffling winds set its surfacewaters to and fro in an irregular way. Where it passes Cape Hatterasit has already lost a large share of its momentum and much of itsheat, and is greatly widened. 

Although the current of the Gulf Stream becomes more languid as we gonorthward, it for a very long time retains its distinction from thewaters of the sea through which it flows. Sailing eastward from themouth of the Chesapeake, the navigator can often observe the momentwhen he enters the waters of this current. This is notable not only inthe temperature, but in the hue of the sea. North of that line thesharpness of the parting wall becomes less distinct, the streamspreads out broadly over the surface of the Atlantic, yet itsthermometric effects are distinctly traceable to Iceland and NovaZembla, and the tropical driftwood which it carries affords theprincipal timber supply of the inhabitants of the first-named isle. Attaining this circumpolar realm, and finally losing the impulse whichbore it on, the water of the Gulf Stream partly returns to thesouthward in a relatively slight current which bears the fluid alongthe coast of Europe until it re-enters the system of tropical windsand the currents which they produce. A larger portion stagnates in thecircumpolar region, in time slowly to return to the tropical districtin a manner afterward to be described. Although the Gulf Stream in theregion north of Cape Hatteras is so indistinct that its presence wasnot distinctly recognised until the facts were subjected to the keeneye of Benjamin Franklin, its effects in the way of climate are sogreat that we must attribute the fitness of northern Europe for theuses of civilized man to its action. But for the heat which thisstream brings to the realm of the North Atlantic, Great Britain wouldbe as sterile as Labrador, and the Scandinavian region, thecradle-land of our race, as uninhabitable as the bleakest parts ofSiberia. 

It is a noteworthy fact that when the equatorial current divides onthe continents against which it flows, the separate streams, althoughthey may follow the shores for a certain distance toward the poles, soon diverge from them, just as the Gulf Stream passes to the seawardfrom the eastern coast of the United States. The reason for thismovement is readily found in the same principle which explains theoblique flow of the trades and counter trades in their passage to andfrom the equatorial belt. The particle of water under the equator, though it flows to the west, has, by virtue of the earth's rotation, an eastward-setting velocity of a thousand miles an hour. Startingtoward the poles, the particle is ever coming into regions of the seawhere the fluid has a less easterly movement, due to the earth'srotation on its axis. Consequently the journeying water by itsmomentum tends to move off in an easterly course. Attaining highlatitudes and losing its momentum, it abides in the realm long enoughto become cooled. 

We have already noted the fact that only a portion of the waters sentnorthward in the Gulf Stream and the other currents which flow fromthe equator to the poles is returned by the surface flow which setstoward the equator along the eastern side of the basins. The largestshare of the tide effects its return journey in other ways. Someportion of this remainder sets equatorward in local cold streams, suchas that which pours forth through Davis Strait into Baffin Bay, flowing under the Gulf Stream waters for an unknown distance towardthe tropics. There are several of these local as yet little knownstreams, which doubtless bring about a certain amount of circulationbetween the polar regions and the tropical districts. Their effect is, however, probably small as compared with that massive drift which wehave now to note. 

The tropical waters when they attain high latitudes are constantlycooled, and are overlaid by the warmer contributions of that tide, andare thus brought lower and lower in the sea. When they start downwardthey have, as observations show, a temperature not much above thefreezing point of salt water. They do not congeal for the reason thatthe salt of the ocean lowers the point at which the water solidifiesto near 28° Fahr. The effect of this action is gradually to press downthe surface cold water until it attains the very bottom in all thecircumpolar regions. At the same time this descending water driftsalong the bottom of the ocean troughs toward the equatorial realm. Asthis cold water is heavier than that which is of higher temperatureand nearer the surface, it has no tendency to rise. Being below thedisturbing influences of any current save its own, it does not tend, except in a very small measure, to mingle with the warmer overlyingfluid. The result is that it continues its journey until it may comewithin the tropics without having gained a temperature of more than35° Fahr. , the increase in heat being due in small measure to thatwhich it receives from the earth's interior and that which it acquiresfrom the overlying warmer water. Attaining the region of the tropicalcurrent, this drift water from the poles gradually rises, to take theplace of that which goes poleward, becomes warm, and again starts onits surface journey toward the arctic and antarctic regions. 

Nothing is known as to the rate of this bottom drift from the polardistricts toward the equator, but, from some computation which he hasmade, the writer is of the opinion that several centuries is doubtlessrequired for the journey from the Arctic Circle to the tropics. Thespeed of the movement probably varies; it may at times require somethousand years for its accomplishment. The effect of the bottom driftis to withdraw from seas in high latitudes the very cold water whichthere forms, and to convey it beneath the seas of middle latitudes toa realm where it is well placed for the reheating process. If all thecold water of circumpolar regions had to journey over the surface tothe equator, the perturbing effect of its flow on the climates ofvarious lands would be far greater than it is at present. Where suchcold currents exist the effect is to chill the air without adding muchto the rainfall; while the currents setting northward not only warmthe regions near which they flow, but by so doing send from the watersurfaces large quantities of moisture which fall as snow or rain. Thusthe Gulf Stream, directly and indirectly, probably contributes morethan half the rainfall about the Atlantic basin. The lack of thisinfluence on the northern part of North America and Asia causes thoselands to be sterilized by cold, although destitute of permanent iceand snow upon their surfaces. 

We readily perceive that the effect of the oceanic circulation uponthe temperatures of different regions is not only great but widelycontrasted. By taking from the equatorial belt a large part of theheat which falls within that realm, it lowers the temperature to thepoint which makes the district fit for the occupancy of man, perhaps, indeed, tenable to all the higher forms of life. This same heatremoved to high latitudes tempers the winter's cold, and thus makes avast realm inhabitable which otherwise would be locked in almostenduring frosts. Furthermore, this distribution of temperatures tendsto reduce the total wind energy by diminishing the trades and countertrades which are due to the variations of heat which are encounteredin passing polarward from the equator. Still further, but for thiscirculation of water in the sea, the oceans about the poles would befrozen to their very bottom, and this vast sheet of ice might beextended southward to within the parallels of fifty degrees north andsouth latitude, although the waters under the equator might at thesame time be unendurably hot and unfit for the occupancy of livingbeings. 

A large part of the difficulties which geologists encounter inendeavouring to account for the changes of the past arise from theevidences of great climatal revolutions which the earth has undergone. In some chapters of the great stone book, whose leaves are the strataof the earth, we find it plainly written in the impressions made byfossils that all the lands beyond the equatorial belt have undergonechanges which can only be explained by the supposition that the heatand moisture of the countries have been subjected to sudden andremarkable changes. Thus in relatively recent times thick-leavedplants which retained their vegetation in a rather tender statethroughout the year have flourished near to the poles, while shortlyafterward an ice sheet, such as now covers the greater part ofGreenland, extended down to the line of the Ohio River at Cincinnati. Although these changes of climate are, as we shall hereafter note, probably due to entangled causes, we must look upon the modificationsof the ocean streams as one of the most important elements in thecausation. We can the more readily imagine such changes to be due tothe alterations in the course and volume of the ocean current when wenote how trifling peculiarities in the geography of theshores--features which are likely to be altered by the endless changeswhich occur in the form of a continent--affect the run of thesecurrents. Thus the growth of coral reefs in southern Florida, and, ingeneral, the formation of that peninsula, by narrowing the exit of thegreat current from the Gulf of Mexico, has probably increased itsvelocity. If Florida should again sink down, that current would goforth into the North Atlantic with the speed of about a mile an hour, and would not have momentum enough to carry its waters over half thevast region which they now traverse. If the lands about the westernborder of the Caribbean Sea, particularly the Isthmus of Darien, should be depressed to a considerable depth below the ocean level, the tropical current would enter the Pacific Ocean, adding to thetemperature of its waters all the precious heat which now vitalizesthe North Atlantic region. Such a geographic accident would not onlyprofoundly alter the life conditions of that part of the world, but itwould make an end of European civilization. 

In the chapter on climatal changes further attention will be given tothe action of ocean currents from the point of view of their influenceon the heat and moisture of different parts of the world. We now haveto consider the last important influence of ocean currents--that whichthey directly exercise on the development of organic life. The moststriking effect of this nature which the sea streams bring about iscaused by the ceaseless transportation to which they subject the eggsand seeds of animals and plants, as well as the bodies of the matureform which are moved about by the flowing waters. But for theexistence of these north and south flowing currents, due to thepresence of the continental barriers, the living tenants of the seaswould be borne along around the earth, always in the same latitude, and therefore exposed to the same conditions of temperature. In thisstate of affairs the influences which now make for change in organicspecies would be far less than they are. Journeying in the greatwhirlpools which the continental barriers make out of the westwardsetting tropical currents, these organic species are ever beingexposed to alterations in their temperature conditions which we knowto be favourable to the creation of those variations on which theadvance of organic life so intimately depends. Thus the ocean currentsnot only help to vary the earth by producing changes in the climate ofboth sea and land, breaking up the uniformity which would otherwisecharacterize regions at the same distance from the equator, but theyinduce, by the consequences of the migrations which they enforce, changes in the organic tenants of the sea. 

Another immediate effect of ocean streams arises where their currentsof warm water come against shores or shallows of the sea. At thesepoints, if the water have a tropical temperature, we invariably find avast and rapid development of marine animals and plants, of which thecoral-making polyps are the most important. In such positions thegrowth of forms which secrete solid skeletons is so rapid that greatwalls of their remains accumulate next the shore, the mass being builtoutwardly by successive growths until the realm of the land may beextended for scores of miles into the deep. In other cases vast moundsof this organic _débris_ may be accumulated in mid ocean until itssurface is interspersed with myriads of islands, all of which mark thework due to the combined action of currents and the marine life whichthey nourish. Probably more than four fifths of all the islands in thetropical belt are due in this way to the life-sustaining action of thecurrents which the trade winds create. 

There are many secondary influences of a less important nature whichare due to the ocean streams. The reader will find on most wall-mapsof the world certain areas in the central part of the oceans which arenoted as Sargassum seas, of which that of the North Atlantic, west andsouth of the Azore Islands, is one of the most conspicuous. In thesetracts, which in extent may almost be compared with the continents, wefind great quantities of floating seaweed, the entangled fronds ofwhich often form a mass sufficiently dense to slightly restrain thespeed of ships. When the men on the caravels of Columbus entered thistangle, they were alarmed lest they should be unable to escape fromits toils. It is a curious fact that these weeds of the sea whilefloating do not reproduce by spores the structures which answer to theseeds of higher plants, but grow only by budding. It seems certainthat they could not maintain their place in the ocean but for theaction of the currents which convey the bits rent off from the shoreswhere the plant is truly at home. This vast growth of plant life inthe Sargassum basins doubtless contributed considerable and importantdeposits of sediment to the sea floors beneath the waters which itinhabits. Certain ancient strata, known as the Devonian black shale, occupying the Ohio valley and the neighbouring parts of North Americato the east and north of that basin, appear to be accumulations whichwere made beneath an ancient Sargassum sea. 

The ocean currents have greatly favoured and in many instancesdetermined the migrations not only of marine forms, but of landcreatures as well. Floating timber may bear the eggs and seeds of manyforms of life to great distances until the rafts are cast ashore in arealm where, if the conditions favour, the creatures may find a newseat for their life. Seeds of plants incased in their often denseenvelopes may, because they float, be independently carried greatdistances. So it comes about that no sooner does a coral or otherisland rise above the waters of the sea than it becomes occupied by avaried array of plants. The migrations of people, even down to thetime of the voyages which discovered America, have in large measurebeen controlled by the run of the ocean streams. The tropical set ofthe waters to the westward helped Columbus on his way, and enabled himto make a journey which but for their assistance could hardly havebeen accomplished. This same current in the northern part of the GulfStream opposed the passage of ships from northern Europe to thewestward, and to this day affects the speed with which their voyagesare made. 

 THE CIRCUIT OF THE RAIN. 

We have now to consider those movements of the water which depend uponthe fact that at ordinary temperatures the sea yields to the air acontinued and large supply of vapour, a contribution which is made inlessened proportion by water in all stages of coldness, and even byice when it is exposed to dry air. This evaporation of the sea wateris proportional to the temperature and to the dryness of the air whereit rests upon the ocean. It probably amounts on the average tosomewhere about three feet per annum; in regions favourably situatedfor the process, as on the west coast of northern Africa, it may bethree or four times as much, while in the cold and humid air about thepoles it may be as little as one foot. When contributed to the air, the water enters on the state of vapour, in which state it tends todiffuse itself freely through the atmosphere by virtue of the motionwhich is developed in particles when in the vaporous or gaseous state. 

The greater part of the water evaporated from the seas probably findsits way as rain at once back into the deep, yet a considerable portionis borne away horizontally until it encounters the land. Theprecipitation of the water from the air is primarily due to thecooling to which it is subjected as it rises in the atmosphere. Overthe sea the ascent is accomplished by the simple diffusion of thevapour or by the uprise through the aërial shaft, such as that nearthe equator or over the centres of the whirling storms. It is when theair strikes the slopes of the land that we find it brought into acondition which most decidedly tends to precipitate its moisture. Lifted upward, the air as it ascends the slopes is brought into coolerand more rarefied conditions. Losing temperature and expanding, itparts with its water for the same reason that it does in the ascendingcurrent in the equatorial belt or in the chimneys of the whirl storms. A general consequence of this is that wherever moisture-laden windsfrom the sea impinge upon a continent they lay down a considerablepart of the water which they contain. 

If all the lands were of the same height, the rain would generallycome in largest proportion upon their coastal belt, or those portionsof the shore-line districts over which the sea winds swept. But asthese winds vary in the amount of the watery vapour which theycontain, and as the surface of the land is very irregular, therainfall is the most variable feature in the climatal conditions ofour sphere. Near the coasts it ranges from two or three inches in aridregions--such as the western part of the Sahara and portions of thecoast regions of Chili and Peru--to eight hundred inches about thehead waters of the Brahmapootra River in northern India, where thehigh mountains are swept over by the moisture-laden airs from theneighbouring sea. Here and there detached mountainous masses produce asingular local increase in the amount of the rainfall. Thus in thelake district in northwestern England the rainfall on the seaward sideof mountains, not over four thousand feet high, is very much greaterthan it is on the other slope, less than a score of miles away. Theselocal variations are common all over the world, though they are butlittle observed. 

In general, the central parts of continents are likely to receive muchless rainfall than their peripheral portions. Thus the centraldistricts of North America, Asia, and Australia--three out of the fivecontinental masses--have what we may call interior deserts. Africa hasone such, though it is north of the centre, and extends to the shoresof the Mediterranean and the Atlantic. The only continent without thiscentral nearly rainless field is South America, where the solecharacteristic arid district is situated on the western slope of theCordilleran range. In this case the peculiarity is due to the factthat the strong westerly setting winds which sweep over the countryencounter no high mountains until they strike the Andean chain. Theyjourney up a long and rather gradual slope, where the precipitation isgradually induced, the process being completed when they strike themountain wall. Passing over its summit, they appear as dry winds onthe Pacific coast. 

Even while the winds frequently blow in from the sea, as along thewestern coast of the Americas, they may come over water which isprevailingly colder than the land. This is characteristically the caseon the western faces of the American continent, where the sea iscooled by the currents setting toward the equator from high latitudes. Such cool sea air encountering the warm land has its temperatureraised, and therefore does not tend to lay down its burden ofmoisture, but seeks to take up more. On this account the rainfall incountries placed under such conditions is commonly small. 

By no means all the moisture which comes upon the earth from theatmosphere descends in the form of rain or snow. A variable, large, though yet undetermined amount falls in the form of dew. Dew is aprecipitation of moisture which has not entered the peculiar statewhich we term fog or cloud, but has remained invisible in the air. Itis brought to the earth through the radiation of heat whichcontinually takes place, but which is most effective during thedarkened half of the day, when the action is not counterbalanced bythe sun's rays. While the sun is high and the air is warm there is aconstant absorption of moisture in large part from the ground or fromthe neighbouring water areas, probably in some part from thosesuspended stores of water, the clouds, if such there be in theneighbourhood. We can readily notice how clouds drifting in from thesea often melt into the dry air which they encounter. Late in theafternoon, even before the sun has sunk, the radiation of heat fromthe earth, which has been going on all the while, but has been lessconsiderable than the incurrent of temperature, in a way overtakesthat influx. The air next the surface becomes cooled from its contactwith the refrigerating earth, and parts with its moisture, forming acoating of water over everything it touches. At the same time themoisture escaping from the warmed under earth likewise drops back uponits cooled surface almost as soon as it has escaped. The thin sheet ofwater precipitated by this method is quickly returned to the air whenit becomes warmed by the morning sunshine, but during the nightquantities of it are absorbed by the plants; very often, indeed, withthe lowlier vegetation it trickles down the leaves and enters theearth about the base of the stem, so that the roots may appropriateit. Our maize, or Indian corn, affords an excellent example of a plantwhich, having developed in a land of droughts, is well contrived, through its capacities for gathering dew, to protect itself againstarid conditions. In an ordinary dew-making night the leaves of asingle stem may gather as much as half a pint of water, which flowsdown their surfaces to the roots. So efficient is this dew supply, this nocturnal cloudless rain, that on the western coast of SouthAmerica and elsewhere, where the ordinary supply of moisture is almostwanting, many important plants are able to obtain from it much of thewater which they need. The effect is particularly striking alongseashores, where the air, although it may not have the humiditynecessary for the formation of rain, still contains enough to formdew. 

It is interesting to note that the quantity of dew which falls upon anarea is generally proportioned to the amount of living vegetationwhich it bears. The surfaces of leaves are very efficient agents ofradiation, and the tangle which they make offers an amount ofheat-radiating area many times as great as that afforded by a surfaceof bared earth. Moreover, the ground itself can not well cool down tothe point where it will wring the moisture out of the air, while thethin membranes of the plants readily become so cooled. Thus vegetationby its own structure provides itself with means whereby it may be in ameasure independent of the accidental rainfall. We should also notethe fact that the dewfall is a concomitant of cloudless skies. Thequantity which is precipitated in a cloudy night is very small, andthis for the reason that when the heavens are covered the heat fromthe earth can not readily fly off into space. Under these conditionsthe temperature of the air rarely descends low enough to favour theprecipitation of dew. 

Having noted the process by which in the rain circuit the waterleaves the sea and the conditions of distribution when it returns tothe earth, we may now trace in more detail the steps in this greatround. First, we should take note of the fact that the water after itenters the air may come back to the surface of the earth in either oftwo ways--directly in the manner of dewfall, or in a longer circuitwhich leads it through the state of clouds. As yet we are not verywell informed as to the law of the cloud-making, but certain featuresin this picturesque and most important process have been tolerablywell ascertained. 

Rising upward from the sea, the vapour of water commonly remainstransparent and invisible until it attains a considerable height abovethe surface, where the cooling tends to make it assume again thevisible state of cloud particles. The formation of these cloudparticles is now believed to depend on the fact that the air is fullof small dust motes, exceedingly small bits of matter derived from themany actions which tend to bring comminuted solid matter into the air, as, for instance, the combustion of meteoric stones, which are greatlyheated by friction in their swift course through the air, theejections of volcanoes, the smoke of forest and other fires, etc. These tiny bits, floating in the air, because of their solid natureradiate their heat, cool the air which lies against them, and therebyprecipitate the water in the manner of dew, exactly as do the leavesand other structures on the surface of the earth. In fact, dewformation is essentially like cloud formation, except that in the onecase the water is gathered on fixed bodies, and in the other onfloating objects. Each little dust raft with its cargo of condensedwater tends, of course, to fall downward toward the earth's surface, and, except for the winds which may blow upward, does so fall, thoughwith exceeding slowness. Its rate of descent may be only a few feet aday. It was falling before it took on the load of water; it will falla little more rapidly with the added burden, but even in a still airit might be months or years before it would come to the ground. Thereason for this slow descent may not at first sight be plain, though alittle consideration will make it so. 

If we take a shot of small size and a feather of the same weight, wereadily note that their rate of falling through the air may vary inthe proportion of ten to one or more. It is easy to conceive that thisdifference is due to the very much less friction which the smallerbody encounters in its motion by the particles of air. With this pointin mind, the student should observe that the surface presented bysolid bodies in relation to their solid contents is the greater thesmaller the diameter. A rough, though not very satisfactory, instanceof this principle may be had by comparing the surface and interiorcontents of two boxes, one ten feet square and the other one footsquare. The larger has six hundred feet of surface to one thousandcubic feet of interior, or about half a square foot of outer surfaceto the cubic foot of contents; while the smaller box has six feet ofsurface for the single cubic foot of interior, or about ten times theproportion of exterior to contents. The result is that the smallerparticles encounter more friction in moving toward the earth, until, in the case of finely divided matter, such as the particles of carbonin the smoke from an ordinary fire, the rate of down-falling may be sosmall as to have little effect in the turbulent conditions ofatmospheric motion. 

[Illustration: _Pocket Creek, Cape Ann, Massachusetts. Note therelatively even size of the pebbles, and the splash wave which setsthem in motion. _]

The little drops of water which gather round dust motes, falling butslowly toward the earth, are free to obey the attractions which theyexercise upon each other--impulses which are partly gravitative andpartly electrical. We have no precise knowledge concerning thesemovements, further than that they serve to aggregate the myriad littlefloats into cloud forms, in which the rafts are brought near together, but do not actually touch each other. They are possibly kept apart byelectrical repulsion. In this state of association without union thedivided water may undergo the curiously modified aggregations whichgive us the varied forms of clouds. As yet we know little as to thecause of cloud shapes. We remark the fact that in the higher of theseagglomerations of condensed vapour, the clouds which float at anelevation of from twenty to thirty thousand feet or more, the massesare generally thin, and arranged more or less in a leaflike form, though even here a tendency to produce spherical clouds is apparent. In this high realm floating water is probably in the frozen state, answering to the form of dew, which we call hoar frost. The lowerclouds, gathering in the still air, show very plainly the tendency toagglomerate into spheres, which appears to be characteristic of allvaporous material which is free to move by its own impulses. It isprobable that the spherical shape of clouds is more or less due to thesame conditions as gathered the stellar matter from the ancientnebular chaos into the celestial spheres. Upon these sphericalaggregations of the clouds the winds act in extremely varied ways. Thecloud may be rubbed between opposite currents, and so flattened outinto a long streamer; it may take the same form by being carried offby a current in the manner of smoke from a fire; the spheres may bekept together, so as to form the patchwork which we call "mackerel"sky; or they may be actually confounded with each other in a vastcommon cloud-heap. In general, where the process of aggregation of twocloud bodies occurs, changes of temperature are induced in the masseswhich are mixed together. If the temperature resulting from thisassociation of cloud masses is an average increase, the cloud maybecome lighter, and in the manner of a balloon move upward. Each ofthe motes in the cloud with its charge of vapour may be compared withthe ballast of the balloon; if they are warmed, they send forth a partof their load of condensed water again to the state of invisiblevapour. Rising to a point where it cools, the vapour gathers back onthe rafts and tends again to weight the cloud downward. The ballast ofan ordinary balloon has to be thrown away from its car; but if somearrangement for condensing the moisture from the air could becontrived, a balloon might be brought into the adjustable state of acloud, going up or down according as it was heated or cooled. 

When the formation of the drop of water or snowflake begins, the massis very small. If in descending it encounters great thickness ofcloud, the bit may grow by further condensation until it becomesrelatively large. Generally in this way we may account for thediversities in the size of raindrops or snowflakes. It often happensthat the particles after taking on the form of snowflakes encounter intheir descent air so warm that they melt into raindrops, or, if onlypartly melted, reach the surface as sleet. Or, starting as raindrops, they may freeze, and in this simple state may reach the earth, orafter freezing they may gather other frozen water about them, so thatthe hailstone has a complicated structure which, from the point ofview of classification, is between a raindrop and a snowflake. 

In the process of condensation--indeed, in the steps which precede theformation of rain and snow--there is often more or less trace ofelectrical action; in fact, a part of the energy which was involved inthe vapourization of water, on its condensation, even on the dustmotes appears to be converted into electrical action, which probablyoperates in part to keep the little aggregates of water asunder. Whenthey coalesce in drops or flakes, this electricity often assumes theform of lightning, which represents the swift passage of the electricstore from a region where it is most abundant to one where it is lessso. The variations in this process of conveying the electricity areprobably great. In general, it probably passes, much as an electriccurrent is conveyed, through a wire from the battery which producesthe force. In other cases, where the tension is high, or, in otherwords, where the discharge has to be hastened, we have the phenomenaof lightning in which the current burns its way along its path, as itmay traverse a slender wire, vapourizing it as it goes. In general, the lightning flash expends its force on the air conductors, or linesof the moist atmosphere along which it breaks its path, its energyreturning into the vapour which it forms or the heat which it producesin the other parts of the air. In some cases, probably not one in thethousand of the flashes, the charge is so heavy that it is not used upin its descent toward the earth, and so electrifies, or, as we say, strikes, some object attached to the earth, through which it passes tothe underlying moisture, where it finds a convenient place to take ona quiet form. Almost all these hurried movements of electrical energywhich intensely heat and light the air which they traverse fly fromone part of a cloud to another, or cross from cloud sphere to cloudsphere; of those which start toward the earth, many are exhaustedbefore they reach its surface, and even those that strike convey but aportion of their original impulse to the ground. 

The wearing-out effect of lightning in its journey along the airconductors in its flaming passages is well illustrated by what happenswhen the charge strikes a wire which is not large enough freely toconvey it. The wire is heated, generally made white hot, often melted, and perhaps scattered in the form of vapour. In doing this work theelectricity may, and often is, utterly dissipated--that is, changedinto heat. It has been proposed to take advantage of this principle inprotecting buildings from lightning by placing in them many thinwires, along which the current will try to make its way, beingexhausted in melting or vaporizing the metal through which it passes. 

There are certain other forms of lightning, or at least of electricaldischarges, which produce light and which may best be described inthis connection. It occasionally happens that the earth becomes socharged that the current proceeds from its surface to the clouds. Morerarely, and under conditions which we do not understand, the electricenergy is gathered into a ball-like form, which may move slowly alongthe surface until it suddenly explodes. It is a common feature of allthese forms of lightning which we have noted that they ordinarily makein their movement considerable noise. This is due to the suddendisplacement of the air which they traverse--displacement due to theaction of heat in separating the particles. It is in all essentialregards similar to the sounds made by projectiles, such as meteors orswift cannon shots, as they fly through the air. It is even morecomparable to the sound produced by exploding gunpowder. The firstsound effect from the lightning stroke is a single rending note, whichendures no longer--indeed, not as long--as the explosion of a cannon. Heard near by, this note is very sharp, reminding one of the soundmade by the breaking of glass. The rolling, continuous sound which wecommonly hear in thunder is, as in the case of the noise produced bycannon, due to echo from the clouds and the earth. Thunder isordinarily much more prolonged and impressive in a mountainous countrythan in a region of plains, because the steeps about the hearerreverberate the original single crash. 

The distribution of thunderstorms is as yet not well understood, butit appears in many cases that they are attendants on the advancingface of cyclones and hurricanes, the area in front of these greatwhirlstorms being subjected to the condensation and irregular airmovements which lead to the development of much electrical energy. There are, however, certain parts of the earth which are particularlysubjected to lightning flashes. They are common in the region near theequator, where the ascending currents bring about heavy rains, whichmean a rapid condensation and consequent liberation of electricalenergy. They diminish in frequency toward the arctic regions. Anobserver at the pole would probably fail ever to perceive strongflashes. For the same reason thunderstorms are more frequent insummer, the time when the difference in temperature between thesurface and the upper air is greatest, when, therefore, the uprushesof air are likely to be most violent. They appear to be more common inthe night than in the daytime, for the reason that condensation isfavoured by the cooling which occurs in the dark half of the day. Itis rare, indeed, that a thunderstorm occurs near midday, a period whenthe air is in most cases taking up moisture on account of the swiftlyincreasing heat. 

There are other forms of electrical discharges not distinctlyconnected with the then existing condensation of moisture. What thesailors call St. Elmo's fire--a brush of electric light from the masttops and other projections of the ship--indicates the passage ofelectrical energy between the vessel and the atmosphere. Similarlights are said sometimes to be seen rising from the surface of thewater. Such phenomena are at present not satisfactorily explained. Perhaps in the same group of actions comes the so-called"Jack-o'-lantern" or "Will-o'-the-wisp" fires flashing from the earthin marshy places, which are often described by the common people, buthave never been observed by a naturalist. If this class ofilluminations really exists, we have to afford them some otherexplanation than that they are emanations of self-inflamedphosphoretted hydrogen, a method of accounting for them whichillogically finds a place in many treatises on atmospheric phenomena. A gas of any kind would disperse itself in the air; it could not danceabout as these lights are said to do, and there is no chemical meansknown whereby it could be produced in sufficient purity and quantityfrom the earth to produce the effects which are described. [3]

[Footnote 3: The present writer has made an extended and careful studyof marsh and swamp phenomena, and is very familiar with the aspect ofthese fields in the nighttime. He has never been able to see any sign ofthe Jack-o'-lantern light. Looking fixedly into any darkness, such as isafforded by the depths of a wood, the eye is apt to imagine theappearance of faint lights. Those who have had to do with outpost dutyin an army know how the anxious sentry, particularly if he is new to thesoldier's trade, will often imagine that he sees lights before him. Sometimes the pickets will be so convinced of the fact that they seelights that they will fire upon the fiction of the imaginations. Thesefacts make it seem probable that the Jack-o'-lantern and his companion, the Will-o'-the-wisp, are stories of the overcredulous. ]

In the upper air, or perhaps even beyond the limits of the fieldwhich deserves the name, in the regions extending from the poles tonear the tropics, there occur electric glowings commonly known as theaurora borealis. This phenomenon occurs in both hemispheres. Theseilluminations, though in some way akin to those of lightning, andthough doubtless due to some form of electrical action, are peculiarin that they are often attended by glows as if from clouds, and bypulsations which indicate movements not at electric speed. As yet butlittle is known as to the precise nature of these curious storms. Ithas been claimed, however, that they are related to the sun spots;those periods when the solar spots are plenty, at intervals of abouteleven years, are the times of auroral discharges. Still further, itseems probable that the magnetic currents of the earth, that circlingenergy which encompasses the sphere, moving round in a general wayparallel to the equator, are intensified during these illuminations ofthe circumpolar skies. 

 GEOLOGICAL WORK OF WATER. 

We turn now to the geological work which is performed by fallingwater. Where the rain or snow returns from the clouds to the sea, theenergy of position given to the water by its elevation above the earththrough the heat which it acquired from the sun is returned to the airthrough which it falls or to the ocean surface on which it strikes. Inthis case the circuit of the rain is short and without geologicalconsequence which it is worth while to consider, except to note thatthe heat thus returned is likely to be delivered in another realm thanthat in which the falling water acquired the store, thus in a smallway modifying the climate. When, however, the precipitation occurs onthe surface of the land, the drops of frozen or fluid water apply apart of their energy in important geological work, the like of whichis not done where they return at once to the sea. 

[Illustration: Fig. 10. --Showing the diverse action of rain on woodedand cleared fields, _a_, wooded area; _b_, tilled ground. ]

We shall first consider what takes place when the water in the form ofdrops of rain comes to the surface of the land. Descending as they dowith a considerable speed, these raindrops apply a certain amount ofenergy to the surface on which they fall. Although the beat of araindrop is proverbially light, the stroke is not ineffective. Observing what happens where the action takes place on the surface ofbare rock, we may notice that the grains of sand or small pebbleswhich generally abound on such surfaces, if they be not too steeplyinclined, dance about under the blows which they receive. If we couldcover hard plate glass, a much firmer material than ordinary stone, with such bits, we should soon find that its surface would becomescratched all over by the friction. Moreover, the raindropsperceptibly urge the small detached bits of stone down the slopestoward the streams. 

If all the earth's surface were bare rocks, the blow of the raindropswould deserve to be reckoned among the important influences which leadto the wearing of land. As it is, when a country is in a state ofNature, only a small part of its surface is exposed to this kind ofwearing. Where there is rain enough to effect any damage, there issure to be sufficient vegetation to interpose a living andself-renewed covering between the rocks and the rain. Even the lichenswhich coat what at first sight often seems to be bare rock afford anample covering for this purpose. It is only where man bares the fieldby stripping away and overturning this protecting vegetation that theraindrops cut away the earth. The effect of their action can often benoted by observing how on ploughed ground a flat stone or a potsherdcomes after a rain to cap a little column. The geologist sometimesfinds in soft sandstones that the same action is repeated in a largerway where a thin fragment of hard rock has protected a column manyfeet in height against the rain work which has shorn down thesurrounding rock. 

When water strikes the moistened surface it at once loses the droplikeform which all fluids assume when they fall through the air. [4]

[Footnote 4: This principle of the spheroidal form in falling fluids isused in making ordinary bird shot. The melted lead drops throughsievelike openings, the resulting spheres of the metal being allowed tofall into water which chills them. Iron shot, used in cutting stone, where they are placed between the saw and the surface of the rock, arealso made in the same manner. The descending fluid divides into dropsbecause it is drawn out by the ever-increasing speed of the fallingparticles, which soon make the stream so thin that it can not holdtogether. ]

When the raindrops coalesce on the surface of the earth, the rôle ofwhat we may call land water begins. Thenceforward until the fluidarrives at the surface of the sea it is continually at work ineffecting a great range of geological changes, only a few of which canwell be traced by the general student. The work of land water is dueto three classes of properties--to the energy with which it is endowedby virtue of its height above the sea, a power due to the heat of thesun; to the capacity it has for taking substances into solution; andto its property of giving some part of its own substance to othermaterials with which it comes in contact. The first of these groups ofproperties may be called dynamical; the others, chemical. 

The dynamic value of water when it falls upon the land is the amountof energy it can apply in going down the slope which separates it fromthe sea. A ton of the fluid, such as may gather in an ordinary rain ona thousand square feet of ground in the highlands of a country--say atan elevation of a thousand feet above the sea--expends before it comesto rest in the great reservoir as much energy as would be required tolift that weight from the ocean's surface to the same height. The waysin which this energy may be expended we shall now proceed in a generalway to trace. 

As soon as the water has been gathered, from its drop to its sheetstate--a process which takes place as soon as it falls--the fluidbegins its downward journey. On this way it is at once parted into twodistinct divisions, the surface water and the ground water: the formercourses more or less swiftly, generally at the rate of a mile or morean hour, in the light of day; the latter enters the interstices of theearth, slowly descends therein to a greater or less depth, andfinally, journeying perhaps at the rate of a mile a year, rejoins thesurface water, escaping through the springs. The proportion of thesetwo classes, the surface and the ground water, varies greatly, and anintermixture of them is continually going on. Thus on the surface ofbare rock or frozen earth all the rain may go away without enteringthe ground. On very sandy fields the heaviest rainfall may be takenup by the porous earth, so that no streams are found. On such surfacesthe present writer has observed that a rainfall amounting to sixinches in depth in two hours produced no streams whatever. We shallfirst follow the history of the surface water, afterward consideringthe work which the underground movements effect. 

If the student will observe what takes place on a level ploughedfield--which, after all, will not be perfectly level, for all fieldsare more or less undulating--he will note that, though the surface mayhave been smoothed by a roller until it appears like a floor, thefirst rain, where the fall takes place rapidly enough to producesurface streams, will create a series of little channels which growlarger as they conjoin, the whole appearing to the eye like a verydetailed map, or rather model, of a river system; it is, indeed, sucha system in miniature. If he will watch the process by which thesestreamlet beds are carved, he will obtain a tolerably clear idea as tothat most important work which the greater streams do in carving theface of the lands. The water is no sooner gathered into a sheet than, guided by the slightest irregularities which it encounters, it beginsto flow. At first the motion is so slow that it does not disturb itsbed, but at some points in the bottom of the sheet the movement soonbecomes swift enough to drag the grains of sand and clay from theiradhesions, bearing them onward. As soon as this beginning of a channelis formed the water moves more swiftly in the clearer way; ittherefore cuts more rapidly, deepening and enlarging its channel, andmaking its motion yet more free. The tiny rills join the greater, alltheir channels sway to and fro as directed this way and that by chanceirregularities, until something like river basins are carved out, those gentle slopes which form broad valleys where the carving hasbeen due to the wanderings of many streams. If the field be large, considerable though temporary brooks may be created, which cutchannels perhaps a foot in depth. At the end of this miniature streamsystem we always find some part of the waste which has been carvedout. If the streamlet discharges into a pool, we find the tinyrepresentative of deltas, which form such an important feature on thecoast line where large rivers enter seas or lakes. Along the lines ofthe stream we may observe here and there little benches, which are theequivalent in all save size of the terraces that are generally to beobserved along the greater streams. In fact, these accidents of anacre help in a most effective way the student to understand thegreater and more complicated processes of continental erosion. 

A normal river--in fact, all the greater streams of theearth--originates in high country, generally in a region of mountains. Here, because of the elevation of the region, the streams have cutdeep gorges or extensive valleys, all of which have slopes leadingsteeply downward to torrent beds. Down these inclined surfaces theparticles worn off from the hard rock by frost and by chemical decaygradually work their way until they attain the bed of the stream. Theagents which assist gravitation in bearing this detritus downward aremany, but they all work together for the same end. The stroke of theraindrop accomplishes something, though but little; the direct washingaction of the brooklets which form during times of heavy rain, but dryout at the close of the storm, do a good deal of the work; thawing andfreezing of the water contained in the mass of detritus help themovement, for, although the thrust is in both directions, it is mosteffective downhill; the wedges of tree roots, which often penetratebetween and under the stones, and there expand in their process ofgrowth, likewise assist the downward motion. The result is that onordinary mountain slopes the layer of fragments constituting the rudesoil is often creeping at the rate of from some inches to some feet ayear toward the torrent bed. If there be cliffs at the top of theslope, as is often the case, very extensive falls of rock may takeplace from it, the masses descending with such speed that theydirectly attain the stream. If the steeps be low and the rock dividedinto vertical joints, especially where there is a soft layer at thebase of the steep, detached masses from the precipice may move slowlyand steadfastly down the slope, so little disturbed in their journeythat trees growing upon their summits may continue to develop for thethousands of years before the mass enters the stream bed. 

Although the fall of rocks from precipices does not often take placein a conspicuously large way, all great mountain regions which havelong been inhabited by man abound in traditions and histories of suchaccidents. Within a century or two there have been a dozen or morecatastrophes of this nature in the inhabited valleys of the Alps. Asthese accidents are at once instructive and picturesque, it is well tonote certain of them in some detail. At Yvorgne, a little parish onthe north shore of the Rhône, just above the lake of Geneva, traditiontells that an ancient village of the name was overwhelmed by the fallof a great cliff. The vast _débris_ forming the steep slope which wasthus produced now bears famous vineyards, but the vintners fancy thatthey from time to time hear deep in the earth the ringing of the bellswhich belonged to the overwhelmed church. In 1806 the district ofGoldau, just north of Lake Lucerne, was buried beneath the ruins of apeak which, resting upon a layer of clay, slipped away like alaunching ship on the surface of the soft material. The _débris_overwhelmed a village and many detached houses, and partly filled aconsiderable lake. The wind produced by this vast rush of falling rockwas so great that people were blown away by it; some, indeed, werekilled in this singular manner. 

The most interesting field of these Swiss mountain falls is a highmountain valley of amphitheatrical form, known as the Diablerets, orthe devil's own district. This great circus, which lies at the heightof about four thousand feet above the sea, is walled around on itsnorthern side by a precipice, above which rest, or rather oncerested, a number of mountain peaks of great bulk. The region has longbeen valued for the excellent pasturage which the head of the valleyaffords. Two costly roads, indeed, have been built into it to affordfootpaths for the flocks and herds and their keepers in the summerseason. Through this human experience with the valley, we have arecord of what has gone on in this part of the mountain wilderness. Within the period of history and tradition, three very great mountainfalls have occurred in this field, each having made its memory good bywidespread disaster which it brought to the people of the _chalets_. The last of these was brought about by the fall of a great peak whichspread itself out in a vast field of ruins in the valley below. Thebelt of destruction was about half a mile wide and three miles long. When the present writer last saw it, a quarter of a century ago, itwas still a wilderness of great rocks, but here and there the processof their decay was giving a foothold for herbage, and in a fewcenturies the field will doubtless be so verdure-clad that its storywill not be told on its face. It is likely, however, to be preservedin the memory of the people, and this through a singular and pathetictradition which has grown up about the place, one which, if not true, comes at least among the legends which we should like to believe. 

As told the present writer by a native of the district, it happenedwhen, in the nighttime the mountain came down, the herdsmen and theircows gathered in the _chalets_--stout buildings which are prepared toresist avalanches of snow. In one of these, which was protected fromcrushing by the position of the stones which covered it, a solitaryherdsman found himself alive in his unharmed dwelling. With him in thedarkness were the cows, a store of food and water, and his provisionsfor the long summer season. With nothing but hope to animate him, heset to work burrowing upward among the rocks, storing the _débris_ inthe room of the _chalet_. He toiled for some months, but finallyemerged to the light of day, blanched by his long imprisonment in thedarkness, but with the strength to bear him to his home. In place ofthe expected warm welcome, the unhappy man found himself received as aghost. He was exorcised by the priest and driven away to the distance. It was only when long afterward his path of escape was discovered thathis history became known. 

Returning to the account of the _débris_ which descends at variedspeed into the torrents, we find that when the detritus encounters theaction of these vigorous streams it is rapidly ground to pieces whileit is pushed down the steep channels to the lower country. Where thestones are of such size that the stream can urge them on, they moverapidly; at least in times when the torrent is raging. They beat overeach other and against the firm-set rocks; the more they wear, thesmaller they become, and the more readily they are urged forward. Where the masses are too large to be stirred by the violent current, they lie unmoved until the pounding of the rolling stones reduces themto the proportions where they may join the great procession. Ordinarily those who visit mountains behold their torrents only intheir shrunken state, when the waters stir no stones, and fail even tobear a charge of mud, all detachable materials having been swept awaywhen the streams course with more vigour. In storm seasons theconditions are quite otherwise; then the swollen torrents, theirwaters filled with clay and sand, bear with them great quantities ofboulders, the collisions of which are audible above the muffled roarof the waters, attesting the very great energy of the action. 

When the waste on a mountain slope lies at a steep angle, particularlywhere the accumulation is due to the action of ancient glaciers, itnot infrequently happens that when the ground is softened with frostgreat masses of the material rush down the slope in the manner oflandslides. The observer readily notes that in many mountain regions, as, for instance, in the White Mountains of New Hampshire, the steepslopes are often seamed by the paths of these great landslides. Theirmovement, indeed, is often begun by sliding snow, which gives animpulse to the rocks and earth which it encounters in its descent. Ata place known as the Wylie Notch, in the White Mountains, in the earlypart of this century, a family of that name was buried beneath a massof glacial waste which had hung on the mountain slope from the ancientdays until a heavy rain, following on a period of thaw, impelled themass down the slope. Although there have been few such catastrophesnoted in this country, it is because our mountains have not been muchdwelt in. As they become thickly inhabited as the Alps are, men aresure to suffer from these accidents. 

As the volume of a mountain torrent increases through the junction ofmany tributaries, the energy of its moving waters becomes sufficientto sweep away the fragments which come to its bed. Before this stageis attained the stream rarely touches the solid under rock of themountain, the base of the current resting upon the larger loose stoneswhich it was unable to stir. In this pebble-paved section, because thestream could not attack the foundation rock, we find no gorges--infact, the whole of this upper section of the torrent system ispeculiarly conditioned by the fact that the streams are dealing notwith bed-rock, but with boulders or smaller loose fragments. If theycut a little channel, the materials from either side slip the faster, and soon repave the bed. But when the streams have by a junctiongained strength, and can keep their beds clear, they soon carve down agorge through which they descend from the upper mountain realm to thelarger valleys, where their conjoined waters take on a riverlikeaspect. It should be noted here that the cutting power of the watermoving in the torrent or in the wave, the capacity it has for abradingrock, resides altogether in the bits of stone or cutting tools withwhich it is armed. Pure water, because of its fluidity, may move overor against firm-set stones for ages without wearing them; but inproportion as it moves rocky particles of any size, the larger theyare, the more effective the work, it wears the rock over which itflows. A capital instance of this may be found where a stream from ahose is used in washing windows. If the water be pure, there is noeffect upon the glass; but if it be turbid, containing bits of sand, in a little while the surface will appear cloudy from the multitude ofline scratches which the hard bits impelled by the water haveinflicted upon it. A somewhat similar case occurs where the wind bearssand against window panes or a bottle which has long lain on theshore. The glass will soon be deeply carved by the action, assumingthe appearance which we term "ground. " This principle is made use ofin the arts. Glass vessels or sheets are prepared for carving bypasting paper cut into figures on their surfaces. The material is thenexposed to a jet of air or steam-impelling sand grains; in a shorttime all the surface which has not been protected by paper has itspolish destroyed and is no longer translucent. 

The passage from the torrent to the river, though not in ageographical way distinct, is indicated to the observant eye by asimple feature--namely, the appearance of alluvial terraces, thosemore or less level heaps of water-borne _débris_ which accumulatealong the banks of rivers, which, indeed, constitute the differencebetween those streams and torrents. Where the mountain waters moveswiftly, they manage to bear onward the waste which they receive. Evenwhere the blocks of stone cling in the bed, it is only a short timebefore they are again set in motion or ground to pieces. If by chancethe detritus accumulates rapidly, the slope is steepened and the workof the torrent made more efficient. As the torrent comes toward thebase of the mountains, where it neither finds nor can create steepslopes over which to flow, its speed necessarily diminishes. With eachreduction in this feature its carrying power very rapidly diminishes. Thus water flowing at the rate of ten miles an hour can urge stonesfour times the mass that it can move when its speed is reduced to halfthat rate. The result is that on the lowlands, with their relativelygentle slopes, the combined torrents, despite the increase in thevolume of the stream arising from their confluence, have to lay down alarge part of their load of detritus. 

If we watch where a torrent enters a mountain river, we observe thatthe main stream in a way sorts over the waste contributed to it, bearing on only those portions which its rate of flow will permit itto carry, leaving the remainder to be built into the bank in the formof a rude terrace. This accumulation may not extend far below thepoint where the torrent which imported the _débris_ joins the mainstream; a little farther down, however, we are sure to find anothersuch junction and a second accumulation of terrace material. As thesecontributions increase, the terrace accumulations soon becomecontinuous, lying on one side or the other of the river, sometimesbordering both banks of the stream. In general, it can be said that solong as the rate of fall of the torrent exceeds one hundred feet tothe mile it does not usually exhibit these shelves of detritus. Belowthat rate of descent they are apt to be formed. Much, however, dependsupon the amount of detritus which the stream bears and the coarsenessof it; moreover, where the water goes through a gorge in the manner ofa flume with steep rocky sides, it can urge a larger amount before itthan when it traverses a wide valley, through which it passes, it maybe, in a winding way. 

At first sight it may seem rather a fine distinction to separatetorrents from rivers by the presence or absence of terraces. As wefollow down the stream, however, and study its action in relation tothese terraces, and the peculiar history of the detritus of which theyare composed, we perceive that these latter accumulations are veryimportant features. Beginning at first with small and imperfectalluvial plains, the river, as it descends toward the sea, gaining instore of water and in the amount of _débris_ which comes with thatwater from the hills, while the rate of fall and consequent speed ofthe current are diminished, soon comes to a stage where it is engagedin an endless struggle with the terrace materials. In times of flood, the walls of the terraces compel the tide to flow over the tops ofthese accumulations. Owing to the relative thinness of the waterbeyond the bed, and to the growth of vegetation there, the currentmoves more slowly, and therefore lays down a considerable deposit ofthe silt and sand which it contains. This may result during a singleflood in lifting the level of the terrace by some inches in height, still further serving to restrict the channel. Along the banks of theMississippi and other large rivers the most of this detritus fallsnear the stream; a little of it penetrates to the farther side of theplains, which often have a width of ten miles or more. The result isthat a broad elevation is constructed, a sort of natural mole orlevee, in a measure damming the flood waters, which can now only enterthe "back swamps" through the channels of the tributary streams. Eachof these back swamps normally discharges into the main stream througha little river of its own, along the banks of which the natural leveesdo not develop. 

We have now to note a curious swinging movement of rivers which wasfirst well observed by the skilful engineers of British India. Thismovement can best be illustrated by its effects. If on any river whichwinds through alluvial plains a jetty is so constructed as to deflectthe stream at any point, the course which it follows will be alteredduring its subsequent flow, it may be, for the distance of hundreds ofmiles. It will be perceived that in its movements a river normallystrikes first against one shore and then against the other. Its waterin a general way moves as does a billiard ball when it flies from onecushion to another. It is true that in a torrent we have the sameconditions of motion; but there the banks are either of hard rock or, if of detritus, they are continually moving into the stream in themanner before described. In the case of the river, however, its pointsof collision are often on soft banks, which are readily undermined bythe washing action of the stream. In the ordinary course of events, the river beginning, we may imagine, with a straight channel, had itscurrent deflected by some obstacle, it may be even by the slightpressure of a tributary stream, is driven against one bank; thence itrebounds and strikes the other. At each point of impinge it cuts thealluvium away. It can bear on only a small portion of that which itthus obtains; the greater part of the material is deposited on theopposite side of the stream, but a little lower down, where it makes ashallow. On these shallows water-loving plants and even certain trees, such as the willows and poplars, find a foothold. When the streamrises, the sediment settles in this tangle, and soon extends thealluvial plain from the neighbouring bank, or in rarer cases the rivercomes to flow on either side of an island of its own construction. Thenatural result of this billiard-ball movement of the waters is thatthe path of the stream is sinuous. The less its rate of fall and thegreater the amount of silt it obtains from its tributaries, the morewinding its course becomes. This gain in those parts of the river'scurvings where deposition tends to take place may be accelerated bytree-planting. Thus a skilful owner of a tract of land on the southbank of the Ohio River, by assiduously planting willow trees on thefront of his property, gained in the course of thirty years more thanan acre in the width of his arable land. When told by the presentwriter that he was robbing his neighbours on the other side of thestream, he claimed that their ignorance of the laws of river motionwas sufficient evidence that they did not deserve to own land. 

In the primitive state of a country the water-loving plants, particularly the trees which flourish in excessively humid conditions, generally make a certain defence against these incursions of thestreams. But when a river has gained an opening in the bank it can, during a flood, extend its width often to the distance of hundreds offeet. During the inundations of the Mississippi the river may at timesbe seen to eat away acres of land in a single day along one of theoutcurves of its banks. The undermined forests falling into the floodjoin the great procession of drift timber, composed of trees whichhave been similarly uprooted, which occupies the middle part of thestream. This driftwood belt often has a width of three or four hundredfeet, the entangled stems and branches making it difficult for a boatto pass from one side of the river to the other. 

[Illustration: Fig. 11. --Oxbows and cut-off. Showing the changes inthe course of a river in its alluvial plain. ]

When the curves of a river have been developed to a certain point (seeFig. 11), when they have attained what is called the "oxbow" form, itoften happens that the stream breaks through the isthmus whichconnects one of the peninsulas with the mainland. Where, as is notinfrequently the case, the bend has a length of ten miles or more, thewater just above and below the new-made opening is apt to differ inheight by some feet. Plunging down the declivity, the stream, flowingwith great velocity, soon enlarges the channel so that its whole tidemay take the easier way. When this result is accomplished, the oldcurve is deserted, sand bars are formed across their mouths, which maygradually grow to broad alluvial plains, so that the long-surviving, crescent-shaped lake, the remnant of the river bed, may be seen farfrom the present course of the ever-changing stream. Gradually theaccumulations of vegetable matter and the silt brought in by floodsefface this moat or oxbow cut-off, as it is so commonly termed. 

As soon as the river breaks through the neck of a peninsula in themanner above described, the current of the stream becomes much swifterfor many miles below and above the opening. Slowly, however, theslopes are rearranged throughout its whole course, yet for a time thestream near the seat of the change becomes straighter than before, andthis for the reason that its swifter current is better able to disposeof the _débris_ which is supplied to it. The effect of a change in thecurrent produced by such new channels as we have described as formingacross the isthmuses of bends is to perturb the course of the streamin all its subsequent downward length. Thus an oxbow cut-off formednear the junction of the Ohio and Mississippi may tend more or less toalter the swings of the Mississippi all the way to the Gulf of Mexico. 

Although the swayings of the streams to and fro in their alluvialplains will give the reader some idea as to the struggle which thegreater rivers have with the _débris_ which is committed to them, thefull measure of the work and its consequences can only be appreciatedby those who have studied the phenomena on the ground. A river suchas the Mississippi is endlessly endeavouring to bear its burden to thesea. If its slope were a uniform inclined plane, the task mightreadily be accomplished; but in this, as in almost all other largewater ways, the slope of the bed is ever diminishing with its onwardcourse. The same water which in the mountain torrent of theAppalachians or Cordilleras rolled along stones several feet indiameter down slopes of a hundred feet or more to the mile can in thelower reaches of the stream move no pebbles which are more than onefourth of an inch in diameter over slopes which descend on the averageabout half a foot in a mile. Thus at every stage from the torrent tothe sea the detritus has from time to time to rest within the alluvialbanks, there awaiting the decay which slowly comes, and which maybring it to the state where it may be dissolved in the water, ordivided into fragments so small that the stream may bear them on. Acomputation which the present writer has made shows that, on theaverage, it requires about forty thousand years for a particle ofstone to make its way down the Mississippi to the sea after it hasbeen detached from its original bed. Of course, some bits may make thejourney straightforwardly; others may require a far greater time toaccomplish the course which the water itself makes at most in a fewweeks. This long delay in the journey of the detritus--a delay causedby its frequent rests in the alluvial plain--brings about importantconsequences which we will now consider. 

As an alluvial plain is constructed, we generally find at the basepebbly material which fell to the bottom in the current of the mainstream as the shores grew outward. Above this level we find thedeposits laid down by the flood waters containing no pebbles, and thisfor the reason that those weightier bits remained in the stream bedwhen the tide flowed over the plain. As the alluvial deposit is laiddown, a good deal of vegetable matter was built into it. Generallythis has decayed and disappeared. On the surface of the plain therehas always been growing abundant vegetation, the remains of whichdecayed on the surface in the manner which we may observe at thepresent day. This decomposing vegetable matter within and upon theporous alluvial material produces large quantities of carbonic acid, agas which readily enters the rain water, and gives it a peculiar powerof breaking up rock matter. Acting on the _débris_, this gas-chargedwater rapidly brings about a decay of the fragments. Much of thematerial passes at once into solution in this water, and drains awaythrough the multitudinous springs which border the river. As thismatter is completely dissolved, as is sugar in water, it goes straightaway to the sea without ever again entering the alluvium. In many, ifnot most, cases this dissolving work which is going on in alluvialterraces is sufficient to render a large part of the materials whichthey contain into the state where it disappears in an unseen manner;thus while the annual floods are constantly laying down accumulationson the surface of these plains, the springs are bearing it away frombelow. 

In this way, through the decomposition which takes place in them, allthose river terraces where much vegetable matter is mingled with themineral substances, become laboratories in which substances arebrought into solution and committed to the seas. We find in the waterof the ocean a great array of dissolved mineral substances; it, indeed, seems probable that the sea water contains some share, thoughusually small, of all the materials which rivers encounter in theirjourney over and under the lands. As the waters of the sea obtain butlittle of this dissolved matter along the coast, it seems likely thatthe greater share of it is brought into the state of solution in thenatural laboratories of the alluvial plains. 

Here and there along the sides of the valleys in which the rivers flowwe commonly find the remains of ancient plains lying at more or lessconsiderable heights above the level of the streams. Generally thesedeposits, which from their form are called terraces, represent thestages of down-wearing by which the stream has carved out its waythrough the rocks. The greater part of these ancient alluvial plainshas been removed through the ceaseless swinging of the stream to andfro in the valley which it has excavated. 

In all the states of alluvial plains, whether they be the fertiledeposits near the level of the streams which built them, or the poorerand ruder surfaced higher terraces, they have a great value tomankind. Men early learned that these lands were of singularly uniformgoodness for agricultural use. They are so light that they were easilydelved with the ancient pointed sticks or stone hoes, or turned by theolden, wooden plough. They not only give a rich return when firstsubjugated, but, owing to the depth of the soil and the frequency withwhich they are visited by fertilizing inundations, they yield richharvests without fertilizing for thousands of years. It is thereforenot surprising that we find the peoples who depended upon tillage forsubsistence first developed on the great river plains. There, indeed, were laid the foundations of our higher civilization; there alonecould the state which demands of its citizens fixed abodes andcontinuous labour take rise. In the conditions which these fields ofabundance afforded, dense populations were possible, and all the artswhich lead toward culture were greatly favoured. Thus it is that thecivilization of China, India, Persia, and Egypt, the beginnings ofman's higher development, began near the mouths of the great rivervalleys. These fields were, moreover, most favourably placed for theinstitution of commerce, in that the arts of navigation, originatingin the sheltered reaches of the streams, readily found its way throughthe estuaries to the open sea. 

Passing down the reaches of a great river as it approaches the sea, wefind that the alluvial plains usually widen and become lower. Atlength we attain a point where the flood waters cover the surface forso large a part of the year that the ground is swampy and untillableunless it is artificially and at great expense of labour won toagriculture in the manner in which this task has been effected in thelower portion of the Rhine Valley. Still farther toward the sea, theplain gradually dips downward until it passes below the level of thewaters. Through this mud-flat section the stream continues to cutchannels, but with the ever-progressive slowing of its motion theburden of fine mud which it carries drops to the bottom, andconstantly closes the paths through which the water escapes. Every fewyears they tend to break a new way on one side or the other of theirformer path. Some of the greatest engineering work done in moderntimes has been accomplished by the engineers engaged in controllingthe exits of large rivers to the sea. The outbreak of the Yellow Riverin 1887, in which the stream, hindered by its own accumulations, forced a new path across its alluvial plains, destroyed a vast deal oflife and property, and made the new exit seventy miles from the pathwhich it abandoned. 

Below the surface of the open water the alluvial deposits spread outinto a broad fan, which slopes gradually to a point where, in themanner of the continental shelf, the bottom descends steeply into deepwater. 

It is the custom of naturalists to divide the lower section of riverdeposits--that part of the accumulation which is near the sea--fromthe other alluvial plains, terming the lower portion the delta. Theword originally came into use to describe that part of the alluviumaccumulated by the Nile near its mouth, which forms a fertileterritory shaped somewhat like the fourth letter of the Greekalphabet. Although the definition is good in the Egyptian instance, and has a certain use elsewhere, we best regard all the detritus in ariver valley which is in the state of repose along the stream to itsutmost branches as forming one great whole. It is, indeed, one of themost united of the large features which the earth exhibits. Thestudent should consider it as a continuous inclined plane ofdiminishing slope, extending from the base of the torrents to thesea, and of course ramifying into the several branches of the riversystem. He should further bear in mind the fact that it is a vastlaboratory where rock material is brought into the soluble state fordelivery to the seas. 

The diversity in the form of river valleys is exceedingly great. Almost all the variety of the landscape is due to this impress ofwater action which has operated on the surface in past ages. Whenfirst elevated above the sea, the surface of the land is but littlevaried; at this stage in the development the rivers have but shallowvalleys, which generally cut rather straight away over the plaintoward the sea. It is when the surface has been uplifted to aconsiderable height, and especially when, as is usually the case, thisuplifting action has been associated with mountain-building, thatvalleys take on their accented and picturesque form. The reason forthis is easily perceived: it lies in the fact that the rocks overwhich the stream flows are guided in the cutting which they effect bythe diversities of hardness in the strata that they encounter. Thework which it does is performed by the hard substances that areimpelled by the current, principally by the sand and pebbles. Thesematerials, driven along by the stream, become eroding tools of veryconsiderable energy. As will be seen when we shortly come to describewaterfalls, the potholes formed at those points afford excellentevidence as to the capacity of stream-impelled bits of stone to cutaway the firmest bed rocks. Naturally the ease with which this carvingwork is done is proportionate to the energy of the currents, and alsoto the relative hardness of the moving bits and the rocks over whichthey are driven. 

So long as the rocks lie horizontally in their natural constructionattitude the course of the stream is not much influenced by thevariations in hardness which the bed exhibits. Where the strata arevery firm there is likely to be a narrow gorge, the steeps of whichrise on either side with but slight alluvial plains; where the bedsare soft the valley widens, perhaps again to contract where in thecourse of its descent it encounters another hard layer. Where, however, the beds have been subjected to mountain-building, and havebeen thrown into very varied attitudes by folding and faulting, thestream now here and now there encounters beds which either restrainits flow or give it freedom. The stream is then forced to cut its wayaccording to the positions of the various underlying strata. Thiseffect upon its course is not only due to the peculiarities ofuplifted rocks, but to manifold accidents of other nature: veins anddikes, which often interlace the beds with harder or softer partitionsthan the country rock; local hardenings in the materials, due tocrystallization and other chemical processes, often createindescribable variations which are more or less completely expressedin the path of the stream. 

When a land has been newly elevated above the sea there is often--wemay say, indeed, generally--a very great difference between the heightof its head waters and the ocean level. In this condition of a countrythe rivers have what we may call a new aspect; their valleys arecommonly narrow and rather steep, waterfalls are apt to abound, andthe alluvial terraces are relatively small in extent. Stage by stagethe torrents cut deeper; the waste which they make embarrasses thecourse of the lower waters, where no great amount of down-cutting ispossible for the reason that the bed of the stream is near sea level. At the same time the alluvial materials, building out to sea, thusdiminish the slope of the stream. In the extreme old age of the riversystem the mountains are eaten down so that the torrent sectiondisappears, and the stream becomes of something like a uniform slope;the higher alluvial plains gradually waste away, until in the end thevalley has no salient features. At this stage in the process, or evenbefore it is attained, the valley is likely to be submerged beneaththe sea, where it is buried beneath the deposits formed on the floor;or a further uplift of the land may occur with the result that thestream is rejuvenated; or once more endowed with the power to createtorrents, build alluvial plains, and do the other interesting work ofa normal river. 

It rarely, if ever, happens that a river valley attains old age beforeit has sunk beneath the sea or been refreshed by further upliftings. In the unstable conditions of the continents, one or the other ofthese processes, sometimes in different places both together, is aptto be going on. Thus if we take the case of the Mississippi and itsprincipal tributaries, the Ohio and Missouri, we find that for manygeological ages the mountains about their sources have frequently, ifnot constantly, grown upward, so that their torrent sections, thoughthey have worn down tens of thousands of feet, are still high abovethe sea level, perhaps on the average as high as they have ever been. At the same time the slight up-and-down swayings of the shore lands, amounting in general to less than five hundred feet, have greatlyaffected the channels of the main river and its tributaries in theirlower parts. Not long ago the Mississippi between Cairo and the Gulfflowed in a rather steep-sided valley probably some hundreds of feetin depth, which had a width of many miles. Then at the close of thelast Glacial period the region sank down so that the sea flooded thevalley to a point above the present junction of the Ohio River withthe main stream. Since then alluvial plains have filled this estuaryto even beyond the original mouth. In many other of our Southernrivers, as along the shore from the Mississippi to the Hudson, thestreams have not brought in enough detritus to fill their drownedvalleys, which have now the name of bays, of which the Delaware andChesapeake on the Atlantic coast, and Mobile Bay on the Gulf ofMexico, are good examples. The failure of Chesapeake and Delaware Baysto fill with _débris_ in the measure exhibited by the more southernvalleys is due to the fact that the streams which flow into them to agreat extent drain from a region thickly covered with glacial waste, amass which holds the flood waters, yielding the supply but slowly tothe torrents, which there have but a slight cutting power. 

In our sketch of river valleys no attention has been given to thephenomena of waterfalls, those accidents of the flow which, as we havenoted, are particularly apt to characterize rivers which have not yetcut down to near the sea level. Where the normal uniform descent whichis characteristic of a river's bed is interrupted by a sudden steep, the fact always indicates the occurrence of one of a number ofgeological actions. The commonest cause of waterfalls is due to asudden change in the character of horizontal or at least nearly levelbeds over which the stream may flow. Where after coursing for adistance over a hard layer the stream comes to its edge and drops on asoft or easily eroded stratum, it will cut this latter bed away, andcreate a more or less characteristic waterfall. Tumbling down the faceof the hard layer, the stream acquires velocity; the _débris_ which itconveys is hurled against the bottom, and therefore cuts powerfully, while before, being only rubbed over the stone as it moved along, itcut but slightly. Masses of ice have the same effect as stones. Bitsdropping from the ledge are often swept round and round by the eddies, so that they excavate an opening which prevents their chance escape. In these confined spaces they work like augers, boring a deep, well-like cavity. As the bits of stone wear out they are replaced byothers, which fall in from above. Working in this way, the fragmentsoften develop regular well-like depressions, the cavities of whichwork back under the cliffs, and by the undermining process deprive theface of the wall of its support, so that it tumbles in ruin to thebase, there to supply more material for the potholing action. 

Waterfalls of the type above described are by far the commonest ofthose which occur out of the torrent districts of a great riversystem. That of Niagara is an excellent specimen of the type, which, though rarely manifested in anything like the dignity of the greatfall, is plentifully shown throughout the Mississippi Valley and thebasin of the Great Lakes. Within a hundred miles of Niagara there areat least a hundred small waterfalls of the same type. Probably threequarters of all the larger accidents of this nature are due to theconditions of a hard bed overlying softer strata. 

Falls are also produced in very many instances by dikes which crossthe stream. So, too, though rarely, only one striking instance beingknown, an ancient coral reef which has become buried in strata mayafford rock of such hardness that when the river comes to cross it itforms a cascade, as at the Falls of the Ohio, at Louisville, Ky. It isa characteristic of all other falls, except those first mentioned, that they rarely plunge with a clean downward leap over the face of aprecipice which recedes at its base, but move downward over anirregular sloping surface. 

In the torrent district of rivers waterfalls are commonly verynumerous, and are generally due to the varying hardness in the rockswhich the streams encounter. Here, where the cutting action is goingon with great rapidity, slight differences in the resistance which therocks make to the work will lead to great variations in the form ofthe bed over which they flow, while on the more gently sloping bottomsof the rivers, where the _débris_ moves slowly, such variations wouldbe unimportant in their effect. When the torrents escape into the mainriver valleys, in regions where the great streams have cut deepgorges, they often descend from a great vertical height, formingwonderful waterfalls, such as those which occur in the famousLauterbrunnen Valley of Switzerland or in that of the Yosemite inCalifornia. This group of cascades is peculiar in that the steep ofthe fall is made not by the stream itself, but by the action of agreater river or of a glacier which may have some time taken itsplace. 

Waterfalls have an economic as well as a picturesque interest in thatthey afford sources of power which may be a very great advantage tomanufacturers. Thus along the Atlantic coast the streams which comefrom the Appalachian highlands, and which have hardly escaped fromtheir torrent section before they attain the sea, afford numerouscataracts which have been developed so that they afford a vast amountof power. Between the James on the south and the Ste. Croix on thenorth more than a hundred of these Appalachian rivers have been turnedto economic use. The industrial arts of this part of the countrydepend much upon them for the power which drives their machinery. Thewhole of the United States, because of the considerable size of itsrivers and their relatively rapid fall, is richly endowed with thissource of energy, which, originating in the sun's heat and conveyedthrough the rain, may be made to serve the needs of man. In view ofthe fact that recent inventions have made it possible to convert thisenergy of falling water into the form of electricity, which may beconveyed to great distances, it seems likely that our rivers will inthe future be a great source of national wealth. 

We must turn again to river valleys, there to trace certain actionsless evident than those already noted, but of great importance indetermining these features of the land. First, we have to note that inthe valley or region drained by a river there is another degrading ordown-wearing action than that which is accomplished by the direct workof the visible stream. All over such a valley the underground waters, soaking through the soil and penetrating through the underlying rock, are constantly removing a portion of the mineral matter which theytake into solution and bear away to the sea. In this way, deprived ofa part of their substance, the rocks are continually settling down byunderwear throughout the whole basin, while they are locally being cutdown by the action of the stream. Hence in part it comes about that ina river basin we find two contrasted features--the general and oftenslight slope of a country toward the main stream and its greatertributaries, and the sharp indentation of the gorge in which thestreams flow, these latter caused by the immediate and recent actionof the streams. 

If now the reader will conceive himself standing at any point in ariver basin, preferably beyond the realms of the torrents, he may withthe guidance of the facts previously noted, with a little use of theimagination, behold the vast perceptive which the history of the rivervalley may unfold to him. He stands on the surface of the soil, that_débris_ of the rocks which is just entering on its way to the ocean. In the same region ten thousand years ago he would have stood upon asurface from one to ten feet higher than the present soil covering. Amillion years ago his station would have been perhaps five hundredfeet higher than the surface. Ten million years in the past, a periodless than the lifetime of certain rivers, such as the French BroadRiver in North Carolina, the soil was probably five thousand feet ormore above its present plane. There are, indeed, cases where rivervalleys appear to have worked down without interruption from thesubsidence of the land beneath the sea to the depth of at least twomiles. Looking upward through the space which the rocks once occupied, we can conceive the action of the forces in their harmoniousco-operation which have brought the surface slowly downward. We canimagine the ceaseless corrosion due to the ground water, bringingabout a constant though slow descent of the whole surface. Again andagain the streams, swinging to and fro under the guidance of theunderlying rock, or from the obstacles which the _débris_ they carriedimposed upon them, have crossed the surface. Now and then perhaps thewearing was intensified by glacial action, for an ice sheet often cutswith a speed many times as great as that which fluid water canaccomplish. On the whole, this exercise of the constructiveimagination in conceiving the history of a river valley is one of themost enlarging tasks which the geologist can undertake. 

Where in a river valley there are many lateral streams, and especiallywhere the process of solution carried on by the underground waters ismost effective, as compared with erosive work done in the bed of themain river, we commonly find the valley sloping gently toward itscentre, the rivers having but slight steeps near their banks. On theother hand, where, as occasionally happens, a considerable stream fedby the rain and snow fall in its torrent section courses for a greatdistance over high, arid plains, on which the ground water and thetributaries do but little work, the basin may slope with very slightdeclivity to the river margins, and there descend to great depths, forming very deep gorges, of which the Colorado Cañon is the mostperfect type. As instances of these contrasted conditions, we maytake, on the one hand, the upper Mississippi, where the grades towardthe main stream are gentle and the valley gorge but slightlyexhibited; on the other, the above-mentioned Colorado, which bears agreat tide of waters drawn from the high and relatively rainy regionof the Rocky Mountains across the vast plateau lying in an almostrainless country. In this section nearly all the down-wearing has beenbrought about in the direct path of the stream, which has worn theelevated plain into a deep gorge during the slow uprising of thetable-land to its present height. In this way a defile nearly a milein depth has been created in a prevailingly rather flat country. Thisgorge has embranchments where the few great tributaries have done likework, but, on the whole, this river flows in an almost unbrokenchannel, the excavation of which has been due to its swift, pebble-bearing waters. 

The tendency of a newly formed river is to cut a more or less distinctcañon. As the basin becomes ancient, this element of the gorge tendsto disappear, the reason for this being that, while the river bed ishigh above the sea, the current is swift and the down-cutting rapid, while the slow subsidence of the country on either side--a processwhich goes on at a uniform rate--causes the surface of that region tobe left behind in the race for the sea level. As the stream bed comesnearer the sea level its rate of descent is diminished, and so theoutlying country gradually overtakes it. 

In regions where the winters are very cold the effect of ice on thedevelopment of the stream beds both in the torrent and river sectionsof the valley is important. This work is accomplished in severaldiverse ways. In the first place, where the stream is clear and thecurrent does not flow too swiftly, the stones on the bottom radiatetheir heat through the water, and thus form ice on their surfaces, which may attain considerable thickness. As ice is considerablylighter than water, the effect is often to lift up the stones of thebed if they be not too large; when thus detached from the bottom, theyare easily floated down stream until the ice melts away. The ice whichforms on the surface of the water likewise imprisons the pebbles alongthe banks, and during the subsequent thaw may carry them hundreds ofmiles toward the sea. It seems likely, from certain observations madeby the writer, that considerable stones may thus be carried from theAlleghany River to the main Mississippi. 

Perhaps the most important effect of ice on river channels isaccomplished when in a time of flood the ice field which covered thestream, perhaps to the depth of some feet, is broken up into vastfloes, which drift downward with the current. When, as on the Ohio, these fields sometimes have the area of several hundred acres, theyoften collide with the shores, especially where the stream makes asharp bend. Urged by their momentum, these ice floes pack into thesemblance of a dam, which may have a thickness of twenty, thirty, oreven fifty feet. Beginning on the shore, where the collision takesplace, the dam may swiftly develop clear across the stream, so that ina few minutes the way of the waters is completely blocked. Theon-coming ice shoots up upon the accumulation, increases its height, and extends it up stream, so that in an hour the mass completely barsthe current. The waters then heap up until they break their way overthe obstacle, washing its top away, until the whole is light enoughto be forced down the stream, where, by the friction it encounters onthe bottom and sides of the channel, it is broken to pieces. It iseasy to see that such moving dams of ice may sweep the bed of a riveras with a great broom. 

Sometimes where the gorges do not form a stationary dam large cakes ofice become turned on edge and pack together so that they roll down thestream like great wheels, grinding the bed rock as they go. 

In high northern countries, as in Siberia, the rivers, even thedeepest, often become so far frozen that their channels are entirelyobstructed. Where, as in the case of these Siberian rivers, the flowis from south to north, it often happens that the spring thaw sets inbefore the more northern beds of the main stream are released fromtheir bondage of frost. In this case the inundations have to find newpaths on either side of the obstructed way. The result is a type ofvalleys characterized by very irregular and changeable stream beds, the rivers having no chance to organize themselves into the shapelycurves which they ordinarily follow. 

The supply which finds its way to a river is composed, as has beenalready incidentally noted, in part of the water which coursesunderground for a greater or less distance before it emerges to thesurface, and in part of that which moves directly over the ground. These two shares of water have somewhat different histories. On theshare of these two depends the stability of the flow. Where, as in NewEngland and other glaciated countries, the surface of the earth iscovered with a thick layer of sand and gravel, which, except whenfrozen, readily admits the water; the rainfall is to a very greatextent absorbed by the earth, and only yielded slowly to the streams. In these cases floods are rare and of no great destructive power. Again, where also the river basin is covered by a dense mantle offorests, the ground beneath which is coated, as is the case inprimeval woods, with a layer of decomposing vegetation a foot or morein depth, this spongy mass retains the water even more effectivelythan the open-textured glacial deposits above referred to. When thewoods, however, are removed from such an area, the rain may descend tothe streams almost as speedily as it finds its way to the gutters fromthe house roofs. It thus comes about that all regions, when reduced totillage, and where the rainfall is enough to maintain a goodagriculture, are, except when they have a coating of glacial waste, exceedingly liable to destructive inundations. 

Unhappily, the risk of river floods is peculiarly great in all theregions of the United States lying much to the east of the RockyMountains, except in the basin of the Great Lakes and in the districtof New England, where the prevalence of glacial sands and gravelsaffords the protection which we have noted. Throughout this region therainfall is heavy, and the larger part of it is apt to come after theground has become deeply snow-covered. The result is a succession ofdevastating floods which already are very damaging to the works ofman, and promise to become more destructive as time goes on. More thanin any other country, we need the protection which forests can give usagainst these disastrous outgoings of our streams. 

 LAKES. 

In considering the journey of water from the hilltops to the sea, weshould take some account of those pauses which it makes on its waywhen for a time it falls into the basin of a lake. These arrests inthe downward motion of water, which we term lakes, are exceedinglynumerous; their proper discussion would, indeed, require aconsiderable volume. We shall here note only the more important oftheir features, those which are of interest to the general student. 

The first and most noteworthy difference in lakes is that whichseparates the group of dead seas from the living basins of freshwater. When a stream attains a place where its waters have to expandinto the lakelike form, the current moves in a slow manner, and thebroad surface exposed to the air permits a large amount ofevaporation. If the basin be large in proportion to the amount of theincurrent water, this evaporation may exceed the supply, and produce asea with no outlet, such as we find in the Dead Sea of Judea, in thatat Salt Lake, Utah, and in a host of other less important basins. Ifthe rate of evaporation be yet greater in proportion to the flow, thelake may altogether dry away, and the river be evaporated before itattains the basin where it might accumulate. In that case the river issaid to sink, but, in place of sinking into the earth, its watersreally rise into the air. Many such sinks occur in the central portionof the Rocky Mountain district. It is important to note that theprocess of evaporation we are describing takes place in the case ofall lakes, though only here and there is the air so dry that theevaporation prevents the basin from overflowing at the lowest point onits rim, forming a river which goes thence to the sea. Even in thecase of the Great Lakes of North America a considerable part of thewater which flows into them does not go to the St. Lawrence and thenceto the sea. As long as the lake finds an outlet to the sea its waterscontain but little more dissolved mineral matter than that we find inthe rivers. But because all water which has been in contact with theearth has some dissolved mineral substances, while that which goesaway by evaporation is pure water, a lake without an outlet graduallybecomes so charged with these materials that it can hold no more insolution, but proceeds to lay them down in deposits of that compoundsubstance which from its principal ingredient we name salt. The waterof dead seas, because of the additional weight of the substances whichit holds, is extraordinarily buoyant. The swimmer notes a differencein this regard in the waters of rivers and fresh-water lakes and thoseof the sea, due to this same cause. But in those of dead seas, saturated with saline materials, the human body can not sink as itdoes in the ordinary conditions of immersion. It is easy to understandhow the salt deposits which are mined in many parts of the world havegenerally, if not in all cases, been formed in such dead seas. [5]

[Footnote 5: In some relatively rare cases salt deposits are formed inlagoons along the shores of arid lands, where the sea occasionallybreaks over the beach into the basin, affording waters which areevaporated, leaving their salt behind them. ]

It is an interesting fact that almost all the known dead seas have inrecent geological times been living lakes--that is, they poured overtheir brims. In the Cordilleras from the line between Canada and theUnited States to central Mexico there are several of these basins. Allof those which have been studied show by their old shore lines thatthey were once brimful, and have only shrunk away in modern times. These conditions point to the conclusion that the rainfall indifferent regions varies greatly in the course of the geologic ages. Further confirmation of this is found in the fact that very great saltdeposits exist on the coast of Louisiana and in northernEurope--regions in which the rainfall is now so great in proportion tothe evaporation that dead seas are impossible. 

Turning now to the question of how lake basins are formed, we note agreat variety in the conditions which may bring about theirconstruction. The greatest agent, or at least that which operates inthe construction of the largest basins, are the irregular movements ofthe earth, due to the mountain-building forces. Where this work goeson on a large scale, basin-shaped depressions are inevitably formed. If all those which have existed remained, the large part of the landswould be covered by them. In most cases, however, the cutting actionof the streams has been sufficient to bring the drainage channels downto the bottom of the trough, while the influx of sediments has servedto further the work by filling up the cavities. Thus at the close ofthe Cretaceous period there was a chain of lakes extending along theeastern base of the Rocky Mountains, constituting fresh-water seasprobably as large as the so-called Great Lakes of North America. Butthe rivers, by cutting down and tilling up, have long sinceobliterated these water areas. In other cases the tiltings of thecontinent, which sometimes oppose the flow of the streams, may for atime convert the upper part of a river basin which originally slopedgently toward the sea into a cavity. Several cases of this descriptionoccurred in New England in the closing stages of the Glacial period, when the ground rose up to the northward. 

We have already noted the fact that the basin of a dead sea becomes incourse of time the seat of extensive salt deposits. These may, indeed, attain a thickness of many hundred feet. If now in the later historyof the country the tract of land with the salt beneath it weretraversed by a stream, its underground waters may dissolve out thesalt and in a way restore the basin to its original unfilledcondition, though in the second state that of a living lake. It seemsvery probable that a portion at least of the areas of Lakes Ontario, Erie, and Huron may be due to this removal of ancient salt deposits, remains of which lie buried in the earth in the region bordering thesebasins. 

By far the commonest cause of lake basins is found in theirregularities of the surface which are produced by the occupation ofthe country by glaciers. When these great sheets of ice lie over aland, they are in motion down the slopes on which they rest; they wearthe bed rocks in a vigorous manner, cutting them down in proportion totheir hardness. As these rocks generally vary in the resistance whichthey oppose to the ice, the result is that when the glacier passesaway the surface no longer exhibits the continued down slope which therivers develop, but is warped in a very complicated way. Thesedepressions afford natural basins in which lakes gather; they may varyin extent from a few square feet to many square miles. When a glacieroccupies a country, the melting ice deposits on the surface of theearth a vast quantity of rocky _débris_, which was contained in itsmass. This detritus is irregularly accumulated; in part it is disposedin the form of moraines or rude mounds made at the margin of theglacier, in part as an irregular sheet, now thick, now thin, whichcovers the whole of the field over which the ice lay. The result ofthis action is the formation of innumerable pools, which continue toexist until the streams have cut channels through which their watersmay drain away, or the basins have become filled with detritusimported from the surrounding country or by peat accumulations whichthe plants form in such places. 

Doubtless more than nine tenths of all the lake basins, especiallythose of small size, which exist in the world are due toirregularities of the land surface which are brought about by glacialaction. Although the greater part of these small basins have beenobliterated since the ice left this country, the number stillremaining of sufficient size to be marked on a good map isinconceivably great. In North America alone there are probably over ahundred and fifty thousand of these glacial lakes, although by far thegreater part of those which existed when the glacial sheet disappearedhave been obliterated. 

Yet another interesting group of fresh-water lakes, or rather weshould call them lakelets from their small size, owes its origin tothe curious underground excavations or caverns which are formed inlimestone countries. The water enters these caverns through what aretermed "sink holes"--basins in the surface which slope gently toward acentral opening through which the water flows into the depths below. The cups of the sink holes rarely exceed half a mile in diameter, andare usually much smaller. Their basins have been excavated by thesolvent and cutting actions of the rain water which gathers in them tobe discharged into the cavern below. It often happens that after asink hole is formed some slight accident closes the downward-leadingshaft, so that the basin holds water; thus in parts of the UnitedStates there are thousands of these nearly circular pools, which incertain districts, as in southern Kentucky, serve to vary thelandscape in much the same manner as the glacial lakes of morenorthern countries. 

Some of the most beautiful lakes in the world, though none more than afew miles in diameter, occupy the craters of extinct volcanoes. Whenfor a time, or permanently, a volcano ceases to do its appointed workof pouring forth steam and molten rock from the depths of the earth, the pit in the centre of the cone gathers the rain water, forming adeep circular lake, which is walled round by the precipitous faces ofthe crater. If the volcano reawakens, the water which blocks itspassage may be blown out in a moment, the discharge spreading in somecases to a great distance from the cone, to be accumulated again whenthe vent ceases to be open. The most beautiful of these volcanic lakesare to be found in the region to the north and south of Rome. Theoriginal seat of the Latin state was on the shores of one of thesecrater pools, south of the Eternal City. Lago Bolsena, which lies tothe northward, and is one of the largest known basins of this nature, having a diameter of about eight miles, is a crater lake. The volcaniccone to which it belongs, though low, is of great size, showing thatin its time of activity, which did not endure very long, this craterwas the seat of mighty ejections. The noblest specimen of this groupof basins is found in Crater Lake, Oregon, now contained in one of thenational parks of the United States. 

Inclosed bodies of water are formed in other ways than thosedescribed; the list above given includes all the important classes ofaction which produce these interesting features. We should now notethe fact that, unlike the seas, the lakes are to be regarded astemporary features in the physiography of the land. One and all, theyendure for but brief geologic time, for the reason that the streamswork to destroy them by filling them with sediment and by carving outchannels through which their waters drain away. The nature of thisaction can well be conceived by considering what will take place inthe course of time in the Great Lakes of North America. As NiagaraFalls cut back at the average rate of several feet a year, it will bebut a brief geologic period before they begin to lower the waters ofLake Erie. It is very probable, indeed, that in twenty thousand yearsthe waters of that basin will be to a great extent drained away. Whenthis occurs, another fall or rapid will be produced in the channelwhich leads from Lake Huron to Lake Erie. This in turn will go throughits process of retreat until the former expanse of waters disappears. The action will then be continued at the outlets of Lakes Michigan andSuperior, and in time, but for the interposition of some actions whichrecreate these basins, their floors will be converted into dry land. 

It is interesting to note that lakes owe in a manner the preservationof their basins to an action which they bring about on the waters thatflow into them. These rivers or torrents commonly convey greatquantities of sediment, which serve to rasp their beds and thus tolower their channels. In all but the smaller lakelets these turbidwaters lay down all their sediment before they attain the outlet ofthe basin. Thus they flow away over the rim rock in a perfectly purestate--a state in which, as we have noted before, water has nocapacity for abrading firm rock. Thus where the Niagara River passesfrom Lake Erie its clean water hardly affects the stone over which itflows. It only begins to do cutting work where it plunges down theprecipice of the Falls and sets in motion the fragments which areconstantly falling from that rocky face. These Falls could not havebegun as they did on the margin of Lake Ontario except for the factthat when the Niagara River began to flow, as in relatively moderntimes, it found an old precipice on the margin of Lake Ontario, formedby the waves of the lake, down which the waters fell, and where theyobtained cutting tools with which to undermine the steep which formsthe Falls. 

Many great lakes, particularly those which we have just beenconsidering, have repeatedly changed their outlets, according as thesurface of the land on which they lie has swayed up and down invarious directions, or as glacial sheets have barred or unbarred theoriginal outlets of the basins. Thus in the Laurentian Lakes aboveOntario the geologist finds evidence that the drainage lines haveagain and again been changed. For a time during the Glacial period, when Lake Ontario and the valley of the St. Lawrence was possessed bythe ice, the discharge was southward into the upper Mississippi or theOhio. At a later stage channels were formed leading from Georgian Bayto the eastern part of Ontario. Yet later, when the last-named lakewas bared, an ice dam appears to have remained in the St. Lawrence, which held back the waters to such a height that they dischargedthrough the valley of the Mohawk into the Hudson. Furthermore, at sometime before the Glacial period, we do not know just when, thereappears to have been an old Niagara River, now filled with drift, which ran from Lake Erie to Ontario, a different channel from thatoccupied by the present stream. 

The effects of lakes on the river systems with which they areconnected is in many ways most important. Where they are ofconsiderable extent, or where even small they are very numerous, theyserve to retain the flood waters, delivering them slowly to theexcurrent streams. In rising one foot a lake may store away more waterthan the river by its consequent rise at the point of outflow willcarry away in many months, and this for the simple reason that thelake may be many hundred or even thousand times as wide as the stream. Moreover, as before noted, the sediment gathered by the stream abovethe level of the lake is deposited in its basin, and does not affectthe lower reaches of the river. The result is that great rivers, suchas drain from the Laurentian Lakes, flow clear water, are exempt fromfloods, are essentially without alluvial plains or terraces, and formno delta deposits. In all these features the St. Lawrence Riveraffords a wonderful contrast to the Mississippi. Moreover, owing tothe clear waters, though it has flowed for a long time, it has neverbeen able to cut away the slight obstructions which form its rapids, barriers which probably would have been removed if its waters had beencharged with sediment. 

[Illustration: _Muir Glacier, Alaska, showing crevasses and dustlayer on surface of ice. _]

 CHAPTER VI. 

 GLACIERS. 

We have already noted the fact that the water in the clouds is verycommonly in the frozen state; a large part of that fluid which isevaporated from the sea attains the solid form before it returns tothe earth. Nevertheless, in descending, at least nine tenths of theprecipitation returns to the fluid state, and does the kind of workwhich we have noted in our account of water. Where, however, the waterarrives on the earth in the frozen condition, it enters on a rôletotally different from that followed by the fluid material. 

Beginning its descent to the earth in a snowflake, the little massfalls slowly, so that when it comes against the earth the blow whichit strikes is so slight that it does no effective work. In the stateof snow, even in the separate flakes, the frozen water contains arelatively large amount of air. It is this air indeed, which, bydividing the ice into many flakes that reflect the light, gives it thewhite colour. This important point can be demonstrated by breakingtransparent ice into small bits, when we perceive that it has the hueof snow. Much the same effect is given where glass is powdered, andfor the same reason. 

As the snowflakes accumulate layer on layer they imbed air betweenthem, so that when the material falls in a feathery shape--say to thedepth of a foot--more than nine tenths of the mass is taken up by theair-containing spaces. As these cells are very small, the circulationin them is slight, and so the layer becomes an admirablenon-conductor, having this quality for the same reason that feathershave it--i. E. , because the cells are small enough to prevent thecirculation of the air, so that the heat which passes has to go byconduction, and all gases are very poor conductors. The result is thata snow coating is in effect an admirable blanket. When the sun shinesupon it, much of the heat is reflected, and as the temperature doesnot penetrate it to any depth, only the superficial part is melted. This molten water takes up in the process of melting a great deal ofheat, so that when it trickles down into the mass it readilyrefreezes. On the other hand, the heat going out from the earth, thestore accumulated in its superficial parts in the last warm season, together with the small share which flows out from the earth'sinterior, is held in by this blanket, which it melts but slowly. Thusit comes about that in regions of long-enduring snowfall the ground, though frozen to the depth of a foot or more at the time when theaccumulation took place, may be thawed out and so far warmed that thevegetation begins to grow before the protecting envelope of snow hasmelted away. Certain of the early flowers of high latitudes, indeed, begin to blossom beneath the mantle of finely divided ice. 

In those parts of the earth which for the most part receive only atemporary coating of snow the effect of this covering isinconsiderable. The snow water is yielded to the earth, from which ithas helped to withdraw the frost, so that in the springtime, thegrowing season of plants, the ground contains an ample store ofmoisture for their development. Where the snowfall accumulates to agreat thickness, especially where it lodges in forests, the influenceof the icy covering is somewhat to protract the winter and thus toabbreviate the growing season. 

Where snow rests upon a steep slope, and gathers to the depth ofseveral feet, it begins to creep slowly down the declivity in a mannerwhich we may often note on house roofs. This motion is favoured by thegradual though incomplete melting of the flakes as the heatpenetrates the mass. Making a section through a mass of snow which hasaccumulated in many successive falls, we note that the top may stillhave the flaky character, but that as we go down the flakes arereplaced by adherent shotlike bodies, which have arisen from thepartial melting and gathering to their centres of the originalexpanded crystalline bits. In this process of change the mass can moveparticle by particle in the direction in which gravity impels it. Theenergy of its motion, however, is slight, yet it can urge loose stonesand forest waste down hill. Sometimes, as in the cemetery at Augusta, Me. , where stone monuments or other structures, such as iron railings, are entangled in the moving mass, it may break them off and conveythem a little distance down the slope. 

So long as the summer sun melts the winter's snow, even if the groundbe bare but for a day, the rôle of action accomplished by the snowfallis of little geological consequence. When it happens that a portion ofthe deposit holds through the summer, the region enters on the glacialstate, and its conditions undergo a great revolution, the consequencesof which are so momentous that we shall have to trace them in somedetail. Fortunately, the considerations which are necessary are notrecondite, and all the facts are of an extremely picturesque nature. 

Taking such a region as New England, where all the earth islife-bearing in the summer season, and where the glacial period of thewinter continues but for a short time, we find that here and there onthe high mountains the snow endures throughout most of the summer, butthat all parts of the surface have a season when life springs intoactivity. On the top of Mount Washington, in the White Mountains ofNew Hampshire, in a cleft known as Tuckerman's Ravine, where thedeposit accumulates to a great depth, the snow-ice remains untilmidsummer. It is, indeed, evident that a very slight change in theclimatal conditions of this locality would establish a permanentaccumulation of frozen water upon the summit of the mountain. If thecrest were lifted a thousand feet higher, without any general changein the heat or rainfall of the district, this effect would beproduced. If with the same amount of rainfall as now comes to theearth in that region more of it fell as snow, a like condition wouldbe established. Furthermore, with an increase of rainfall to somethinglike double that which now descends the snow bore the same proportionto the precipitation which it does at present, we should almostcertainly have the peak above the permanent snow line, that levelbelow which all the winter's fall melts away. These propositions arestated with some care, for the reason that the student should perceivehow delicate may be--indeed, commonly is--the balance of forces whichmake the difference between a seasonal and a perennial snow covering. 

As soon as the snow outlasts the summer, the region which it occupiesis sterilized to life. From the time the snow begins to hold over thewarm period until it finally disappears, that field has to be reckonedout of the habitable earth, not only to man, but to the lowliestorganisms. [6]

[Footnote 6: In certain fields of permanent snow, particularly near theirboundaries, some very lowly forms of vegetable life may develop on afrozen surface, drawing their sustenance from the air, and supplied withwater by the melting which takes place during the summertime. Theseforms include the rare phenomenon termed red snow. ]

If the snow in a glaciated region lay where it fell, the result wouldbe a constant elevation of the deposit year by year in proportion tothe annual excess of deposition over the melting or evaporation of thematerial. But no sooner does the deposit attain any considerablethickness than it begins to move in the directions of leastresistance, in accordance with laws which the students of glaciers arejust beginning to discern. In small part this motion is accomplishedby avalanches or snow slides, phenomena which are in a way important, and therefore merit description. Immediately after a heavy snowfall, in regions where the slopes are steep, it often happens that thedeposit which at first clung to the surface on which it lay becomes soheavy that it tends to slide down the slope; a trifling action, theslipping, indeed, of a single flake, may begin the movement, which atfirst is gradual and only involves a little of the snow. Gatheringvelocity, and with the materials heaped together from the junction ofthat already in motion with that about to be moved, the avalanche insliding a few hundred feet down the slope may become a deep stream ofsnow-ice, moving with great celerity. At this stage it begins to breakoff masses of ice from the glaciers over which it may flow, or even tomove large stones. Armed with these, it rends the underlying earth. After it has flowed a mile it may have taken up so much earth andmaterial that it appears like a river of mud. Owing to the fact thatthe energy which bears it downward is through friction converted intoheat, a partial melting of the mass may take place, which converts itinto what we call slush, or a mixture of snow and water. Finally, thetorrent is precipitated into the bottom of a valley, where in time thefrozen water melts away, leaving only the stony matter which it boreas a monument to show the termination of its flow. 

It was the good fortune of the writer to see in the Swiss Oberland onevery great avalanche, which came from the high country through adescent of several thousand feet to the surface of the UpperGrindelwald Glacier. The first sign of the action was a vague tremorof the air, like that of a great organ pipe when it begins to vibrate, but before the pulsations come swiftly enough to make an audible note. It was impossible to tell when this tremor came, but the wary guide, noting it before his charge could perceive anything unusual, madehaste for the middle of the glacier. The vibration swelled to a roar, but the seat of the sound amid the echoing cliffs was indeterminable. Finally, from a valley high up on the southern face of the glacier, there leaped forth first a great stone, which sprang with successiverebounds to the floor of ice. Then in succession other stones andmasses of ice which had outrun the flood came thicker and thicker, until at the end of about thirty seconds the steep front of theavalanche appeared like a swift-moving wall. Attaining the cliffs, itshot forth as a great cataract, which during the continuance of theflow--which lasted for several minutes--heaped a great mound ofcommingled stones and ice upon the surface of the glacier. The massthus brought down the steep was estimated at about three thousandcubic yards, of which probably the fiftieth part was rock material. Anavalanche of this volume is unusual, and the proportion of stonymatter borne down exceptionally great; but by these sudden motions ofthe frozen water a large part of the snow deposited above the zone ofcomplete melting is taken to the lower valleys, where it may disappearin the summer season, and much of the erosion accomplished in themountains is brought about by these falls. 

In all Alpine regions avalanches are among the most dreaded accidents. Their occurrence, however, being dependent upon the shape of thesurface, it is generally possible to determine in an accurate way theliability of their happening in any particular field. The Swiss takeprecaution to protect themselves from their ravages as other folk doto procure immunity from floods. Thus the authorities of many of themountain hamlets maintain extensive forests on the sides of thevillages whence the downfall may be expected, experience having shownthat there is no other means so well calculated to break the blowwhich these great snowfalls can deliver, as thick-set trees which, though they are broken down for some distance, gradually arrest thestream. 

As long as the region occupied by permanent snow is limited to sharpmountain peaks, relief by the precipitation of large masses to thelevel below the snow line is easily accomplished, but manifestly thiskind of a discharge can only be effective from a very small field. Where the relief is not brought about by these tumbles of snow, another mode of gravitative action accomplishes the result, though ina more roundabout way, through the mechanism of glaciers. 

We have already noted the fact that the winter's snow upon ourhillsides undergoes a movement in the direction of the slope. What wehave now to describe in a rather long story concerning glaciers restsupon movements of the same nature, though they are in certain featurespeculiarly dependent on the continuity of the action from year toyear. It is desirable, however, that the student should see that thereis at the foundation no more mystery in glacial motion than there isin the gradual descent of the snow after it has lain a week on ahillside. It is only in the scale and continuity of the action thatthe greatest glacial envelope exceeds those of our temporarywinters--in fact, whenever the snow falls the earth it covers entersupon an ice period which differs only in degree from that from whichour hemisphere is just escaping. 

Where the reader is so fortunate as to be able to visit a region ofglaciers, he had best begin his study of their majestic phenomena byascending to those upper realms where the snow accumulates from yearto year. He will there find the natural irregularities of the rocksurface in a measure evened over by a vast sheet of snow, from whichonly the summits of the greater mountains rise. He may soon satisfyhimself that this sheet is of great depth, for here and there it isintersected by profound crevices. If the visit is made in the seasonwhen snow falls, which is commonly during most of the year, he mayobserve, as before noted in our winter's snow, that the deposit, though at first flaky, attains at a short distance below the surface asomewhat granular character, though the shotlike grains fall apartwhen disturbed. Yet deeper, ordinarily a few feet below the surface, these granules are more or less cemented together; the mass thus losesthe quality of snow, and begins to appear like a whitish ice. Lookingdown one of the crevices, where the light penetrates to the depth of ahundred feet or more, he may see that the bluish hue somewhatincreases with the depth. A trace of this colour is often visible evenin the surface snow on the glacier, and sometimes also in our ordinarywinter fields. In a hole made with a stick a foot or more in depth afaint cerulean glimmer may generally be discerned; but the increasedblueness of the ice as we go down is conspicuous, and readily leads usto the conclusion that the air, to which, as we before noted, thewhiteness of the snow is due, is working out of the mass as theprocess of compaction goes on. In a glacial district this snow massabove the melting line is called the _névé_. 

Remembering that the excess of snow beyond the melting in a _névé_district amounts, it may be, to some feet of material each year, weeasily come to the conclusion that the mass works down the slope inthe manner which it does even where the coating is impermanent. Thissupposition is easily confirmed: by observing the field we find thatthe sheet is everywhere drawing away from the cliffs, leaving a deepfissure between the _névé_ and the precipices. This crevice is calledby the German-Swiss guides the _Bergschrund_. Passage over it isoften one of the most difficult feats to accomplish which the Alpineexplorer has to undertake. In fact, the very appearance of thesurface, which is that of a river with continuous down slopes, issufficient evidence that the mass is slowly flowing toward thevalleys. Following it down, we almost always come to a place where itpasses from the upper valleys to the deeper gorges which pierce theskirts of the mountain. In going over this projection the mass ofsnow-ice breaks to pieces, forming a crowd of blocks which march downthe slope with much more speed than they journeyed when united in thehigher-lying fields. In this condition and in this part of themovement the snow-ice forms what are called the _seracs_, or curds, asthe word means in the French-Swiss dialect. Slipping and tumblingdown the steep slope on which the _seracs_ develop, the ice becomesbroken into bits, often of small size. These fragments are quicklyreknit into the body of ice, which we shall hereafter term theglacier, and in this process the expulsion of the air goes on morerapidly than before, and the mass assumes a more transparent icelikequality. 

The action of the ice in the pressures and strains to which it issubjected in joining the main glacier and in the further part of itscourse demand for their understanding a revision of those notions asto rigidity and plasticity which we derive from our common experiencewith objects. It is hard to believe that ice can be moulded bypressure into any shape without fracturing, provided the motion isslowly effected, while at the same time it is as brittle as ice to asudden blow. We see, however, a similar instance of contrastedproperties in the confection known as molasses candy, a stick of whichmay be indefinitely bent if the flexure is slowly made, but will flyto pieces like glass if sharply struck. Ice differs from the sugarysubstance in many ways; especially we should note that while it may besqueezed into any form, it can not be drawn out, but fractures on theapplication of a very slight tension. The conditions of its movementwe will inquire into further on, when we have seen more of its action. 

Entering on the lower part of its course, that where it flows into theregion below the snow line, the ice stream is now confined between thewalls of the valley, a channel which in most cases has been shapedbefore the ice time, by a mountain torrent, or perhaps by a slowerflowing river. In this part of its course the likeness of a glacialstream to one of fluid water is manifest. We see that it twists withthe turn of the gorge, widens where the confining walls are far apart, and narrows where the space is constricted. Although the surface ishere and there broken by fractures, it is evident that the movement ofthe frozen current, though slow, is tolerably free. By placing stakesin a row across the axis of a glacier, and observing their movementfrom day to day, or even from hour to hour if a good theodolite isused for the purpose, we note that the movement of the stream isfastest in the middle parts, as in the case of a river, and that itslows toward either shore, though it often happens, as in a stream ofmolten water, that the speediest part of the current is near one side. Further observations have indicated that the movement is most rapid onthe surface and least at the bottom, in which the stream is alsoriverlike. It is evident, in a word, that though the ice is not fluidin strict sense, the bits of which it is made up move in substantiallythe manner of fluids--that is, they freely slip over each other. Wewill now turn our attention to some important features of a detailedsort which glaciers exhibit. 

If we visit a glacier during the part of the year when the wintersnows are upon it, it may appear to have a very uninterrupted surface. But as the summer heat advances, the mask of the winter coating goesaway, and we may then see the structure of the ice. First of all wenote in all valley glaciers such as we are observing that the streamis overlaid by a quantity of rocky waste, the greater part of whichhas come down with the avalanches in the manner before described, though a small part may have been worn from the bed over which the iceflows. In many glaciers, particularly as we approach theirtermination, this sheet of earth and rock materials often covers theice so completely that the novice in such regions finds it difficultto believe that the ice is under his feet. If the explorer is mindedto take the rough scramble, he can often walk for miles on thesemasses of stone without seeing, much less setting foot on any frozenwater. In some of the Alaskan glaciers this coating may bear a forestgrowth. In general, this material, which is called moraine, isdistributed in bands parallel to the sides of the glaciers, and thestrips may amount to a half dozen or more. Those on the sides of theice have evidently been derived from the precipices which they havepassed. Those in the middle have arisen from the union of the morainesformed in two or more tributary valleys. 

[Illustration: Fig. 12. --Map of glaciers and moraines near Mont Blanc. ]

Where the avalanches fall most plentifully, the stones lie buried withthe snow, and only melt out when the stream attains the region wherethe annual waste of its surface exceeds the snowfall. In this sectionwe can see how the progressive melting gradually brings the rocky_débris_ into plain view. Here and there we will find a boulderperched on a pedestal of ice, which indicates a recent down-wearing ofthe field. A frequent sound in these regions arises from the tumble ofthe stones from their pedestals or the slipping of the masses from thesharp ridge which is formed by the protection given to the ice throughthe thick coating of detritus on its surface. These movements of themoraines often distribute their waste over the glacier, so that in itslower part we can no longer trace the contributions from the severalvalleys, the whole area being covered by the _débris_. At the end ofthe ice stream, where its forward motion is finally overcome by thewarmth which it encounters, it leaves in a rude heap, extending oftenlike a wall across the valley, all the coarse fragments which itconveys. This accumulation, composed of all the lateral moraines whichhave gathered on the ice by the fall of avalanches, is called theterminal moraine. As the ice stream itself shrinks, a portion of thedetritus next the boundary wall is apt to be left clinging againstthose slopes. It is from the presence of these heaps in valleys nowabandoned by glaciers that we obtain some information as to the formergreater extent of glacial action. 

The next most noticeable feature is the crevasse. These fracturesoften exist in very great numbers, and constitute a formidable barrierin the explorer's way. The greater part of these ruptures below the_serac_ zone run from the sides of the stream toward the centrewithout attaining that region. These are commonly pointed up stream;their formation is due to the fact that, owing to the swifter motionin the central parts of the stream, the ice in that section draws awayfrom the material which is moving more slowly next the shore. Asbefore noted, these ice fractures when drawn out naturally formfissures at right angles to the direction of the strain. In the middleportions of the ice other fissures form, though more rarely, whichappear to depend on local strains brought about through theirregularity of the surface over which the ice is flowing. 

If the observer is fortunate, he may in his journey over the glacierhave a chance to see and hear what goes on when crevasses are formed. First he will hear a deep, booming sound beneath his feet, whichmerges into a more splintering note as the crevice, which begins atthe bottom or in the distance, comes upward or toward him. When thesound is over, he may not be able to see a trace of the fracture, which at first is very narrow. But if the break intersect any of thenumerous shallow pools which in a warm summer's day are apt to cover alarge part of the surface, he may note a line of bubbles rushing upthrough the water, marking the escape of the air from the glacier, some remnant of that which is imprisoned in the original snow. Evenwhere this indication is wanting, he can sometimes trace the creviceby the hissing sound of the air streams where they issue from the ice. If he will take time to note what goes on, he can usually in an houror two behold the first invisible crack widen until it may be half aninch across. He may see how the surface water hastens down theopening, a little river system being developed on the surface of theice as the streams make their way to one or more points of descent. Indoing this work they excavate a shaft which often becomes many feet indiameter, down which their waters thunder to the base of the glacier. This well-like opening is called a _moulin_, or mill, a name which, aswe shall see, is well deserved from the work which falling watersaccomplish. Although the institution of the _moulin_ shaft dependsupon the formation of a crevice, it often happens that as the icemoves farther on its journey its walls are again thrust together, soldered in the manner peculiar to ice, so that no trace of therupture remains except the shaft which it permitted to form. Likeeverything else in the glacier, the _moulin_ slowly moves down theslope, and remains open as long as it is the seat of descending watersproduced by the summer melting. When it ceases to be kept open fromthe summer, its walls are squeezed together in the fashion that thecrevices are closed. 

Forming here and there, and generally in considerable numbers, thecrevices of a glacier entrap a good deal of the morainal _débris_, which falls through them to the bottom of the glacier. Smaller bitsare washed into the _moulin_, by the streams arising from the meltingice, which is brought about by the warm sun of the summer, andparticularly by the warm rains of that season. On those glacierswhere, owing to the irregularity of the bottom over which the iceflows, these fractures are very numerous, it may happen that all thedetritus brought upon the surface of the glacier by avalanches findsits way to the floor of the ice. 

Although it is difficult to learn what is going on at the undersurface of the glacier, it is possible directly and indirectly toascertain much concerning the peculiar and important work which isthere done. The intrepid explorer may work his way in through thelateral fissures, and even with care safely descend some of thefissures which penetrate the central parts of a shallow ice stream. There, it may be at the depth of a hundred feet or more, he will finda quantity of stones, some of which may be in size like to a smallhouse held in the body of the ice, but with one side resting upon thebed rock. He may be so fortunate as to see the stone actually inprocess of cutting a groove in the bed rock as it is urged forward bythe motion of the glacier. The cutting is not altogether in the fixedmaterial, for the boulder itself is also worn and scored in the work. Smaller pebbles are caught in the space between the erratic and themotionless rock and ground to bits. If in his explorations the studentfinds his way to the part of the floor on which the waters of a_moulin_ fall, he may have a chance to observe how the stones set inmotion serve to cut the bed rock, forming elongated potholes much asin the case of ordinary waterfalls, or at the base of those shaftswhich afford the beginnings of limestone caverns. 

The best way to penetrate beneath the glacier is through the arch ofthe stream which always flows from the terminal face of the ice river. Even in winter time every large glacier discharges at its end aconsiderable brook, the waters of which have been melted from the icein small part by the outflow of the earth's heat; mainly, however, bythe warmth produced in the friction of the ice on itself and on itsbottom--in other words, by the conversion of that energy of position, of which we have often to speak, into heat. In the summer time thissubglacial stream is swollen by the surface waters descending throughthe crevices and the _moulins_ which come from them, so that theoutflow often forms a considerable river, and thus excavates in theice a large or at least a long cavern, the base of which is the bedrock. In the autumn, when the superficial melting ceases, this gallerycan often be penetrated for a considerable distance, and affords anexcellent way to the secrets of the under ice. The observer may heresee quantities of the rock material held in the grip of the ice, andforced to a rude journey over the bare foundation stones. Now and thenhe may find the glacial mass in large measure made up of stones, theadmixture extending many feet above the bottom of the cavern, perhapsto the very top of the arch. He may perchance find that these stonesare crushing each other where they are in contact. The result will bebrought about by the difference in the rate of advance of the ice, which moves the faster the higher it is above the surface over whichit drags, and thus forces the stones on one level over those below. Where the waters of the subglacial stream have swept the bed rockclean of _débris_ its surface is scored, grooved, and here and therepolished in a manner which is accomplished only by ice action, thoughsome likeness to it is afforded where stones have been swept over forages by blowing sand. Here and there, often in a way which interruptsthe cavern journey, the shrunken stream, unable to carry forward the_débris_, deposits the material in the chamber, sometimes filling thearch so completely that the waters are forced to make a detour. Thisaction is particularly interesting, for the reason that in regionswhence glaciers have disappeared the deposits formed in the old icearches often afford singularly perfect moulds of those caverns whichwere produced by the ancient subglacial streams. These moulds aretermed _eskers_. 

If the observer be attentive, he will note the fact that the watersemerging from beneath the considerable glacier are very much chargedwith mud. If he will take a glass of the water at the point of escape, he will often find, on permitting it to settle, that the sedimentamounts to as much as one twentieth of the volume. While the greaterpart of this detritus will descend to the bottom of the vessel in thecourse of a day, a portion of it does not thus fall. He may also notethat this mud is not of the yellowish hue which he is accustomed tobehold in the materials laid down by ordinary rivers, but has awhitish colour. Further study will reveal the fact that the differenceis due to the lack of oxidation in the case of the glacial detritus. River muds forming slowly and during long-continued exposure to theaction of the air have their contained iron much oxidized, which givesthem a part of their darkened appearance. Moreover, they are somewhatcoloured with decayed vegetable matter. The waste from beneath theglacier has been quickly separated from the bed rock, all the faces ofthe grains are freshly fractured, and there is no admixture of organicmatter. The faces of the particles thus reflect light in substantiallythe same way as powdered glass or pulverized ice, and consequentlyappear white. 

A little observation will show the student that this very muddycharacter of waters emerging from beneath the glacier is essentiallypeculiar to such streams as we have described. Ascending any of theprincipal valleys of Switzerland, he may note that some of the streamsflow waters which carry little sediment even in times when they aremuch swollen, while others at all seasons have the whitish colour. Alittle further exploration, or the use of a good map, will show himthat the pellucid streams receive no contributions of glacial water, while those which look as if they were charged with milk come, in partat least, from the ice arches. From some studies which the writer hasmade in Swiss valleys, it appears that the amount of erosionaccomplished on equal areas of similar rock by the descent of thewaters in the form of a glacier or in that of ordinary torrentsdiffers greatly. Moving in the form of ice, or in the state ofice-confined streams, the mass of water applies very many times asmuch of its energy of position to grinding and bearing away the rocksas is accomplished where the water descends in its fluid state. 

The effect of the intense ice action above noted is rapidly to wearaway the rocks of the valley in which the glacier is situated. Thiswork is done not only in a larger measure but in a different way fromthat accomplished by torrents. In the case of the latter, the streambed is embarrassed by the rubbish which comes into it; only here andthere can it attack the bed rock by forcing the stones over itssurface. Only in a few days of heavy rain each year is its work at alleffective; the greater part of the energy of position of its waters isexpended in the endless twistings and turnings of its stream, whichresult only in the development of heat which flies away into theatmosphere. In the ice stream, owing to its slow movement and to thedetritus which it forces along the bottom, a vastly greater part ofthe energy which impels it down the slope is applied to rock cutting. None of the boulders, even if they are yards in diameter, obstruct itsmotion; small and great alike are to it good instruments wherewith toattack the bed rocks. The fragments are never left to waste byatmospheric decay, but are to a very great extent used up inmechanical work, while the most of the detritus which comes to atorrent is left in a coarse state when it is delivered to the stream;the larger part of that which the glacier transports is worn out inits journey. To a great extent it is used up in attacking the bedrock. In most cases the _débris_ in the terminal moraine is evidentlybut a small part of what entered the ice during its journey from theuplands; the greater part has been worn out in the rude experiences towhich it has been subjected. 

It is evident that even in the regions now most extensively occupiedby glaciers the drainage systems have been shaped by the movement ofordinary streams--in other words, ice action is almost everywhere, even in the regions about the poles, an incidental feature in the workof water, coming in only to modify the topography, which is mainlymoulded by the action of fluid water. When, owing to climatal changes, a valley such as those of the Alps is occupied by a glacial stream, the new current proceeds at once, according to its evident needs, tomodify the shape of its channel. An ordinary torrent, because of theswiftness of its motion, which may, in general, be estimated at fromthree to five miles an hour, can convey away the precipitation over avery narrow bed. Therefore its channel is usually not a hundredth partas wide as the gorge or valley in which it lies. But when thedischarge takes place by a glacier, the speed of which rarely exceedsfour or five feet a day, the ice stream because of its slow motion hasto fill the trough from side to side, it has to be some thousand timesas deep and wide as the torrent. The result is that as soon as theglacial condition arises in a country the ice streams proceed tochange the old V-shaped torrent beds into those which have a broadU-like form. The practised eye can in a way judge how long a valleyhas been subjected to glacial action by the extent to which it hasbeen widened by this process. 

In the valleys of Switzerland and other mountain districts which havebeen attentively studied it is evident that glacial action has playeda considerable part in determining their forms. But the work has beenlimited to that part of the basin in which the ice is abundantlyprovided with cutting tools in the stone which have found their way tothe base of the stream. In the region of the _névé_, where thecontributions of rocky matter to the surface of the deposit made fromthe few bare cliffs which rise above the sheet of snow is small, thesnow-ice does no cutting of any consequence. Where it passes over thesteep at the head of the deep valley into which it drains, and isriven into the _seracs_, such stony matter as it may have gathered isallowed to fall to the bottom, and so comes into a position where itmay do effective work. From this _serac_ section downward the nowdistinct ice river, being in general below the snow line, haseverywhere cliffs, on either side from which the contributions of rockmaterial are abundant. Hence this part of the glacier, though it isthe wasting portion of its length, does all the cutting work of anyconsequence which is performed. It is there that the underrunningstreams become charged with sediment, which, as we have noted, theybear in surprising quantities, and it is therefore in this section ofthe valley that the impress of the ice work is the strongest. Itseffect is not only to widen the valley and deepen it, but also toadvance the deep section farther up the stream and its tributaries. The step in the stream beds which we find at the _seracs_ appears tomark the point in the course of the glacier where, owing to thefalling of stones to its base, as well as to its swifter movements andthe firmer state of the ice, it does effective wearing. 

There are many other features connected with glaciers which richlyrepay the study of those who have a mind to explore in the manner ofthe physicist interested in ice actions the difficult problems whichthey afford; but as these matters are not important from the point ofview of this work, no mention of them will here be made. We will nowturn our attention to that other group of glaciers commonly termedcontinental, which now exist about either pole, and which at varioustimes in the earth's history have extended far toward the equator, mantling over vast extents of land and shallow sea. The differencebetween the ice streams of the mountains and those which we termcontinental depends solely on the areas of the fields and the depth ofthe accumulation. In an ordinary Alpine region the _névé_ districts, where the snow gathers, are relatively small. Owing to the rathersteep slopes, the frozen water is rapidly discharged into the lowervalleys, where it melts away. Both in the _névé_ and in the distinctglacier of the lower grounds there are, particularly in the latter, projecting peaks, from which quantities of stone are brought down byavalanches or in ordinary rock falls, so that the ice is abundantlysupplied with cutting tools, which work from its surface down to itsdepths. 

As the glacial accumulation grows in depth there are fewer peaksemerging from it, and the streams which it feeds rise the higher untilthey mantle over the divides between the valleys. Thus byimperceptible stages valley glaciers pass to the larger form, usuallybut incorrectly termed continental. We can, indeed, in going from themountains in the tropics to the poles, note every step in thistransition, until in Greenland we attain the greatest ice mass in theworld, unless that about the southern pole be more extensive. In theGreenland glacier the ice sheet covers a vast extent of what isprobably a mountain country, which is certainly of this nature in thesouthern part of the island, where alone we find portions of the earthnot completely covered by the deep envelope. Thanks to the labours ofcertain hardy explorers, among whom Nansen deserves the foremostplace, we now know something as to the conditions of this vast icefield, for it has been crossed from shore to shore. The results ofthese studies are most interesting, for they afford us a clew as tothe conditions which prevail over a large part of the earth during theGlacial period from which the planet is just escaping, and in theearlier ages when glaciation was likewise extensive. We shalltherefore consider in a somewhat detailed way the features which theGreenland glacier presents. 

Starting from the eastern shore of that land, if we may thus term aregion which presents itself mainly in the form of ice, we find nextthe shore a coast line not completely covered with ice and snow, buthere and there exhibiting peaks which indicate that if the frozenmantle were removed the country would appear deeply intersected withfiords in the manner exhibited in the regions to the south ofGreenland or the Scandinavian peninsula. The ice comes down to thesea through the valleys, often facing the ocean for great distanceswith its frozen cliffs. Entering on this seaward portion of theglacier, the observer finds that for some distance from the coast linethe ice is more or less rifted with crevices, the formation of whichis doubtless due to irregularities of the rock bottom over which itmoves. These ruptures are so frequent that for some miles back it isvery difficult to find a safe way. Finally, however, a point isattained where these breaks rather suddenly disappear, and thenceinward the ice rises at the rate of upward slope of a few feet to themile in a broad, nearly smooth incline. In the central portion of theregion for a considerable part of the territory the ice has verylittle slope. Thence it declines toward the other shore, exhibitingthe same features as were found on the eastern versant until near thecoast, when again the surface is beset with crevices which continue tothe margin of the sea. 

Although the explorations of the central field of Greenland are as yetincomplete, several of these excursions into or across the interiorhave been made, and the identity of the observations is such that wecan safely assume the whole region to be of one type. We canfurthermore run no risk in assuming that what we find in Greenland, atleast so far as the unbroken nature of the central ice field isconcerned, is what must exist in every land where the glacial envelopebecomes very deep. In Greenland it seems likely that the depth of theice is on the average more than half a mile, and in the central partof the realm the sheet may well have a much greater profundity; it maybe nearly a mile deep. The most striking feature--that of a vastunbroken expanse, bordered by a region where the ice is ruptured--istraceable wherever very extensive and presumably deep deposits of icehave been examined. As we shall see hereafter, these features teach usmuch as to the conditions of glacial action--a matter which we shallhave to examine after we have completed our general survey as to thechanges which occur during glacial periods. 

In the present state of that wonderful complex of actions which weterm climate, glaciers are everywhere, so far as our observationsenable us to judge, generally in process of decrease. In Switzerland, although the ancients even in Roman days were in contact with the ice, they were so unobservant that they did not even remark that the icewas in motion. Only during the last two centuries have we anyobservations of a historic sort which are of value to the geologist. Fortunately, however, the signs written on the rock tell the story, except for its measurement in terms of years, as clearly as anyrecords could give it. From this testimony of the rocks we perceivethat in the geological yesterday, though it may have been some tens ofthousands of years ago, the Swiss glaciers, vastly thickened, and withtheir horizontal area immensely expanded, stretched over the Alpinecountry, so that only here and there did any of the sharper peaks riseabove the surface. These vast glaciers, almost continually united ontheir margins, extended so far that every portion of what is now theSwiss Republic was covered by them. Their front lay on the southernlowlands of Germany, on the Jura district of France; on the south, itstretched across the valley of the Po as far as near Milan. We knowthis old ice front by the accumulations of rock _débris_ which werebrought to it from the interior of the mountain realm. We canrecognise the peculiar kinds of stone, and with perfect certaintytrace them to the bed rock whence they were riven. Moreover, we canfollow back through the same evidence the stages of retreat of theglaciers, until they lost their broad continental character andassumed something like their present valley form. Up the valley of anyof the great rivers, as, for instance, that of the Rhône above thelake of Geneva, we note successive terminal moraines which clearlyindicate stages in the retreat of the ice when for a time it ceased togo backward, or even made a slight temporary readvance. It is easilyseen that on such occasions the stones carried to the ice front wouldbe accumulated in a heap, while during the time when day by day theglacier was retreating the rock waste would be left broadcast over thevalley. 

As we go up from the course of the glacial streams we note that thesuccessive moraines have their materials in a progressively lessdecayed state. Far away from the heap now forming, and in proportionto the distance, the stones have in a measure rotted, and the heapswhich they compose are often covered with soil and occupied byforests. Within a few miles of the ice front the stones still have afresh aspect. When we arrive within, say, half a mile of the morainenow building, we come to the part of the glacial retreat of which wehave some written or traditional account. This is in general to theeffect that the wasting of the glaciers is going on in this century asit went on in the past. Occasionally periods of heavy snow wouldrefresh the ice streams, so that for a little time they pushed theirfronts farther down the valley. The writer has seen during one ofthese temporary advances the interesting spectacle of ice destroyingand overturning the soil of a small field which had been planted ingrain. 

It should be noted that these temporary advances of the ice are notdue to the snowfall of the winter or winters immediately preceding theforward movement. So slow is the journey of the ice from the _névé_field to the end of a long glacier that it may require centuries forthe store accumulated in the uplands to affect the terminal portion ofthe stream. We know that the bodies of the unhappy men who have beenlost in the crevices of the glacier are borne forward at a uniform andtolerably computable rate until they emerge at the front, where theice melts away. In at least one case the remains have appeared aftermany years in the _débris_ which is contributed to the moraine. Onaccount of this slow feeding of the glacial stream, we naturally mayexpect to find, as we do, in fact, that a great snowfall of manyyears ago, and likewise a period when the winter's contribution hasbeen slight, would influence the position of the terminal point of theice stream at different times, according to its length. If the lengthof the flow be five miles, it may require twenty or thirty years forthe effect to be evident; while if the stream be ten miles long, theinfluence may not be noted in less than threescore years. Thus itcomes about that at the present time in the same glacial district somestreams may be advancing while others are receding, though, on thewhole, the ice is generally in process of shrinkage. If the presentrate of retreat should be maintained, it seems certain that at the endof three centuries the Swiss glaciers as a whole will not haveanything like their present area, and many of the smaller streams willentirely disappear. 

Following the method of the illustrious Louis Agassiz, who firstattentively traced the evidence which shows the geologically recentgreat extension of glaciers by studying the evidence of the action infields they no longer occupy, geologists have now inspected a largepart of the land areas with a view to finding the proofs of such icework. So far as these indications are concerned, the indications whichthey have had to trace are generally of a very unmistakable character. Rarely, indeed, does a skilled student of such phenomena have tosearch in any region for more than a day before he obtains indubitableevidence which will enable him to determine whether or not the fieldhas recently been occupied by an enduring ice sheet--one whichsurvives the summer season and therefore deserves the name of glacier. The indications which he has to consider consist in the direction andmanner in which the surface materials have been carried, the physicalconditions of these materials, the shape of the surface of theunderlying rock as regards its general contour, and the presence orabsence of scratches and groovings on its surface. As these records ofice action are of first importance in dealing with this problem, andas they afford excellent subjects for the study of those who dwell inglaciated regions, we shall note them in some detail. 

The geologist recognises several ways in which materials may betransported on the surface of the earth. They may be cast forth byvolcanoes, making their journey by being shot through the air, or byflowing in lava streams; it is always easy at a glance, save in veryrare instances, to determine whether fragments have thus beenconveyed. Again, the detritus may be moved by the wind; this action islimited; it only affects dust, sand, and very small pebbles, and iseasily discriminated. The carriage may be effected by river or marinecurrents; here, again, the size of the fragments moved is small, andthe order of their arrangement distinctly traceable. The fragments maybe conveyed by ice rafts; here, too, the observer can usually limitthe probabilities he has to consider by ascertaining, as he cangenerally do, whether the region which he is observing has been belowa sea or lake. In a word, the before-mentioned agents oftransportation are of somewhat exceptional influence, and in mostcases can, as explanations of rock transportation, be readilyexcluded. When, therefore, the geologist finds a country abundantlycovered with sand, pebbles, and boulders arranged in an irregular way, he has generally only to inquire whether the material has been carriedby rivers or by glaciers. This discrimination can be quickly andcritically effected. In the first place, he notes that rivers only intheir torrent sections can carry large fragments of rock, and that inall cases the fragments move down hill. Further, that where depositsare formed, they have more or less the form of alluvial deposits. Ifnow the observations show that the rock waste occupying the surface ofany region has been carried up hill and down, across the valleys, particularly if there are here and there traces of frontal moraines, the geologist is entitled to suppose--he may, indeed, be sure--thatthe carriage has been effected by a glacial sheet. 

Important corroborative evidence of ice action is generally to befound by inspecting the bed rock below the detritus, which indicatesglacial action. Even if it be somewhat decayed, as is apt to be thecase where the ice sheet long since passed away, the bed rock islikely to have a warped surface; it is cast into ridges and furrows ofa broad, flowing aspect, such as liquid water never produces, which, indeed, can only be created by an ice sheet moving over the surface, cutting its bed in proportion to the hardness of the material. Furthermore, if the bed rock have a firm texture, and be not too muchdecayed, we almost always find upon it grooves or scratches, channelscarved by the stones embedded in the body of the ice, and drawn by itsmotion over the fixed material. Thus the proof of glacial extension inthe last ice epoch is made so clear that accurate maps can be preparedshowing the realm of its action. This task is as yet incomplete, although it is already far advanced. 

While the study of glaciers began in Europe, inquiries concerningtheir ancient extension have been carried further and with moreaccuracy in North America than in any other part of the world. We maytherefore well begin our description of the limits of the ice sheetswith this continent. Imagining a seafarer to have approached Americaby the North Atlantic, as did the Scandinavians, and that his voyagecame perhaps a hundred thousand years or more before that of LeifEricsson, he would have found an ice front long before he attained thepresent shores of the land. This front may have extended from south ofGreenland, off the shores of the present Grand Banks of Newfoundland, thence and westward to central or southern New Jersey. This cliff ofice was formed by a sheet which lay on the bottom of the sea. On theNew Jersey coast the ice wall left the sea and entered on the body ofthe continent. We will now suppose that the explorer, animated withthe valiant scientific spirit which leads the men of our day to seekthe poles, undertook a land journey along the ice front across thecontinent. From the New Jersey coast the traveller would have passedthrough central Pennsylvania, where, although there probably detachedoutlying glaciers lying to the southward as far as central Virginia, the main front extended westward into the Ohio Valley. In southernOhio a tongue of the ice projected southwardly until it crossed theOhio River, where Cincinnati now lies, extending a few miles to thesouthward of the stream. Thence it deflected northwardly, crossing theMississippi, and again the Missouri, with a tongue or lobe which wentfar southward in that State. Then again turning to the northwest, itfollowed in general the northern part of the Missouri basin until itcame to within sight of the Rocky Mountains. There the ice front ofthe main glacier followed the trend of the mountains at some distancefrom their face for an unknown extent to the northward. In theCordilleras, as far south as southern Colorado, and probably in theSierra Nevada to south of San Francisco, the mountain centresdeveloped local glaciers, which in some places were of very greatsize, perhaps exceeding any of those which now exist in Switzerland. It will thus be seen that nearly one half of the present land area ofNorth America was beneath a glacial covering, though, as before noted, the region about the Gulf of Mexico may have swayed upward when thenorthern portion of the land was borne down by the vast load of icewhich rested upon it. Notwithstanding this possible addition to theland, our imaginary explorer would have found the portion of thecontinent fit for the occupancy of life not more than half as great asit is at present. 

In the Eurasian continent there was no such continuous ice sheet as inNorth America, but the glaciers developed from a number of differentcentres, each moving out upon the lowlands, or, if its position wassouthern, being limited to a particular mountain field. One of thesecentres included Scandinavia, northern Germany, Great Britain about asfar south as London, and a large part of Ireland, the ice covering theintermediate seas and extending to the westward, so that the passageof the North Atlantic was greatly restricted between this ice frontand that of North America. Another centre, before noted, was formed inthe Alps; yet another, of considerable area, in the Pyrenees; otherless studied fields existed in the Apennines, in the Caucasus, theUral, and the other mountains of northern Asia. Curiously enough, however, the great region of plains in Siberia does not appear to havebeen occupied by a continuous ice sheet, though the similar region inNorth America was deeply embedded in a glacier. Coincident with thisdevelopment of ice in the eastern part of the continent, the icestreams of the Himalayan Mountains, some of which are among thegreatest of our upland glaciers, appear to have undergone but amoderate extension. Many other of the Eurasian highlands were probablyice-bound during the last Glacial period, but our knowledge concerningthese local fields is as yet imperfect. 

In the southern hemisphere the lands are of less extent and, on thewhole, less studied than in the northern realm. Here and there whereglaciers exist, as in New Zealand and in the southern part of SouthAmerica, observant travellers have noticed that these ice fields haverecently shrunk away. Whether the time of greatest extension and ofretreat coincided with that of the ice sheets in the north is not yetdetermined; the problem, indeed, is one of some difficulty, and maylong remain undecided. It seems, however, probable that the glaciersof the southern hemisphere, like those in the north, are in process ofretreat. If this be true, then their time of greatest extension wasprobably the same as that of the ice sheets about the southern pole. From certain imperfect reports which we have concerning evidences ofglaciation in Central America and in the Andean district in thenorthern part of South America, it seems possible that at one time theupland ice along the Cordilleran chain existed from point to pointalong that system of elevations, so that the widest interval betweenthe fields of permanent snow with their attendant glaciers did notmuch exceed a thousand miles. 

Observing the present gradual retreat of those ice remnants whichremain mere shreds and patches of the ancient fields, it seems atfirst sight likely that the extension and recession of the greatglaciers took place with exceeding slowness. Measured in terms ofhuman life, in the manner in which we gauge matters of man's history, this process was doubtless slow. There are reasons, however, tobelieve that the coming and going were, in a geological sense, swift;they may have, indeed, been for a part of the time of startlingrapidity. Going back to the time of geological yesterday, before theice began its development in the northern hemisphere, all the evidencewe can find appears to indicate a temperate climate extending fartoward the north pole. The Miocene deposits found within twelvedegrees, or a little more than seven hundred miles, of the north pole, and fairly within the realm of lowest temperature which now exists onthe earth, show by the plant remains which they contain that theconditions permitted the growth of forests, the plants having atolerably close resemblance to those which now freely develop in thesouthern portion of the Mississippi Valley. Among them there arespecies which had the habit of retaining their broad, rather softleaves throughout the winter season. The climate appears, in a word, to have been one where the mean annual temperature must have beenthirty degrees or more higher than the present average of that realm. Although such conditions near the sea level are not inconsistent withthe supposition that glaciers existed in the higher mountains of thenorth, they clearly deny the possibility of the realm being occupiedby continental glaciers. 

Although the Pliocene deposits formed in high latitudes have to agreat extent been swept away by the subsequent glacial wearing, theyindicate by their fossils a climatal change in the direction ofgreater cold. We trace this change, though obscurely, in aprogressive manner to a point where the records are interrupted, andthe next interpretable indication we have is that the ice sheet hadextended to somewhere near the limits which we have noted. We are thendriven to seek what we can concerning the sojourn of the ice on theland by the amount of wearing which it has inflicted upon the areaswhich it occupied. This evidence has a certain, though, as we shallsee, a limited value. 

When the students of glacial action first began the great task ofinterpreting these records, they were led to suppose that the amountof rock cutting which was done by the ice was very great. Observingwhat goes on, in the manner we have noted, beneath a valley glaciersuch as those of Switzerland, they saw that the ice work went onrapidly, and concluded that if the ice remained long at work in aregion it must do a vast deal of erosion. They were right in a part oftheir premises, but, as we shall see, probably in another part wrong. Looking carefully over the field where the ice has operated, we notethat, though at first sight the area appears to have lost all trace ofits preglacial river topography, this aspect is due mainly to theirregular way in which the glacial waste is laid down. Close studyshows us that we may generally trace the old stream valleys down tothose which were no larger than brooks. It is true that these channelsare generally and in many places almost altogether filled in withrubbish, but a close study of the question has convinced the writer, and this against a previous view, that the amount of erosion in NewEngland and Canada, where the work was probably as great as anywhere, has not on the average exceeded a hundred feet, and probably was muchless than that amount. 

Even in the region north of Lake Ontario, over which the ice was deepand remained for a long time, the amount of erosion is singularlysmall. Thus north of Kingston the little valleys in the limestonerocks which were cut by the preglacial streams, though somewhatencumbered with drift, remain almost as distinct as they are onsimilar strata in central Kentucky, well south of the field which theice occupied. In fact, the ice sheet appears to have done the greatestpart of its work and to have affected the surface most in the belt ofcountry a few hundred miles in width around the edges of the sheet. Itwas to be expected that in a continental glacier, as in those ofmountain valleys, the most of the _débris_ should be accumulated aboutthe margin where the materials dropped from the ice. But why thecutting action should be greatest in that marginal field is not atfirst sight clear. To explain this and other features as best we may, we shall now consider the probable history of the great ice march inadvance and retreat, and then take up the conditions which broughtabout its development and its disappearance. 

Ice is in many ways the most remarkable substance with which thephysicist has to deal, and among its eminent peculiarities is that itexpands in freezing, while the rule is that substances contract inpassing from the fluid to the solid state. On this account frozenwater acts in a unique manner when subjected to pressure. For eachadditional atmosphere of pressure--a weight amounting to about fifteenpounds to the square inch--the temperature at which the ice will meltis lowered to the amount of sixteen thousandths of a degreecentigrade. If we take a piece of ice at the temperature of freezingand put upon it a sufficient weight, we inevitably bring about a smallamount of melting. Where we can examine the mass under favourableconditions, we can see the fluid gather along the lines of thecrystals or other bits of which the ice is composed. We readily notethis action by bringing two pieces of ice together with a slightpressure; when the pressure is removed, they will adhere. The adhesionis brought about not by any stickiness of the materials, for thesubstance has no such property. It is accomplished by melting alongthe line of contact, which forms a film of water, that at oncerefreezes when the pressure is withdrawn. When a firm snowball ismade by even pressing snow, innumerable similar adhesions grow up inthe manner described. The fact is that, given ice at the temperatureat which it ordinarily forms, pressure upon it will necessarilydevelop melting. 

The consequences of pressure melting as above described are inglaciers extremely complicated. Because the ice is built into theglacier at a temperature considerably below the freezing point, itrequires a great thickness of the mass before the superincumbentweight is sufficient to bring about melting in its lower parts. If weknew the height at which a thermometer would have stood in the surfaceice of the ancient glacier which covered the northern part of NorthAmerica, we could with some accuracy compute how thick it must havebeen before the effect of pressure alone would have brought aboutmelting; but even then we should have to reckon the temperaturederived from the grinding of the ice over the floor and the crushingof rocks there effected, as well as the heat which is constantlythough slowly coming forth from the earth's interior. The result isthat we can only say that at some depth, probably less than a mile, the slowly accumulating ice would acquire such a temperature that, subjected to the weight above it, the material next the bottom wouldbecome molten, or at least converted into a sludgelike state, in whichit could not rub against the bottom, or move stones in the manner ofordinary glaciers. 

As fast as the ice assumed this liquid or softened state, it would besqueezed out toward the region where, because of the thinning of theglacier, it would enter a field where pressure melting did not occur. It would then resume the solid state, and thence journey to the marginof the ice in the ordinary manner. We thus can imagine how such aglacier as occupied the northern part of this continent could havemoved from the central parts toward its periphery, as we can not do ifwe assume that the glacier everywhere lay upon the bed rock. There isno slope from Lake Erie to the Ohio River at Cincinnati. Knowing thatthe ice moved down this line, there are but two methods of accountingfor its motion: either the slope of the upper surface to the northwardwas so steep that the mass would have been thus urged down, the upperparts dragging the bottom along with them, or the ice sheet for thegreater part of its extent rested upon pressure-molten water, orsludge ice, which was easily squeezed out toward the front. The firstsupposition appears inadmissible, for the reason that the ice wouldhave to be many miles deep at Hudson Bay in order that its uppersurface should have slope enough to overcome the rigidity of thematerial and bring about the movement. We know that any such depth isnot supposable. 

The recent studies in Greenland supply us with strong corroborativeevidence for the support of the view which is here urged. The widecentral field of that area, where the ice has an exceeding slightdeclivity, and is unruptured by crevices, can not be explained excepton the supposition that it rests on pressure-molten water. The thinnersection next the shore, where the glacier is broken up by thoseirregular movements which its wrestle with the bottom inevitablyinduces, shows that there it is in contact with the bed rock, for itbehaves exactly as do the valley glaciers of like thickness. 

The view above suggested as to the condition of continental glaciersenables us to explain not only their movements, but the relativelyslight amount of wearing which they brought about on the lands theyoccupied. Beginning to develop in mountain regions, or near the poleson the lowlands, these sheets, as soon as they attained the thicknesswhere the ice at their bottom became molten, would rapidly advance forgreat distances until they attained districts where the meltingexceeded the supply of frozen material. In this excursion only themarginal portion of the glacier would do erosive work. This wouldevidently be continued for the greatest amount of time near the frontor outer rim of the ice field, for there, we may presume, that forthe longest time the cutting rim would rest upon the bed rock of thecountry. As the ice receded, this rim would fall back; thus in theretreat as in the advance the whole of the field would be subjected toa certain amount of erosion. On this supposition we should expect tofind that the front of a continental glacier, fed with pressure-moltenwater from all its interior district, which became converted into ice, would attain much warmer regions than the valley streams, where allthe flow took place in the state of ice, and, furthermore, that thespeed of the going on the margin would be much more rapid than in theAlpine streams. These suppositions are well borne out by the study ofexisting continental ice sheets, which move with singular rapidity attheir fronts, and by the ancient glaciers, which evidently extendedinto rather warm fields. Thus, when the ice front lay at the site ofCincinnati, at six hundred feet above the sea, there were no glaciersin the mountains of North Carolina, though those rise more than fivethousand feet higher in the air, and are less than two hundred milesfarther south. It is therefore evident that the continental glacier atthis time pushed southward into a comparatively warm country in a waythat no stream moving in the manner of a valley glacier could possiblyhave done. 

The continental glaciers manage in many cases to convey detritus froma great distance. Thus, when the ice sheet advanced southwardly fromthe regions north of the Great Lakes, they conveyed quantities of the_débris_ from that section as far south as the Ohio River. In partthis rubbish was dragged forward by the ice as the sheet advanced; inpart it was urged onward by the streams of liquid water formed by theordinary process of ice melting. Such subglacial rivers appear to havebeen formed along the margins of all the great glaciers. We cansometimes trace their course by the excavation which they have made, but more commonly by the long ridges of stratified sand and gravelwhich were packed into the caverns excavated by these subglacialrivers, which are known to glacialists as _eskers_, or as serpentkames. In many cases we can trace where these streams flowed up streamin the old river valleys until they discharged over their head waters. Thus in the valley of the Genesee, which now flows from Pennsylvania, where it heads against the tributaries of the Ohio and Susquehanna, toLake Ontario, there was during the Glacial epoch a considerable riverwhich discharged its waters into those of the Ohio and the Susquehannaover the falls at the head of its course. 

[Illustration: _Front of Muir Glacier, showing ice entering the sea;also small icebergs. _]

The effect of widespread glacial action on a country such as NorthAmerica appears to have been, in the first place, to disturb theattitude of the land by bearing down portions of its surface, aprocess which led to the uprising of other parts which lay beyond therealm of the ice. Within the field of glaciation, so far as the icerested bodily on the surface, the rocks were rapidly worn away. Agreat deal of the _débris_ was ground to fine powder, and went farwith the waters of the under-running streams. A large part wasentangled in the ice, and moved forward toward the front of theglacier, where it was either dropped at the margin or, during therecession of the glacier, was laid upon the surface as the ice meltedaway. The result of this erosion and transportation has been to changethe conditions of the surface both as regards soil and drainage. Asthe reader has doubtless perceived, ordinary soil is, outside of theriver valleys, derived from the rock beneath where it lies. Inglaciated districts the material is commonly brought from aconsiderable distance, often from miles away. These ice-made soils arerarely very fertile, but they commonly have a great endurance fortillage, and this for the reason that the earth is refreshed by thedecay of the pebbles which they contain. Moreover, while the tillableearth of other regions usually has a limited depth, verging downwardinto the semisoil or subsoil which represent the little changed bedrocks, glacial deposits can generally be ploughed as deeply as mayprove desirable. 

The drainage of a country recently affected by glaciers is alwaysimperfect. Owing to the irregular erosion of the bed rocks, and to theyet more irregular deposition of the detritus, there are very numerouslakes which are only slowly filled up or by erosion provided withdrainage channels. Though several thousand years have passed by sincethe ice disappeared from North America, the greater part of the areaof these fresh-water basins remains, the greater number of them, mostly those of small size, have become closed. 

Where an ice stream descends into the sea or into a large lake, thedepth of which is about as great as the ice is thick, the relativelightness of the ice tends to make it float, and it shortly breaks offfrom the parent mass, forming an iceberg. Where, as is generally thecase in those glaciers which enter the ocean, a current sweeps by theplace where the berg is formed, it may enter upon a journey which maycarry the mass thousands of miles from its origin. The bergs separatedfrom the Greenland glaciers, and from those about the south pole, areoften of very great size; sometimes, indeed, they are some thousandfeet in thickness, and have a length of several miles. It oftenhappens that these bergs are formed of ice, which contains in itslower part a large amount of rock _débris_. As the submerged portionof the glacier melts in the sea water, these stones are graduallydropped to the bottom, so that the cargo of one berg may be strewedalong a line many hundred miles in length. It occasionally happensthat the ice mass melts more slowly in those parts which are in theair than in its under-water portions. It thus becomes top-heavy andoverturns, in which case such stony matter as remains attains aposition where it may be conveyed for a greater distance than if theglacier were not capsized. It is likely, indeed, that now and thenfragments of rock from Greenland are dropped on the ocean floor in thepart of the Atlantic which is traversed by steamers between ourAtlantic ports and Great Britain. 

Except for the risks which they bring to navigators, icebergs have noconsiderable importance. It is true they somewhat affect thetemperature of sea and air, and they also serve to convey fragments ofstone far out to sea in a way that no other agent can effect; but, onthe whole, their influence on the conditions of the earth isinconsiderable. 

Icebergs in certain cases afford interesting indices as to the motionof oceanic currents, which, though moving swiftly at a depth below thesurface, do not manifest themselves on the plain of the sea. Thus inthe region about Greenland, particularly in Davis Strait, bergs havebeen seen forcing their way southward at considerable speed throughordinary surface ice, which was either at rest or moving in theopposite direction. The train of these bergs, which moves upward fromthe south polar continent, west of Patagonia, indicates also in a veryemphatic way the existence of a very strong northward-setting currentin that part of the ocean. 

 * * * * *

We have now to consider the causes which could bring about such greatextensions of the ice sheet as occurred in the last Glacial period. Here again we are upon the confines of geological knowledge, and in afield where there are no well-cleared ways for the understanding. Infacing this problem, we should first note that those who are of theopinion that a Glacial period means a very cold climate in the regionswhere the ice attained its extension are probably in error. Natural asit may seem to look for exceeding cold as the cause of glaciation, thefacts show us that we can not hold this view. In Siberia and in theparts of North America bordering on the Arctic Sea the average cold isso intense that the ground is permanently frozen--as it is, forinstance, in the Klondike district--to the depth of hundreds of feet, only the surface thawing out during the warm summers. All this regionis cold enough for glaciers, but there is not sufficient snowfall tomaintain them. On the other hand, in Greenland, and in a less thoughconspicuous degree in Scandinavia, where the waters of the NorthAtlantic somewhat diminish the rigour of the cold, and at the sametime bring about a more abundant snowfall, the two actions beingintimately related, we have very extensive glaciers. Such facts, whichcould be very much extended, make it clear that the climate of glacialperiods must have been characterized by a great snowfall, and not bythe most intense cold. 

It is evident that what would be necessary again to envelop the borealparts of North America with a glacial sheet would not be aconsiderable decrease of heat, but an increase in the winter'scontribution of frozen water. Even if the heat released by thissnowfall elevated the average temperature of the winter, as itdoubtless would in a considerable measure, it would not melt off thesnow. That snowfall tends to warm the air by setting free the heatwhich was engaged in keeping the water in a state of vapour isfamiliarly shown by the warming which attends an ordinary snowstorm. Even if the fall begin with a temperature of about 0° Fahr. , the airis pretty sure to rise to near the freezing point. 

It is evident that no great change of temperature is required in orderto bring about a very considerable increase in the amount of snowfall. In the ordinary succession of seasons we often note the occurrence ofwinters during which the precipitation of snow is much above theaverage, though it can not be explained by a considerable climatalchange. We have to account for these departures from the normalweather by supposing that the atmospheric currents bring in more thanthe usual amount of moisture from the sea during the period when greatfalls of snow occur. In fact, in explaining variations in the humidityof the land, whether those of a constant nature or those that are tobe termed accidental, we have always to look to those features whichdetermine the importation of vapour from the great field of the oceanwhere it enters the air. We should furthermore note that thesepeculiarities of climate are dependent upon rather slight geographicaccidents. Thus the snowfall of northern Europe, which serves tomaintain the glaciation of that region, and, curiously enough, in somemeasure its general warmth, depends upon the movement of the GulfStream from the tropics to high latitudes. If by any geographicalchange, such as would occur if Central America were lowered so as tomake a free passage for its waters to the westward, the glaciers ofGreenland and of Scandinavia would disappear, and at the same time thetemperature of those would be greatly lowered. Thus the most evidentcause of glaciation must be sought in those alterations of the landwhich affect the movement of the oceanic currents. 

Applying this principle to the northern hemisphere, we can in a wayimagine a change which would probably bring about a return of such anice period as that from which the boreal realm is now escaping. Let ussuppose that the region of not very high land about Bering Straitshould sink down so as to afford the Kuro Siwo, or North Pacificequivalent of our Gulf Stream, an opportunity to enter the Arctic Seawith something like the freedom with which the North Atlantic currentis allowed to penetrate to high latitudes. It seems likely that thisPacific current, which in volume and warmth is comparable to that ofthe Atlantic, would so far elevate the temperature of the arcticwaters that their wide field would be the seat of a great evaporation. Noting once again the fact that the Greenland glaciers, as well asthose of Norway, are supplied from seas warmed by the Gulf Stream, weshould expect the result of this change would be to develop similarice fields on all the lands near that ocean. 

Applying the data gathered by Dr. Croll for the Gulf Stream, it seemslikely that the average annual temperature induced in the Arctic Seaby the free entrance of the Japan current would be between 20° and 30°Fahr. This would convert this wide realm of waters into a field ofgreat evaporation, vastly increasing the annual precipitation. Itseems also certain that the greater part of this precipitation wouldbe in the form of snow. It appears to the writer that this cause alonemay be sufficient to account for the last Glacial period in thenorthern hemisphere. As to the probability that the region aboutBering Strait may have been lowered in the manner required by thisview, it may be said that recent studies on the region about Mount St. Elias show that during or just after the ice epoch the shores in thatportion of Alaska were at least four thousand feet lower than atpresent. As this is but a little way from the land which we shouldhave to suppose to be lowered in order to admit the Japan current, wecould fairly conclude that the required change occurred. As for thecause of the land movement, geologists are still in doubt. They know, however, that the attitudes of the land are exceedingly unstable, andthat the shores rarely for any considerable time maintain theirposition. It is probable that these swayings of the earth's surfaceare due to ever-changing combinations of the weight in different partsof the crust and the strains arising from the contraction of its innerparts. 

In the larger operations of Nature the effects which we behold, however simple, are rarely the products of a single cause. In fact, there are few actions so limited that they can fairly be referred toone influence. It is therefore proper to state that there are manyother actions besides those above noted which probably enter intothose complicated equations which determine the climatal conditions ofthe earth. To have these would carry us into difficult and speculativeinquiries. 

As before remarked, all the regions which have been subjected toglaciation are still each year brought temporarily into the glacialstate. This fact serves to show us that the changes necessary toproduce great ice sheets are not necessarily of a startling nature, however great the consequences may be. Assuming, then, that relativelyslight alterations of climate may cause the ice sheet to come and go, we may say that all the influences which have been suggested by thestudents of glaciation, and various other slighter causes which cannot be here noted, may have co-operated to produce the peculiarresult. In this equation geographic change has affected the course ofthe ocean currents, and has probably been the most influential, or atleast the commonest, cause to which we must attribute the extension ofice sheets. Next, alterations of the solar heat may be looked to as achange-bringing action; unfortunately, however, we have no directevidence that this is an efficient cause. Thirdly, the variations inthe eccentricity of the earth's orbit, combined with the precession ofthe equinoxes and the rotation of the apsides, may be regarded asoperative. The last of all, changes in the constitution of theatmosphere, have to be taken into account. To these must be added, asbefore remarked, many less important actions which influence thismarvellously delicate machine, the work of which is expressed in thephenomena assembled under the name of climate. 

Evidence is slowly accumulating which serves to show that glacialperiods of greater or less importance have been of frequent occurrenceat all stages in the history of the earth of which we have a distinctrecord. As these accidents write their history upon the ground alone, and in a way impermanently, it is difficult to trace the ice times ofancient geological periods. The scratches on the bed rocks, and theaccumulations of detritus formed as the ice disappeared, have alikebeen worn away by the agents of decay. Nevertheless, we can trace hereand there in the older strata accumulations of pebbly matter oftencontaining large boulders, which clearly were shaped and broughttogether by glacial action. These are found in some instances farsouth of the region occupied by the glaciers during the last iceepoch. They occur in rocks of the Cambrian or Silurian age in easternTennessee and western North Carolina; they are also found in Indiabeyond the limits to which glaciers have attained in modern times. 

In closing this inadequate account of glacial action, a story whichfor its complete telling would require many volumes, it is well forthe reader to consider once again how slight are the changes ofclimate which may alternately withdraw large parts of the land fromthe uses of life, and again quickly restore the fields to the serviceof plants and animals. He may well imagine that these changes, bydriving living creatures to and fro, profoundly affect the history oftheir development. This matter will be dealt with in the volumeconcerning the history of organic beings. 

When the ice went off from the northern part of this continent, thesurface of the country, which had been borne down by the weight of theglacier, still remained depressed to a considerable depth below thelevel of the sea, the depression varying from somewhere about onehundred feet in southern New England to a thousand feet or more inhigh latitudes. Over this region, which lay beneath the level of thesea, the glacier, when it became thin enough to float, was doubtlessbroken up into icebergs, in the manner which we now behold along thecoast of Greenland. Where the shore was swept by a strong current, these bergs doubtless drifted away; but along the most of the coastline they appear to have lain thickly grouped next the shores, gradually delivering their loads of stones and finer _débris_ to thebottom. These masses of floating ice in many cases seem to haveprevented the sea waves from attaining the shore, and thus hinderedthe formation of those beaches which in their present elevatedcondition enable us to interpret the old position of the sea alongcoast lines which have been recently elevated. Here and there, however, from New Jersey to Greenland, we find bits of these ancientshores which clearly tell the story of that down-sinking of the landbeneath the burden of the ice which is such an instructive feature inthe history of that period. 

 CHAPTER VII. 

 THE WORK OF UNDERGROUND WATER. 

We have already noted two means by which water finds its wayunderground. The simplest and largest method by which this action iseffected is by building in the fluid as the grains of the rock arelaid down on the floors of seas or lakes. The water thus imprisoned isfirmly inclosed in the interstices of the stone, it in time takes upinto its mass a certain amount of the mineral materials which arecontained in the deep-buried rocks. The other portion of the groundwater--that with which we are now to be specially concerned--arisesfrom the rain which descends into the crevices of the earth; it istherefore peculiar to the lands. For convenience we shall term theoriginal embedded fluid _rock water_, and that which originates fromthe rain _crevice water_, the two forming the mass of the earth water. 

The crevice water of the earth, although forming at no time more thana very small fraction of the hidden fluid, is an exceedingly potentgeological agent, doing work which, though unseen, yet affords thevery foundations on which rest the life alike of land and sea. Whenthis water enters the earth, though it is purified of all mineralmaterials, it has already begun to acquire a share of a gaseoussubstance, carbonic acid, or, as chemists now term it, carbon dioxide, which enables the fluid to begin its rôle of marvellous activities. Inits descent as rain, probably even before it was gathered in drops inthe cloud realm, the water absorbs a certain portion of this gas fromthe atmosphere. Entering the realm of the soil, where the decayingorganic matter plentifully gives forth carbon dioxide, a further storeof the gas is acquired. At the ordinary pressure of the air, water maytake in many times its bulk of the gas. 

The immediate effect of carbonic acid when it is absorbed by water isgreatly to increase the capacity which that fluid has for takingmineral matters into solution. When charged with this gas, in themeasure in which it may be in the soil, water is able to dissolveabout fifty times as much limestone as it can in its perfectly pureform take up. A familiar instance of this peculiar capacity which thegas gives may often be seen where the water from a soda-water fountaindrips upon the marble slab beneath. In a few years this slab will beconsiderably corroded, though pure water would in the same time havehad no effect upon it. 

The first and by far the most important effect of crevice water isexercised upon the soil, which is at once the product of this action, and the laboratory where the larger part of the work is done. Penetrating between the grains of the detrital covering, held in largequantities in the coating, and continually in slow motion, thegas-charged water takes a host of substances into solution, and bringsthem into a condition where they may react upon each other in thechemical manner. These materials are constantly being offered to theroots of plants and brought in contact with the underlying rock whichhas not passed into the state of soil. The changes induced in thisstony matter lead to its breaking up, or at least to its softening tothe point where the roots can penetrate it and complete itsdestruction. Thus it comes about that the water which to a greatextent divides the rocks into the state of soil, which is continuallywearing away the material on the surface, or leaching it out throughthe springs, is also at work in restoring the layer from beneath. 

The greater part of the water which enters the soil does notpenetrate to any great depth in the underlying rocks, but finds itsway to the surface after no long journey in the form of small springs. Generally those superficial springs do not emerge through distinctchannels, but move, though slowly, in a massive way down the slopesuntil they enter a water course. Along the banks of any river, howeversmall, or along the shores of the sea, a pit a few inches deep justabove the level of the water will be quickly filled by a flow fromthis sheet which underlies the earth. At a distance from the streamthis sheet spring is in contact with the bed rocks, and may be manyfeet below the surface, but it comes to the level of the river or thesea near their margins. Here and there the shape of the bed rocks, being like converging house roofs, causes the superficial springs toform small pipelike channels for the escape of their gathered waters, and the flow emerges at a definite point. Almost all these sources ofconsiderable flow are due to the action of the water on the underlyingrock, where we shall now follow that portion of the crevice waterwhich penetrates deeply into the earth. 

Almost all rocks, however firm they may appear to be, are divided bycrevices which extend from the soil level it may be to the depths ofthousands of feet. These rents are in part due to the strains ofmountain-building, which tend to disrupt the firmest stone, leavingopen fractures. They are also formed in other ways, as by theimperfectly understood agencies which produce joint planes. It oftenhappens that where rocks are highly tilted water finds its waydownward between the layers, which are imperfectly soldered together, or a bed of coarse material, such as sandstone or conglomerate, mayafford an easy way by which the water may descend for miles beneaththe surface. Passing through rocks which are not readily soluble, thewater, already to a great extent supplied with mineral matter by itsjourney through the soil, may not do much excavating work, and evenafter a long time may only slightly enlarge the spaces in which itmay be stored or the channels by which it discharges to the surface. Hence it comes about that in many countries, even where the waterspenetrate deeply, they do not afford large springs. It is otherwisewhere the crevice waters enter limestones composed of materials whichare readily dissolved. In such places we find the rain so readilyentering the underlying rock that no part of the fall goes at once tothe brooks, but all has a long underground journey. 

In any limestone district where the beds of the material are thick andtolerably pure--as, for instance, in the cavern district of southernKentucky--the traveller who enters the region notes at once that theusual small streams which in every region of considerable rainfall heis accustomed to see intersecting the surface of the country areentirely absent. In their place he notes everywhere pitlikedepressions of bowl-shaped form, the sink holes to which we havealready adverted. Through the openings in the bottom of these the rainwaters descend into the depths of the earth. Although the most ofthese depressions have but small openings in their bottom, now andthen one occurs with a vertical shaft sufficiently large to permit theexplorer to descend into it, though he needs to be lowered down in themanner of a miner who is entering a shaft. In fact, the journey isnearly always one of some hazard; it should not be undertaken savewith many precautions to insure safety. 

When one is lowered away through an open sink hole, though the descentmay at first be somewhat tortuous, the explorer soon finds himselfswinging freely in the air, it may be at a point some hundred feetabove the base of the bottle-shaped shaft or dome into which he hasentered. Commonly the neck of the bottle is formed where the water hasworked its way through a rather sandy limestone, a rock which was notreadily dissolved by the water. In the pure and therefore easily cutlimestone layers the cavity rapidly expands until the light of thelantern may not disclose its walls. Farther down there is apt to be ashelf composed of another impure limestone, which extends off near themiddle of the shaft. If the explorer can land upon this shelf, he issure to find that from this imperfect floor the cavern extends off inone or more horizontal galleries, which he may follow for a greatdistance until he comes to the point where there is again a well-likeopening through the hard layer, with another dome-shaped base beneath. Returning to the main shaft, the explorer may continue his descentuntil he attains the base of this vertical section of the cave, wherehe is likely to find himself delivered in a pool of water of no greatdepth, the bottom of which is occupied by a quantity of small, hardstones of a flinty nature, which have evidently come from the upperparts of the cavern. The close observer will have noted that here andthere in the limestone there are flinty bits, such as those which hefinds in the pool. From the bottom of the dome a determined inquirercan often make his way along the galleries which lead from that level, though it may be after a journey of miles to the point where heemerges from the cavern on the banks of an open-air river. 

Although a journey by way of the sink holes through a cavern system isto be commended for the reason that it is the course of the caverningwaters, it is, on the whole, best to approach the cave through theirexits along the banks of a stream or through the chance openings whichare here and there made by the falling in of their roofs. Oneadvantage of this cavity of entrance is that we can thus approach thecavern in times of heavy rain when the processes which lead to theirconstruction are in full activity. Coming in this way to one of thedomes formed beneath a sink hole, we may observe in rainy weather thatthe water falling down the deep shaft strikes the bottom with greatforce; in many of the Kentucky caves it falls from a greater heightthan Niagara. At such times the stones in the basin at the bottom ofthe shaft are vigorously whirled about, and in their motion they cutthe rocks in the bottom of the basin--in fact, this cavity is a greatpot hole, like those at the base of open-air cascades. It is now easyto interpret the general principles which determine the architectureof the cavern realm. 

When it first enters the earth all the work which the water does inthe initial steps of cavern formation is effected by solution. As thecrevice enlarges and deepens, the stream acquires velocity, and beginsto use the bits of hard rock in boring. It works downward in this wayby the mixed mechanical and chemical action until it encounters a hardlayer. Then the water creeps horizontally through the soft stratum, doing most of its work by solution, until it finds a crevice in thefloor through which it can excavate farther in the downward direction;so it goes on in the manner of steps until it burrows channels to theopen stream. In time the vertical fall under the sink hole will cutthrough the hard layer, when the water, abandoning the first line ofexit, will develop another at a lower level, and so in time it comesabout that there may be several stories of the cave, the lowest beingthe last to be excavated. Of the total work thus done, only a smallpart is accomplished by the falling of the water, acting through theboring action of its tools, the bits of stone before mentioned; theprincipal part of the task is done by the solvent action of thecarbonated waters on the limestone. In the system of caverns known asthe Mammoth Cave, in Kentucky, the writer has estimated that at leastnine tenths of the stone was removed in the state of solution. 

When first excavated, the chambers of a limestone cavern have littlebeauty to attract the eye. The curves of the walls are sometimesgraceful, but the aspect of the chambers, though in a measure grand, is never charming. When, however, the waters have ceased to carve theopenings, when they have been drained away by the formation ofchannels on a lower level, there commonly sets in a process known asstalactitization, which transforms the scene into one of singularbeauty. We have already noted the fact that everywhere in ordinaryrocks there are crevices through which water, moving under thepressure of the fluid which is above, may find its way slowlydownward. In the limestone roofs of caverns, particularly in those ofthe upper story, this ooze of water passes through myriads of unseenfissures at a rate so slow that it often evaporates in the dry airwithout dropping to the floor. When it comes out of the rocks thewater is charged with various salts of lime; when it evaporates itleaves the material behind on the roof. Where the outflow is so slightthat the fluid does not gather into drops, it forms an incrustation oflimy matter, which often gathers in beautiful flowerlike forms, orperhaps in the shape of a sheet of alabaster. Where drops are formed, a small, pendent cone grows downward from the ceiling, over which thewater flows, and on which it evaporates. This cone grows slowlydownward until it may attain the floor of the chamber, which has aheight of thirty feet or more. If all the water does not evaporate, that which trickles off the apex of the cone, striking on the floor, is splashed out into a thin sheet, so that it evaporates in a speedymanner, lays down its limestone, and thus builds another and rudercone, which grows upward toward that which is pendent above it. Finally, they grow together, enlarged by the process which constructedthem, until a mighty column may be formed, sculptured as if by thehands of a fantastic architect. 

[Illustration: Fig. 13. --Stalactites and stalagmites on roof and floorof a cavern. The arrows show the direction of the moving water. ]

All the while that subterranean streams are cutting the cavernsdownward the open-air rivers into which they discharge are deepeningtheir beds, and thereby preparing for the construction of yet lowerstories of caves. These open-air streams commonly flow in steep-sided, narrow valleys, which themselves were caves until the galleries becameso wide that they could no longer support the roof. Thus we often findthat for a certain distance the roof over a large stream has fallenin, so that the water flows in the open air. Then it will plungeunder an arch and course, it may be, for some miles, before it againarrives at a place where the roof has disappeared, or perhaps attainsa field occupied by rocks of another character, in which caverns werenot formed. At places these old river caverns are abandoned by thestreams, which find other courses. They form natural tunnels, whichare not infrequently of considerable length. One such in southwesternVirginia has been made useful for a railway passing from one valley toanother, thus sparing the expense of a costly excavation. Where theremnant of the arch is small, it is commonly known as a naturalbridge, of which that in Rockbridge County, in Virginia, is a verynoble example. Arches of this sort are not uncommon in many caverncountries; five such exist in Carter County, Kentucky, a district inthe eastern part of that State which abounds in caverns, though noneof them are of conspicuous height or beauty. [7]

[Footnote 7: It is reported that one of these natural bridges of CarterCounty has recently fallen down. This is the natural end of thesefeatures. As before remarked, they are but the remnants of much moreextensive roofs which the processes of decay have brought to ruin. ]

At this stage of his studies on cavern work the student will readilyconceive that, as the surface of the country overlying the cave isincessantly wearing down, the upper stories of the system arecontinually disappearing, while new ones are forming at the presentdrainage level of the country. In fact, the attentive eye can in sucha district find here and there evidences of this progressivedestruction. Not only do the caves wear out from above, but theirroofs are constantly falling to their floors, a process which isgreatly aided by the growth of stalactites. Forming in the crevices orjoints between the stones, these rock growths sometimes prize offgreat blocks. In other cases the weight of the pendent stalactitedrags the ill-supported masses of the roof to the floor. In this way agallery originally a hundred feet below the surface may work its wayupward to the light of day. The entrance by which the Mammoth Cave isapproached appears to have been formed in this manner, and at severalpoints in that system of caverns the effect of this action may bedistinctly observed. 

We must now go a step further on the way of subterranean water, andtrace its action in the depths below the plane of ordinary caves, which, as we have noted, do not extend below the level of the mainstreams of the cavern district. The first group of facts to beattended to is that exhibited by artesian wells. These occur whererocks have been folded down into a basinlike form. It often happensthat in such a basin the rocks of which it is composed are some ofthem porous, and others impervious to water, and that the porouslayers outcrop on the high margins of the depression and havewater-tight layers over them. These conditions can be well representedby supposing that we have two saucers, one within the other, with anintervening layer of sand which is full of water. If now we bore anopening in the bottom of the uppermost saucer, we readily conceivethat the water will flow up through it. In Nature we often find thesebasins with the equivalent of the sandy layer in the model justdescribed rising hundreds of feet above the valley, so that theartesian well, so named from the village of Artois, near Paris, wherethe first opening of this nature was made, may yield a stream whichwill mount upward, especially where piped, to a great height. At manyplaces in the world it is possible by such wells to obtain a largesupply of tolerably pure water, but in general it is found to containtoo large a supply of dissolved mineral matter or sulphuretted gasesto be satisfactory for domestic purposes. It may be well to note thefact that the greater part of the so-called artesian wells, or boringswhich deliver water to a height above the surface, are not trueartesian sources, in that they do not send up the water by the actionof gravitation, but under the influence of gaseous pressure. 

Where, as in the case of upturned porous beds, the crevice waterpenetrates far below the earth's surface or the open-air streams whichdrain the water away, the fluid acquires a considerable increase oftemperature, on the average about one degree Fahrenheit for eacheighty feet of descent. It may, indeed, become so heated that if itwere at the earth's surface it would not only burst into steam with avast explosive energy, but would actually shine in the manner ofheated solids. As the temperature of water rises, and as the pressureon it increases, it acquires a solvent power, and takes in rockymatter in a measure unapproached at the earth's surface. At the depthof ten miles water beginning as inert rain would acquire theproperties which we are accustomed to associate with strong acids. Passing downward through fissures or porous strata in the mannerindicated in the diagram, the water would take up, by virtue of itsheat and the gases it contained, a share of many mineral substanceswhich we commonly regard as insoluble. Gold and even platinum--thelatter a material which resists all acids at ordinarytemperatures--enters into the solution. If now the water thus chargedwith mineral stores finds in the depths a shorter way to the surfacethan that which it descended, which may well happen by way of a deeprift in the rocks, it will in its ascent reverse the process which itfollowed on going down. It will deposit the several minerals in theorder of their solubilities--that is, the last to be taken in will bethe first to be crystallized on the walls of the fissure through whichthe upflow is taking place. The result will be the formation of a veinbelonging to the variety known as fissure veins. 

[Illustration: Fig. 14. --Diagram of vein. The different shadings showthe variations in the nature of the deposits. ]

A vein deposit such as we are considering may, though rarely, becomposed of a single mineral. Most commonly we find the depositarranged in a banded form in the manner indicated in the figure (seediagram 14). Sometimes one material will abound in the lower portionsof the fissure and another in its higher parts, a feature which isaccounted for by the progressive cooling and relinquishment ofpressure to which the water is subjected on its way to the surface. With each decrement of those properties some particular substance goesout of the fluid, which may in the end emerge in the form of a warm orhot spring, the water of which contains but little mineral matter. Where, however, the temperature is high, some part of the deposit, even a little gold, may be laid down just about the spring in thedeposits known as sinter, which are often formed at such places. 

In many cases the ore deposits are formed not only in the main channelof the fissure, but in all the crevices on either side of that way. Inthis manner, much as in the case of the growth of stalactitic matterbetween the blocks of stone in the roofs of a cavern, large fragmentsof rock, known as "horses, " are often pushed out into the body of thevein. In some instances the growth of the vein appears to enlarge thefissure or place of the deposit as the accumulation goes on, theprocess being analogous to that by which a growing root widens thecrevice into which it has penetrated. In other instances the fissureformed by the force has remained wide open, or at most has been butpartly filled by the action of the water. 

It not infrequently happens that the ascending waters of hot springsentering limestones have excavated extensive caves far below thesurface of the earth, these caverns being afterward in part filled bythe ores of various metals. We can readily imagine that the water atone temperature would excavate the cavern, and long afterward, when ata lower heat, they might proceed to fill it in. At a yet later stage, when the surface of the country had worn down many thousands of feetbelow the original level, the mineral stores of the caverns may bebrought near the surface of the earth. Some of the most importantmetalliferous deposits of the Cordilleras are found in this group ofhot-water caverns. These caverns are essentially like those producedby cold water, with the exception of the temperature of the fluidwhich does the work and the opposite direction of the flow. 

In following crevice water which is free to obey the impulses ofgravitation far down into the earth, we enter on a realm where therock or construction water, that which was built into the stone atthe time of its formation, is plentiful. Where these two groups ofwaters come in contact an admixture occurs, a certain portion of therock water joining that in the crevices. Near the surface of theground we commonly find that all the construction water has beenwashed out by this action. Yet if the rocks be compact, or if theyhave layers of a soft and clayey nature, we may find the constructionwater, even in very old deposits, remaining near the surface of theground. Thus in the ancient Silurian beds of the Ohio Valley a boringcarried a hundred feet below the level of the main rivers commonlydiscovers water which is clearly that laid down in the crevices of thematerial at the time when the rocks were formed in the sea. In allcases this water contains a certain amount of gases derived from thedecomposition of various substances, but principally from thealteration of iron pyrite, which affords sulphuretted hydrogen. Thusthe water is forced to the surface with considerable energy, and thewell is often named artesian, though it flows by gas pressure on theprinciple of the soda-water fountain, and not by gravity, as in thecase of true artesian wells. 

The passage between the work done by the deeply penetrating surfacewater and that due to the fluid intimately blended with the rock builtinto the mass at the time of its formation is obscure. We are, however, quite sure that at great depths beneath the earth theconstruction water acts alone not only in making veins, but inbringing about many other momentous changes. At a great depth thiswater becomes intensely heated, and therefore tends to move in anydirection where a chance fissure or other accident may lessen thepressure. Creeping through the rocks, and moving from zones of onetemperature to another, these waters bring about in the fineinterstices chemical changes which lead to great alterations in theconstitution of the rock material. It is probably in part to theseslow driftings of rock water that beds originally made up of small, shapeless fragments, such as compose clay slates, sandstones, andlimestones, may in time be altered into crystalline rocks, where thereis no longer a trace of the original bits, all the matter having beentaken to pieces by the process of dissolving, and reformed in theregular crystalline order. In many cases we may note how a crystalafter being made has been in part dissolved away and replaced byanother mineral. In fact, many of our rocks appear to have been againand again made over by the slow-drifting waters, each particular statein their construction being due to some peculiarity of temperature orof mineral contents which the fluid held. These metamorphic phenomena, though important, are obscure, and their elucidation demands someknowledge of petrographic science, that branch of geology whichconsiders the principles of rock formation. They will therefore not befurther considered in this work. 

 VOLCANOES. 

Of old it was believed that volcanoes represented the outpouring offluid rock which came forth from the central realm of the earth, aregion which was supposed still to retain the liquid state throughwhich the whole mass of our earth has doubtless passed. Recentstudies, however, have brought about a change in the views ofgeologists which is represented by the fact that we shall treatvolcanic phenomena in connection with the history of rock water. 

In endeavouring to understand the phenomena of volcanoes it is verydesirable that the student should understand what goes on in a normaleruption. The writer may, therefore, be warranted in describing someobservations which he had an opportunity to make at an eruption ofVesuvius in 1883, when it was possible to behold far more than canordinarily be discerned in such outbreaks--in fact, the opportunity ofa like nature has probably not been enjoyed by any other personinterested in volcanic action. In the winter of 1882-'83 Vesuvius wassubjected to a succession of slight outbreaks. At the time of theobservations about to be noted the crater had been reduced to a cupabout three hundred feet in diameter and about a hundred feet deep. The vertical shaft at the bottom, through which the outbursts weretaking place, was about a hundred feet across. Taking advantage of aheavy gale from the northwest, it was practicable, notwithstanding theexplosions, to climb to the edge of the crater wall. Looking down intothe throat of the volcano, although the pit was full of whirlingvapours and the heat was so great that the protection of a mask wasnecessary, it was possible to see something of what was going on atthe moment of an explosion. 

The pipe of the volcano was full of white-hot lava. Even in a day ofsunshine, which was only partly obscured by the vapours which hungabout the opening, the heat of the lava made it very brilliant. Thismass of fluid rock was in continuous motion, swaying violently up anddown the tube. From four to six times a minute, at the moment of itsupswaying, it would burst as by the explosion of a gigantic bubble. The upper portion of the mass was blown upward in fragments, thedischarge being like that of shot from a fowling piece; the fragments, varying in size from small, shotlike bits to masses larger than aman's head, were shot up sometimes to the height of fifteen hundredfeet above the point of ejection. The wind, blowing at the rate ofabout forty miles an hour, drove the falling bits of rock to theleeward, so that there was no considerable danger to be apprehendedfrom them. Some seconds after the explosion they could be heardrattling down on the farther slope of the cone. Observations on theinterval between the discharge and the fall of the fragments made iteasy to compute the height to which they were thrown. 

At the moment when the lava in the pipe opened for the passage of thevapour which created the explosion the movement, though performed ina fraction of a second, was clearly visible. At first the vapour wascolourless; a few score feet up it began to assume a faint, bluishhue; yet higher, when it was more expanded, the tint changed to thatof steam, which soon became of the ordinary aspect, and gathered inswift-revolving clouds. The watery nature of the vapour was perfectlyevident by its odour. Though commingled with sulphurous-acid gas, itstill had the characteristic smell of steam. For a half hour it waspossible to watch the successive explosions, and even to make roughsketches of the scene. Occasionally the explosions would come in quicksuccession, so that the lava was blown out of the tube; again, thepool would merely sway up and down in a manner which could beexplained only by supposing that great bubbles of vapour were workingtheir way upward toward the point where they could burst. Each ofthese bubbles probably filled a large part of the diameter of thepipe. In general, the phenomena recalled the escape of the jet from ageyser, or, to take a familiar instance, that of steam from the pipeof a high-pressure engine. When the heat is great, steam may often beseen at the mouth of the pipe with the same transparent appearancewhich was observed in the throat of the crater. In the cold air of themountain the vapour was rapidly condensed, giving a rainbow hue in theclouds when they were viewed at the right angle. The observations wereinterrupted by the fact that the wind so far died away that largeballs of the ejected lava began to fall on the windward side of thecone. These fragments, though cooled and blackened on their outside bytheir considerable journey up and down through the air, were still sosoft that they splashed when they struck the surface of cinders. 

Watching the cone from a distance, one could note that from time totime the explosions, increasing in frequency, finally attained a pointwhere the action appeared to be continuous. The transition wascomparable to that which we may observe in a locomotive which, when itfirst gets under way, gives forth occasional jets of steam, but, slowly gaining speed, finally pours forth what to eye and ear alikeseem to be a continuous outrush. All the evidence that we haveconcerning volcanic outbreaks corroborates that just cited, and is tothe effect that the essence of the action consists in the outbreak ofwater vapour at a high temperature, and therefore endowed with verygreat expansive force. Along with this steam there are many othergases, which always appear to be but a very small part of the wholeescape of a vaporous nature--in fact, the volcanic steam, so far asits chemical composition has been ascertained, has the compositionwhich we should expect to find in rock water which had been forced outfrom the rock by the tensions that high temperature creates. 

Because of its conspicuous nature, the lava which flows from mostvolcanoes, or is blown out from them in the form of finely dividedash, is commonly regarded as the primary feature in a volcanicoutbreak. Such is not really the case. Volcanic explosions may occurwith very little output of fluid rock, and that which comes forth mayconsist altogether of the finely divided bits of rock to which we givethe name of ash. In fact, in all very powerful explosions we mayexpect to find no lava flow, but great quantities of this finelydivided rock, which when it started from the depths of the earth wasin a fluid state, but was blown to pieces by the contained vapour asit approached the surface. 

If the student is so fortunate as to behold a flood of lava comingforth from the flanks of a volcano, he will observe that even at thevery points of issue, where the material is white-hot and appears tobe as fluid as water, the whole surface gives forth steam. On a stillday, viewed from a distance, the path of a lava flow is marked by adense cloud of this vapour which comes forth from it. Even after thelava has cooled so that it is safe to walk upon it, every crevicecontinues to pour forth steam. Years after the flowing has ceased, andwhen the rock surface has become cool enough for the growth ofcertain plants upon it, these crevices still yield steam. It isevident, in a word, that a considerable part of a lava mass, evenafter it escapes from the volcanic pipes, is water which is intimatelycommingled with the rock, probably lying between the very finestgrains of the heated substance. Yet this lava which has come forthfrom the volcano has only a portion of the water which it originallycontained; a large, perhaps the greater part, has gone forth in theexplosive way through the crater. It is reasonably believed that thefluidity of lava is in considerable measure due to the water which itcontains, and which serves to give the mass the consistence of paste, the partial fluidity of flour and rock grains being alike broughtabout in the same manner. 

So much of the phenomena of volcanoes as has been above noted isintended to show the large part which interstitial water plays involcanic action. We shall now turn our attention again to the state ofthe deeply buried rock water, to see how far we may be able by it toaccount for these strange explosive actions. When sediments are laiddown on the sea floor the materials consist of small, irregularlyshaped fragments, which lie tumbled together in the manner of a massof bricks which have been shot out of a cart. Water is buried in theplentiful interspaces between these bits of stone; as before remarked, the amount of this construction water varies. In general, it is atfirst not far from one tenth part of the materials. Besides the fluidcontained in the distinct spaces, there is a share which is held ascombined water in the intimate structure of the crystals, if suchthere be in the mass. When this water is built into the stone it hasthe ordinary temperature of the sea bottom. As the depositing actionscontinue to work, other beds are formed on the top of that which weare considering, and in time the layer may be buried to the depth ofmany thousand feet. There are reasons to believe that on the floors ofthe oceans this burial of beds containing water may have brought greatquantities of fluid to the depth of twenty miles or more below theouter surface of the rocks. 

[Illustration: Fig. 15. --Flow of lava invading a forest. A tree in thedistance is not completely burned, showing that the molten rock hadlost much of its original heat. ]

The effect of deep burial is to increase the heat of strata. Thisresult is accomplished in two different ways. The direct effectarising from the imposition of weight, that derived from the mass ofstratified material, is, as we know, to bring about a down-sinking ofthe earth's crust. In the measure of this falling, heat is engenderedprecisely as it is by the falling of a trip-hammer on the anvil, withwhich action, as is well known, we may heat an iron bar to a hightemperature. It is true that this down-sinking of the surface underweight is in part due to the compression of the rocks, and in part tothe slipping away of the soft underpinning of more or less fluid rock. Yet further it is in some measure brought about by the wrinkling ofthe crust. But all these actions result in the conversion of energy ofposition into heat, and so far serve to raise the temperature of therocks which are concerned in the movements. By far the largest sourceof heat, however, is that which comes forth from the earth's interior, and which was stored there in the olden day when the matter formingthe earth gathered into the mass of our sphere. This, which we mayterm the original heat, is constantly flowing forth into space, butmakes its way slowly, because of the non-conductive, or, as we mayphrase it, the "blanketing" effect of the outer rock. The effect ofthe strata is the same as that exercised by the non-conductivecoatings which are put on steam boilers. A more familiar comparisonmay be had from the blankets used for bedclothing. If on top of thefirst blanket we put a second, we keep warmer because the temperatureof the lower one is elevated by the heat from our body which is heldin. In the crust of the earth each layer of rock resists the outflowof heat, and each addition lifts the temperature of all the layersbelow. 

When water-bearing strata have been buried to the depth of ten miles, the temperature of the mass may be expected to rise to somewherebetween seven hundred and a thousand degrees Fahrenheit. If the depthattained should be fifty miles, it is likely that the temperature willbe five times as great. At such a heat the water which the rockscontain tends in a very vigorous way to expand and pass into the stateof vapour. This it can not readily do, because of its closeimprisonment; we may say, however, that the tendency toward explosionis almost as great as that of ignited gunpowder. Such powder, if heldin small spaces in a mass of cast steel, could be fired withoutrending the metal. The gases would be retained in a highly compressed, possibly in a fluid form. If now it happens that any of the strain inthe rocks such as lead to the production of faults produce fissuresleading from the surface into this zone of heated water, the tendencyof the rocks containing the fluid, impelled by its expansion, will beto move with great energy toward the point of relief or lessenedpressure which the crevice affords. Where rocks are in any waysoftened, pressure alone will force them into a cavity, as is shown bythe fact that beds of tolerably hard clay stones in deep coal minesmay be forced into the spaces by the pressure of the rocks whichoverlie them--in fact, the expense of cutting out these in-creepingrocks is in some British mines a serious item in the cost of theproduct. 

The expansion of the water contained in the deep-lying heated rocksprobably is by far the most efficient agent in urging them toward theplane of escape which the fissure affords. When the motion begins itpervades all parts of the rock at once, so that an actual flow isinduced. So far as the movement is due to the superincumbent weight, the tendency is at once to increase the temperature of the movingmass. The result is that it may be urged into the fissure perhaps evenhotter than when it started from the original bed place. In proportionas the rocky matter wins its way toward the surface, the pressure uponit diminishes, and the contained vapours are freer to expand. Takingon the vaporous form, the bubbles gather to each other, and when theyappear at the throat of the volcano they may, if the explosions beinfrequent, assume the character above noted in the little eruption ofVesuvius. Where, however, the lava ascends rapidly through thechannel, it often attains the open air with so much vapour in it, andthis intimately mingled with the mass, that the explosion rends thematerials into an impalpably fine powder, which may float in the airfor months before it falls to the earth. With a less violent movementthe vapour bubbles expand in the lava, but do not rend it apart, thusforming the porous, spongy rock known as pumice. With a yet slowerascent a large part of the steam may go away, so that we may have aflow of lava welling forth from the vent, still giving forth steam, but with a vapour whose tension is so lowered that the matter is notblown apart, though it may boil violently for a time after it escapesinto the air. 

Although the foregoing relatively simple explanation of volcanicaction can not be said as yet to be generally accepted by geologists, the reasons are sufficient which lead us to believe that it accountsfor the main features which we observe in this class of explosions--inother words, it is a good working hypothesis. We shall now proceed inthe manner which should be followed in all natural inquiry to see ifthe facts shown in the distribution of volcanoes in space and timeconfirm or deny the view. 

The most noteworthy feature in the distribution of volcanoes is that, at the present time at least, all active vents are limited to the seafloors or to the shore lands within the narrow range of three hundredmiles from the coast. Wherever we find a coast line destitute ofvolcanoes, as is the case with the eastern coast of North and SouthAmerica, it appears that the shore has recently been carried into theland for a considerable distance--in other words, old coast lines arenormally volcanic; that is, here and there have vents of this nature. Thus the North Atlantic, the coasts of which appear to have goneinland for a great distance in geologically recent times, isnon-volcanic; while the Pacific coast, which for a long time hasremained in its present position, has a singularly continuous line ofcraters near the shore extending from Alaska to Tierra del Fuego. Souninterrupted is this line of volcanoes that if they were all ineruption it would very likely be possible to journey down the coastwithout ever being out of sight of the columns of vapour which theywould send forth. On the floor of the sea volcanic peaks appear to bevery widely distributed; only a few of them--those which attain thesurface of the water--are really known, but soundings show long linesof elevations which doubtless represent cones distributed along faultlines, none of the peaks of sufficient height to break the surface ofthe sea. It is likely, indeed, that for one marine volcano whichappears as an island there are scores which do not attain the surface. Volcanic islands exist and generally abound in the ocean and greaterseas; every now and then we observe a new one forming as a smallisland, which is apt to be washed away by the sea shortly after theeruption ceases, the disappearance being speedy, for the reason thatthe volcanic ashes of which these cones are composed drift away likesnow before the movement of the waves. 

If the waters of the ocean and seas were drained away so that we couldinspect the portion of the earth's surface which they cover as readilyas we do the dry lands, the most conspicuous feature would be theinnumerable volcanic eminences which lie hidden in these wateryrealms. Wherever the observer passed from the centres of the presentlands he would note within the limits of those fields only mountains, much modified by river action; hills which the rivers had left inscarfing away the strata; and dales which had been carved out by theflowing waters. Near the shore lines of the vanished seas he wouldbegin to find mountains, hills, and vales occasionally commingled withvolcanic peaks, those structures built from the materials ejected fromthe vents. Passing the coast line to the seaward, the hills and daleswould quickly disappear, and before long the mountains would vanishfrom his way, and he would gradually enter on a region of vast rollingplains beset by volcanic peaks, generally accumulated in long ranges, somewhat after the manner of mountains, but differing from thoseelevations not only in origin but in aspect, the volcanic set of peaksbeing altogether made up of conical, cup-topped elevations. 

A little consideration will show us that the fact of volcanoes beingin the limit to the sea floors and to a narrow fringe of shore nextcertain ocean borders is reconcilable with the view as to theirformation which we have adopted. We have already noted the fact thatthe continents are old, which implies that the parts of the earthwhich they occupy have long been the seats of tolerably continuouserosion. Now and then they have swung down partly beneath the sea, andduring their submersion they received a share of sediments. But, onthe whole, all parts of the lands except strips next the coast may bereckoned as having been subjected to an excess of wearing action farexceeding the depositional work. Therefore, as we readily see, underneath such land areas there has been no blanketing process goingon which has served to increase the heat in the deep underlying rocks. On the contrary, it would be easy to show, and the reader may see ithimself, that the progressive cooling of the earth has probablybrought about a lowering of the temperature in all the section fromthe surface to very great depths, so that not only is the rock waterunaffected by increase of heat, but may be actually losingtemperature. In other words, the conditions which we assume bringabout volcanic action do not exist beneath the old land. 

Beneath the seas, except in their very greatest depths, and perhapseven there, the process of forming strata is continually going on. Next the shores, sometimes for a hundred or two miles away to seaward, the principal contribution may be the sediment worn from the lands bythe waves and the rivers. Farther away it is to a large extent made upof the remains of animals and plants, which when dying give theirskeletons to form the strata. Much of the materials laid down--perhapsin all more than half--consist of volcanic dust, ashes, and pumice, which drifts very long times before it finds its way to the bottom. Wehave as yet no data of a precise kind for determining the average rateof accumulation of sediments upon the sea floor, but from what isknown of the wearing of the lands, and the amount of volcanic wastewhich finds its way to the seas, it is probably not less than about afoot in ten thousand years; it is most likely, indeed, much to exceedthis amount. From data afforded by the eruptions in Java and in otherfields where the quantity of volcanic dust contributed to the seas canbe estimated, the writer is disposed to believe that the average rateof sedimentation on the sea floors is twice as great as the estimateabove given. 

Accumulating at the average rate of one foot in ten thousand years, itwould require a million years to produce a hundred feet of sediments;a hundred million to form ten thousand feet, and five hundred millionto create the thickness of about ten miles of bed. At the rate of twofeet in ten thousand years, the thickness accumulated would be abouttwenty miles. When we come to consider the duration of the earth'sgeologic history, we shall find reasons for believing that theformation of sediment may have continued for as much as five hundredmillion years. 

The foregoing inquiries concerning the origin of volcanoes show thatat the present time they are clearly connected with some process whichgoes on beneath the sea. An extension of the inquiry indicates thatthis relation has existed in earlier geological times; for, althoughthe living volcanoes are limited to places within three hundred milesof the sea, we find lava flows, ashes, and other volcanicaccumulations far in the interior of the continents, though the energywhich brought them forth to the earth's surface has ceased to operatein those parts of the land. In these cases of continental volcanoes itgenerally, if not always, appears that the cessation of the activityattended the removal of the shore line of the ocean or thedisappearance of great inland seas. Thus the volcanoes of theYellowstone district may have owed their activity to the immensedeposits of sediment which were formed in the vast fresh-water lakeswhich during the later Cretaceous and early Tertiary times stretchedalong the eastern face of the Rocky Mountains, forming a MediterraneanSea in North America comparable to that which borders southern Europe. It thus appears that the arrangement of volcanoes with reference tosea basins has held for a considerable period in the past. Stillfurther, when we look backward through the successive formations ofthe earth's crust we find here and there evidences in old lava flows, in volcanic ashes, and sometimes in the ruins of ancient cones whichhave been buried in the strata, that igneous activity such as is nowdisplayed in our volcanoes has been, since the earliest days of whichwe have any record, a characteristic feature of the earth. There is noreason to suppose that this action has in the past been any greater orany less than in modern days. All these facts point to the conclusionthat volcanic action is due to the escape of rock water which has beenheated to high temperatures, and which drives along with it as itjourneys toward a crevice the rock in which it has been confined. 

We will now notice some other explanations of volcanic action whichhave obtained a certain credence. First, we may note the view thatthese ejections from craters are forced out from a supposed liquidinterior of the earth. One of the difficulties of this view is that wedo not know that the earth's central parts are fluid--in fact, manyconsiderations indicate that such is not the case. Next, we observethat we not infrequently find two craters, each containing fluid lava, with the fluid standing at differences of height of several thousandfeet, although the cones are situated very near each other. If theselavas came from a common internal reservoir, the principles whichcontrol the action of fluids would cause the lavas to be at the sameelevation. Moreover, this view does not provide any explanation of thefact that volcanoes are in some way connected with actions which go onon the floors of great water basins. There is every reason to believethat the fractures in the rocks under the land are as numerous anddeep-going as those beneath the sea. If it were a mere question ofaccess to a fluid interior, volcanoes should be equally distributed onland and sea floors. Last of all, this explanation in no wise accountsfor the intermixture of water with the fluid rock. We can not wellbelieve that water could have formed a part of the deeper earth in theold days of original igneous fusion. In that time the water must havebeen all above the earth in the vaporous state. 

Another supposition somewhat akin to that mentioned is that the waterof the seas finds its way down through crevices beneath the floors ofthe ocean, and, there coming in contact with an internal molten mass, is converted into steam, which, along with the fluid rock, escapesfrom the volcanic vent. In addition to the objections urged to thepreceding view, we may say concerning this that the lava, if it cameforth under these circumstances, would emerge by the short way, thatby which the water went down, and not by the longer road, by which itmay be discharged ten thousand feet or more above the level of thesea. 

The foregoing general account of volcanic action should properly befollowed by some account of what takes place in characteristiceruptions. This history of these matters is so ample that it wouldrequire the space of a great encyclopædia to contain them. We shalltherefore be able to make only certain selections which may serve toillustrate the more important facts. 

By far the best-known volcanic cone is that of Vesuvius, which hasbeen subjected to tolerably complete record for about twenty-fourhundred years. About 500 B. C. The Greeks, who were ever on the searchfor places where they might advantageously plant colonies, settled onthe island of Ischia, which forms the western of what is now termedthe Bay of Naples. This island was well placed for tillage as well asfor commerce, but the enterprising colonists were again and againdisturbed by violent outbreaks of one or more volcanoes which lie inthe interior of this island; at one time it appears that the peoplewere driven away by these explosions. 

In these pre-Christian days Vesuvius, then known as Monte Somma, wasnot known to be a volcano, it never having shown any trace oferuption. It appeared as a regularly shaped mountain, somewhat overtwo thousand feet high, with a central depression about three miles indiameter at the top, and perhaps two miles over at the bottom, whichwas plainlike in form, with some lakes of bitter water in the centre. The most we know of this central cavity is connected with theinsurrection of the slaves led by Spartacus, the army of the revoltershaving camped for a time on the plain encircled by the crater walls. The outer slopes of the mountain afforded then a remarkably fertilesoil; some traces, indeed, of the fertility have withstood the moderneruptions which have desolated its flanks. This wonderful Bay ofNaples became the seat of the fairest Roman culture, as well as of avery extended commerce. Toward the close of the first century of ourera the region was perhaps richer, more beautifully cultivated, andthe seat of a more elaborate luxury than any part of the shore line ofEurope at the present day. At the foot of the mountain, on the easternborder of the bay, the city of Pompeii, with a population of aboutfifty thousand souls, was a considerable port, with an extensivecommerce, particularly with Egypt. The charming town was also a placeof great resort for rich Egyptians who cared to dwell in Europe. Onthe flanks of the mountain there was at least one large town, Herculaneum, which appears to have been an association of rich men'sresidences. On the eastern side of the bay, at a point now known asBaiæ, the Roman Government had a naval station, which in the year 79was under the command of the celebrated Pliny, a most voluminousthough unscientific writer on matters of natural history. With him inthat year there was his nephew, commonly known as the younger Pliny, then a student of eighteen years, but afterward himself an author. These facts are stated in some detail, for they are all involved inthe great tragedy which we are now to describe. 

For many years there had been no eruption about the Bay of Naples. Thevolcanoes on Ischia had been still for a century or more, and thevarious circular openings on the mainland had been so far quiet thatthey were not recognised as volcanoes. Even the inquisitive Pliny, with his great learning, was so little of a geologist that he did notknow the signs which indicate the seat of volcanic action, though theyare among the most conspicuous features which can meet the eye. TheGreeks would doubtless have recognised the meaning of these physicalsigns. In the year 63 the shores of the Bay of Naples were subjectedto a distinctive earthquake. Others less severe followed in subsequentyears. In an early morning in the year 79, a servant aroused the elderPliny at Baiæ with the news that there was a wonderful cloud risingfrom Monte Somma. The younger Pliny states that in form it was like apine tree, the common species in Italy having a long trunk with acrown of foliage on its summit, shaped like an umbrella. This crown ofthe column grew until it spread over the whole landscape, darkeningthe field of view. Shortly after, a despatch boat brought a message tothe admiral, who at once set forth for the seat of the disturbance. Heinvited his nephew to accompany him, but the prudent young man relatesin his letters to Tacitus, from whom we know the little concerning theeruption which has come down to us, that he preferred to do somereading which he had to attend to. His uncle, however, went straightforward, intending to land at some point on the shore at the foot ofthe cone. He found the sea, however, so high that a landing wasimpossible; moreover, the fall of rock fragments menaced the ship. Hetherefore cruised along the shore for some distance, landing at astation probably near the present village of Castellamare. At thispoint the fall of ashes and pumice was very great, but the sturdy oldRoman had his dinner and slept after it. There is testimony that hesnored loudly, and was aroused only when his servants began to fearthat the fall of ashes and stones would block the way out of hisbedchamber. When he came forth with his attendants, their headsprotected by planks resting on pillows, he set out toward Pompeii, which was probably the place where he sought to land. After going somedistance, the brave man fell dead, probably from heart disease; it issaid that he was at the time exceedingly asthmatic. No sooner were hisservants satisfied that the life had passed from his body than theyfled. The remains were recovered after the eruption had ceased. Theyounger Pliny further relates that after his uncle left, the cloudfrom the mountain became so dense that in midday the darkness was thatof midnight, and the earthquake shocks were so violent that wagonsbrought to the courtyard of the dwelling to bear the members of thehousehold away were rolled this way and that by the quakings of theearth. 

Save for the above-mentioned few and unimportant details concerningthe eruption, we have no other contemporaneous account. We have, indeed, no more extended story until Dion Cassius, writing long afterthe event, tells us that Herculaneum and Pompeii were overwhelmed; buthe mixes his story with fantastic legends concerning the appearance ofgods and demons, as is his fashion in his so-called history. Of allthe Roman writers, he is perhaps the most untrustworthy. Fortunately, however, we have in the deposits of ashes which were thrown out at thetime of this great eruption some basis for interpreting the eventswhich took place. It is evident that for many hours the Vesuviancrater, which had been dormant for at least five hundred years, blewout with exceeding fury. It poured forth no lava streams; the energyof the uprushing vapours was too great for that. The molten rock intheir path was blown into fine bits, and all the hard material castforth as free dust. In the course of the eruption, which probably didnot endure more than two days, possibly not more than twenty-fourhours, ash enough was poured forth to form a thick layer which spreadfar over the neighbouring area of land and sea floor. It covered thecities of Herculaneum and Pompeii to a depth of more than twenty feet, and over a circle having a diameter of twenty miles the averagethickness may have been something like this amount. So deep was itthat, although almost all the people of these towns survived, it didnot seem to them worth while to undertake to excavate their dwellingplaces. At Pompeii the covering did not overtop the higher of the lowhouses. An amount of labour which may be estimated at not over onethirtieth of the value, or at least the cost which had been incurredin building the city, would have restored it to a perfectlyinhabitable state. The fact that it was utterly abandoned probablyindicates a certain superstitious view in connection with theeruption. 

The fact that the people had time to flee from Herculaneum andPompeii, bearing with them their more valuable effects, is proved bythe excavations at these places which have been made in modern times. The larger part of Pompeii and a considerable portion of Herculaneumhave been thus explored; only rarely have human remains been found. Here and there, particularly in the cellars, the labourers engaged inthe work of disinterring the cities note that their picks enter acavity; examining the space, they find they have discovered theremains of a human skeleton. It has recently been learned that bypouring soft plaster of Paris into these openings a mould may beobtained which gives in a surprisingly perfect manner the originalform of the body. The explanation of this mould is as follows: Alongwith the fall of cinders in an eruption there is always a greatdescent of rain, arising from the condensation of the steam whichpours forth from the volcano. This water, mingling with the ashes, forms a pasty mud, which often flows in vast streams, and issometimes known as mud lava. This material has the qualities ofcement--that is, it shortly "sets" in a manner comparable to plasterof Paris or ordinary mortar. During the eruption of 79 this mudpenetrated all the low places in Pompeii, covering the bodies of thepeople, who were suffocated by the fumes of the volcanic emanations. We know that these people were not drowned by the inundation; theirattitudes show that they were dead before the flowing matterpenetrated to where they lay. 

It happened that Pompeii lay beyond the influence of the subsequentgreat eruptions of Vesuvius, so that it afterward received only slightash showers. Herculaneum, on the other hand, has century by centurybeen more and more deeply buried until at the present time it iscovered by many sheets of lava. This is particularly to be regretted, for the reason that, while Pompeii was a seaport town of no greatwealth or culture, Herculaneum was the residence place of the gentry, people who possessed libraries, the records of which can be in manycases deciphered, and from which we might hope to obtain some of thelost treasures of antiquity. The papyrus rolls on which the books ofthat day were written, though charred by heat and time, are stillinterpretable. 

After the great explosion of 79, Vesuvius sank again into repose. Itwas not until 1056 that vigorous eruptions again began. From time totime slight explosions occurred, none of which yielded lava flows; itwas not until the date last mentioned that this accompaniment of theeruption began to appear. In 1636, after a repose of nearly a centuryand a half, there came a very great outbreak, which desolated a wideextent of country on the northwestern side of the cone. At this stagein the history of the crater the volcanic flow began to attain thesea. Washing over the edge of the old original crater of Monte Somma, and thus lowering its elevation, these streams devastated, during theeruption just mentioned and in various other outbreaks, a wide fieldof cultivated land, overwhelming many villages. The last considerableeruption which yielded large quantities of lava was that of 1872, which sent its tide for a distance of about six miles. 

Since 1636 the eruptions of Vesuvius have steadily increased infrequency, and, on the whole, diminished in violence. In the earlyyears of its history the great outbreaks were usually separated byintervals of a century or more, and were of such energy that the lavawas mostly blown to dust, forming clouds so vast that on two occasionsat least they caused a midnight darkness at Constantinople, nearlytwelve hundred miles away. This is as if a volcano at Chicago shouldcompletely hide the sun in the city of Boston. In the present state ofVesuvius, the cone may be said to be in slight, almost continuouseruption. The old central valley which existed before the eruption of79, and continued to be distinct for long after that time, has beenfilled up by a smaller cone, bearing a relatively tiny crater of vent, the original wall being visible only on the eastern and northern partsof its circuit, and here only with much diminished height. On thewestern face the slope from the base of the mountain to the summit ofthe new cone is almost continuous, though the trained eye can tracethe outline of Monte Somma--its position in a kind of bench, which istraceable on that side of the long slope leading from the summit ofthe new cone to the sea. The fact that the lavas of Vesuvius havebroken out on the southwestern side, while the old wall of the conehas remained unbroken on the eastern versant, has a curiousexplanation. The prevailing wind of Naples is from the southwest, being the strong counter trades which belong in that latitude. In theold days when the Monte Somma cone was constructed these winds causedthe larger part of the ashes to fall on the leeward side of the cone, thus forming a thicker and higher wall around that part of the crater. 

From the nature of the recent eruptions of Vesuvius it appears likelythat the mountain is about to enter on a second period of inaction. The pipes leading through the new cone are small, and the mass of thiselevation constitutes a great plug, closing the old crater mouth. Togive vent to a large discharge of steam, the whole of this great mass, having a depth of nearly two thousand feet, would have to be blownaway. It seems most likely that when the occasion for such a dischargecomes, the vapours of the eruption will seek a vent through some otherof the many volcanic openings which lie to the westward of this greatcone. The history of these lesser volcanoes points to the conclusionthat when the path by way of Vesuvius is obstructed they may giverelief to the steam which is forcing its course to the surface. Two orthree times since the eruption of Pliny, during periods when Vesuviushad long been quiet, outbreaks have taken place on Ischia or in thePhlægræn Fields, a region dotted with small craters which lies to thewest of Naples. The last of these occurred in 1552, and led to theformation of the beautiful little cone known as Monte Nuovo. Thiseruption took place near the town of Puzzuoli, a place which was thenthe seat of a university, the people of which have left us records ofthe accident. 

[Illustration: Fig. 16. --Diagrammatic sections through Mount Vesuvius, showing changes in the form of the cone. (From Phillips. )]

The outbreak which formed Monte Nuovo was slight but verycharacteristic. It occurred in and beside a circular pool known as theLucrine Lake, itself an ancient crater. At the beginning of thedisturbance the ground opened in ragged cavities, from which mud andashes and great fragments of hard rock were hurled high in the air, some of the stones ascending to a height of several thousand feet. With slight intermissions this outbreak continued for some days, resulting in the formation of a hill about five hundred feet high, with a crater in its top, the bottom of which lay near the level ofthe sea. Although this volcanic elevation, being made altogether ofloose fragments, is rapidly wearing down, while the crater is fillingup, it remains a beautiful object in the landscape, and is alsonoteworthy for the fact that it is the only structure of this naturewhich we know from its beginning. In the Phlægræn Field there are anumber of other craters of small size, with very low cones about them. These appear to have been the product of brief, slight eruptions. Thatknown as the Solfatara, though not in eruption during the historicperiod, is interesting for the fact that from the crevices of therocks about it there comes forth a continued efflux of carbonic-acidgas. This substance probably arises from the effect of heat containedin old lavas which are in contact with limestone in the deepunder-earth. We know such limestones are covered by the lavas ofVesuvius, for the reason that numerous blocks of the rock are thrownout during eruptions, and are often found embedded in the lavastreams. It is an interesting fact that these craters of thePhlægræn Field, lying between the seats of vigorous eruption onIschia and at Vesuvius, have never been in vigorous eruption. Theirslight outbreaks seem to indicate that they have no permanentconnection with the sources whence those stronger vents obtain theirsupply of heated steam. 

The facts disclosed by the study of the Vesuvian system of volcanoesafford the geologist a basis for many interesting conclusions. 

In the first place, he notes that the greater part of the cones, allthose of small size, are made up of finely divided rock, which mayhave been more or less cemented by the processes of change whichgo on within it. It is thus clear that the lava flows areunessential--indeed, we may say accidental--contributions to the mass. In the case of Vesuvius they certainly do not amount to as much as onetenth of the elevation due to the volcanic action. The share of thelava in Vesuvius is probably greater than the average, for during thelast six centuries this vent has been remarkably lavigerous. [8]Observation on the volcanoes of other districts show that the Vesuviangroup is in this regard not peculiar. Of nearly two hundred coneswhich the writer has examined, not more than one tenth disclosedistinct lavas. 

[Footnote 8: I venture to use this word in place of the phrase"lava-yielding" for the reason that the term is needed in thedescription of volcanoes. ]

An inspection of the old inner wall of Monte Somma in that portionwhere it is best preserved, on the north side of the Atria delCavallo, or Horse Gulch--so called for the reason that those whoascended Vesuvius were accustomed to leave their saddle animalsthere--we perceive that the body of the old cone is to a considerableextent interlaced with dikes or fissures which have been filled withmolten lava that has cooled in its place. It is evident that duringthe throes of an eruption, when the lava stands high in the crater, these rents are frequently formed, to be filled by the fluid rock. Infact, lava discharges, though they may afterward course for longdistances in the open air, generally break their way undergroundthrough the cindery cone, and first are disclosed at the distance of amile or more from the inner walls of the crater. Their path isprobably formed by riftings in the compacted ashes, such as we traceon the steep sides of the Atria del Cavallo, as before noted. For thefurther history of these fissures, we shall have to refer to factswhich are better exhibited in the cone of Ætna. 

The amount of rock matter which has been thrown forth from thevolcanoes about the Bay of Naples is very great. Only a portion of itremains in the region around these cones; by far the greater part hasbeen washed or blown away. After each considerable eruption a widefield is coated with ashes, so that the tilled grounds appear as ifentirely sterilized; but in a short time the matter in good partdisappears, a portion of it decays and is leached away, and the mostof the remainder washes into the sea. Only the showers, whichaccumulate a deep layer, are apt to be retained on the surface of thecountry. A great deal of this powdered rock drifts away in the wind, sometimes in great quantities, as in those cases where it darkened thesky more than a thousand miles from the cone. Moreover, the water ofthe steam which brought about the discharges and the other gases whichaccompanied the vapour have left no traces of their presence, exceptin the deep channels which the rain of the condensing steam haveformed on the hillsides. Nevertheless, after all these subtractionsare made, the quantity of volcanic matter remaining on the surfaceabout the Bay of Naples would, if evenly distributed, form a layerseveral hundred feet in thickness--perhaps, indeed, a thousand feet indepth--over the territory in which the vents occur. All this matterhas been taken in relatively recent times from the depths of theearth. The surprising fact is that no considerable and, indeed, nopermanent subsidence of the surface has attended this excavation. Wecan not believe that this withdrawal of material from the under-earthhas resulted in the formation of open underground spaces. We know fullwell that any such, if it were of considerable size, would quickly becrushed in by the weight of the overlying rocks. We have, indeed, tosuppose that these steam-impelled lavas, which are driven toward thevent whence they are to go forth in the state of dust or fluid, comeunderground from distances away, probably from beneath the floors ofthe sea to the westward. 

Although the shores of the Bay of Naples have remained in general withunchanged elevation for about two thousand years, they have here andthere been subjected to slight oscillations which are most likelyconnected with the movement of volcanic matter toward the vents whereit is to find escape. The most interesting evidence of this nature isafforded by the studies which have been made on the ruins of theTemple of Serapis at Puzzuoli. This edifice was constructed inpre-Christian times for the worship of the Egyptian god Serapis, whoseintervention was sought by sick people. The fact that this divinity ofthe Nile found a residence in this region shows how intimate was therelation between Rome and Egypt in this ancient day. The Serapeium wasbuilt on the edge of the sea, just above its level. When in moderndays it began to be studied, its floor was about on its originallevel, but the few standing columns of the edifice afford indubitableevidence that this part of the shore has been lowered to the amount oftwenty feet or more and then re-elevated. The subsidence is proved bythe fact that the upper part of the columns which were not protectedby the _débris_ accumulated about them have been bored by certainshellfish, known as _Lithodomi_, which have the habit of excavatingshelters in soft stone, such as these marble columns afford. Atpresent the floor on which the ruin stands appears to be graduallysinking, though the rate of movement is very slow. 

Another evidence that the ejections may travel for a great distanceunderground on their way to the vent is afforded by the fact thatVesuvius and Ætna, though near three hundred miles apart, appear toexchange activities--that is, their periods of outbreak are notsimultaneous. Although these elements of the chronology of the twocones may be accidental, taken with similar facts derived from otherfields, they appear to indicate that vents, though far separated fromeach other, may, so to speak, be fed from a common subterraneansource. It is a singular fact in this connection that the volcano ofStromboli, though situated between these two cones, is in a state ofalmost incessant activity. This probably indicates that the last-namedvent derives its vapours from another level in the earth than thegreater cones. In this regard volcanoes probably behave like springs, of which, indeed, they may be regarded as a group. The reader isdoubtless aware that hot and cold springs often escape very neartogether, the difference in the temperature being due to the depthfrom which their waters come forth. 

As the accidents of volcanic explosion are of a nature to be verydamaging to man, as well as to the lower orders of Nature, it is fitthat we should note in general the effect of the Neapolitan eruptionson the history of civilization in that region. As stated above, thefirst Greek settlements in this vicinity--those on the island ofIschia--were much disturbed by volcanic outbreaks, yet the islandbecame the seat of a permanent and prosperous colony. The greateruption of 79 probably cost many hundred lives, and led to theabandonment of two considerable cities, which, however, could at smallcost have been recovered to use. Since that day various eruptions havetemporarily desolated portions of the territory, but only in verysmall fields have the ravages been irremediable. Where the ground wascovered with dust, it has in most places been again tillable, and sorapid is the decay of the lavas that in a century after their flow hasceased vines can in most cases be planted on their surfaces. The cityof Naples, which lies amid the vents, though not immediately incontact with any of them, has steadfastly grown and prospered from thepre-Christian times. It is doubtful if any lives have ever been lostin the city in consequence of an eruption, and no great inconveniencehas been experienced from them. Now and then, after a great ashshower, the volcanic dust has to be removed, but the labour is lessserious than that imposed on many northern cities by a snowstorm. Through all these convulsions the tillage of the district has beenmaintained. It has ever been the seat of as rich and profitable ahusbandry as is afforded by any part of Italy. In fact, the ashshowers, as they import fine divided rock very rich in substancesnecessary for the growth of plants, have in a measure served tomaintain the fertility of the soil, and by this action have in somedegree compensated for the injury which they occasionally inflict. Comparing the ravages of the eruptions with those inflicted by war, unnecessary disease, or even bad politics, and we see that thesenatural accidents have been most merciful to man. Many a tyrant hascaused more suffering and death than has been inflicted by these rudeoperations of Nature. 

From the point of view of the naturalist, Ætna is vastly moreinteresting than Vesuvius. The bulk of the cone is more than twentytimes as great as that of the Neapolitan volcano, and the magnitude ofits explosions, as well as the range of phenomena which they exhibit, incomparably greater. It happens, however, that while human history ofthe recorded kind has been intimately bound up with the tiny Vesuviancone, partly because the relatively slight nature of its disturbancespermitted men to dwell beside it, the larger Ætna has expelled culturefrom the field near its vent, and has done the greater part of itswork in the vast solitude which it has created. [9]

[Footnote 9: In part the excellent record of Vesuvius is due to the factthat since the early Christian centuries the priests of St. Januarius, the patron of Naples, have been accustomed to carry his relics inprocession whenever an eruption began. The cessation of the outbreak hasbeen written down to the credit of the saint, and thus we are providedwith a long story of the successive outbreaks. ]

Ætna has been in frequent eruption for a very much longer time thanVesuvius. In the odes of Pindar, in the sixth century before Christ, we find records of eruptions. It is said also that the philosopherEmpedocles sought fame and death by casting himself into the fierycrater. There has thus in the case of this mountain been no such longperiod of repose as occurred in Vesuvius. Though our records of theoutbreaks are exceedingly imperfect, they serve to show that the venthas maintained its activity much more continuously than is ordinarilythe case with volcanoes. Ætna is characteristically a lava-yieldingcone; though the amount of dust put forth is large, the ratio of thefluid rock which flows away from the crater is very much greater thanat Vesuvius. Nearly half the cone, indeed, may be composed of thismaterial. Our space does not permit anything like a consecutive storyof the Ætnean eruptions since the dawn of history, or even a fullaccount of its majestic cone; we can only note certain features of aparticularly instructive nature which have been remarked by the manyable men who have studied this structure and the effects of itsoutbreak. 

The most important feature exhibited by Ætna is the vast size of itscone. At its apex its height, though variable from the frequentdestruction and rebuilding of the crater walls, may be reckoned asabout eleven thousand feet. The base on which the volcanic materiallies is probably less than a thousand feet above the sea, so that themaximum thickness of the heap of volcanic ejections is probably abouttwo miles. The average depth of this coating is probably about fivethousand feet, and, as the cone has an average diameter of aboutthirty miles, we may conclude that the cone now contains about athousand cubic miles of volcanic materials. Great as is this mass, it is only a small part of the ejected material which has gone forthfrom the vent. All the matter which in its vaporous state went forthwith the eruption, the other gases and vapours thus discharged, havedisappeared. So, too, a large part of the ash and much of the lava hasbeen swept away by the streams which drain the region, and which intimes of eruption are greatly swollen by the accompanying torrentialrains. The writer has estimated that if all the emanations from thevolcano--solid, fluid, and gaseous--could be heaped on the cone, theywould form a mass of between two and three thousand cubic miles incontents. Yet notwithstanding this enormous outputting of earthymatter, the earth on which the Ætnean cone has been constructed hasnot only failed to sink down, but has been in process of continuous, slow uprising, which has lifted the surface more than a thousand feetabove the level which it had at the time when volcanic action began inthis field. Here, even more clearly than in the case of Vesuvius, wesee that the materials driven forth from the crater are derived notfrom just beneath its foundation, but from a distance, from realmswhich in the case of this insular volcano are beneath the sea floors. It is certain that here the migration of rock matter, impelled by theexpansion of its contained water toward the vent, has so far exceededthat which has been discharged through the crater that an uprising ofthe surface such as we have observed has been brought about. 

[Illustration: _Mount Ætna, seen from near Catania. The imperfectcones on the sky line to the left are those of small secondaryeruptions. _]

There are certain peculiarities of Mount Ætna which are due in part toits great size and in part to the climatal conditions of the region inwhich it lies. The upper part of the mountain in winter is deeplysnow-clad; the frozen water often, indeed, forms great drifts in thegorges near the summit. Here it has occasionally happened that a layerof ashes has deeply buried the mass, so that it has been preserved foryears, becoming gradually more inclosed by the subsequent eruptions. At one point where this compact snow--which has, indeed, taken on theform of ice--has been revealed to view, it has been quarried andconveyed to the towns upon the seacoast. It is likely that there aremany such masses of ice inclosed between the ash layers in the upperpart of the mountain, where, owing to the height, the climate is verycold. This curious fact shows how perfect a non-conductor the ash bedsof a volcano are to protect the frozen water from the heat of therocks about the crater. 

The furious rains which beset the mountain in times of great eruptionsexcavate deep channels on its sides. The lava outbreaks which attendalmost every eruption, and which descend from the base of the cindercone at the height of from five to eight thousand feet above the sea, naturally find their way into these channels, where they course in themanner of rivers until the lower and less valleyed section of the coneis reached. 

Such a lava flow naturally begins to freeze on the surface, the lavaat first becoming viscid, much in the manner of cream on the surfaceof milk. Urged along by the more fluid lava underneath, this viscidcoating takes a ropy or corrugated form. As the freezing goes deeper, a firm stone roof may be formed across the gorge, which, when thecurrent of lava ceases to flow from the crater, permits the lower partof the stream to drain away, leaving a long cavern or scries of cavesextending far up the cone. The nature of this action is exactlycomparable to that which we may observe when on a frosty morning afterrain we may find the empty channels which were occupied by rills ofwater roofed over with ice; the ice roofs are temporary, while thoseof lava may endure for ages. Some of these lava-stream caves have beendisclosed, in the manner of ordinary caverns, by the falling of theirroofs; but the greater part are naturally hidden beneath theever-increasing materials of the cone. 

The lava-stream caves of Ætna are not only interesting because oftheir peculiarities of form, which we shall not undertake to describe, but also for the reason that they help us to account for a verypeculiar feature in the history of the great cone. On the slopes ofthe volcano, below the upper cindery portion, there are severalhundred lesser cones, varying from a few score to seven hundred feetin height. Each of these has its appropriate crater, and has evidentlybeen the seat of one or more eruptions. As the greater part of thesecones are ancient, many of them being almost effaced by the rain orburied beneath the ejections which have surrounded their bases sincethe time they were formed, we are led to believe that many thousandsof them have been formed during the history of the volcano. Thehistory of these subsidiary cones appears to be connected with thelava caves noted above. These caverns, owing to the irregularities oftheir form, contain water. They are, in fact, natural cisterns, wherethe abundant rainfall of the mountain finds here and there storage. When, during the throes of an eruption, dikes such as we know often topenetrate the mountain, are riven outward from the crater through themass of the cone, and filled with lava, the heated rock must oftencome in contact with these masses of buried water. The result of thiswould inevitably be the local generation of steam at a hightemperature, which would force its way out in a brief but vigorouseruption, such as has been observed to take place when theseperipheral volcanoes are formed. Sometimes it has happened that afterthe explosion the lava has found its way in a stream from the fissurethus opened. That this explanation is sufficient is in a measure shownby observations on certain effects of lava flows from Vesuvius. Thewriter was informed by a very judicious observer, a resident ofNaples, who had interested himself in the phenomena of that volcano, that the lava streams when they penetrated a cistern, such as theyoften encounter in passing over villages or farmsteads, vaporized thewater, and gave rise, through the action of the steam, to smalltemporary cones, which, though generally washed away by the furtherflow of the liquid rock, are essentially like those which we find onÆtna. Such subsidiary, or, as they are sometimes called, parasiticcones, are known about other volcanoes, but nowhere are they socharacteristic as on the flanks of that wonderful volcano. 

A very conspicuous feature in the Ætnean cone consists of a greatvalley known as the Val del Bove, or Bull Hollow, which extends fromthe base of the modern and ever-changeable cinder cone down the flanksof the older structure to near its base. This valley has steep sides, in places a thousand or more feet high, and has evidently been formedby the down-settling of portions of the cone which were left withoutsupport by the withdrawal from beneath them of materials cast forth ina time of explosion. In an eruption this remarkable valley was theseat of a vast water flood, the fluid being cast forth from the craterat the beginning of the explosion. In the mouths of this and othervolcanoes, after a long period of repose, great quantities of water, gathering from rains or condensed from the steam which slowly escapesfrom these openings, often pours like a flood down the sides of themountains. In the great eruption of Galongoon, in Java, such a mass ofwater, cast forth by a terrific explosion, mingled with ashes, so thatthe mass formed a thick mud, was shot forth with such energy that itravaged an area nearly eighty miles in diameter, destroying theforests and their wild inhabitants, as well as the people who dweltwithin the range of the amazing disaster. So powerfully was this waterdriven from the crater that the districts immediately at the base ofthe cone were in a manner overshot by the vast stream, and escapedwith relatively little injury. 

When it comes forth from the base of the cinder cone, or from one ofthe small peripheral craters, the lava stream usually appears to bewhite hot, and to flow with almost the ease of water. It does notreally have that measure of fluidity; its condition is rather that ofthin paste; but the great weight of the material--near two and a halftimes that of water--causes the movement down the slope to be speedy. The central portion of the lava stream long retains its hightemperature; but the surface, cooling, is first converted into a toughsheet, which, though it may bend, can hardly be said to flow. Furtherhardening converts these outlying portions of the current into hard, glassy stone, which is broken into fragments in a way resembling theice on the surface of a river. It thus comes about that the advancingfront of the lava stream becomes covered, and its motion hindered bythe frozen rock, until the rate of ongoing may not exceed a few feetan hour, and the appearance is that of a heap of stone slowly rollingdown a slope. Now and then a crevice is formed, through which a thinstream of liquid lava pours forth, but the material, having alreadyparted with much of its heat, rapidly cools, and in turn becomescovered with the coating of frozen fragments. In this state of thestream the lava flow stands on all sides high above the slope which itis traversing; it is, in fact, walled in by its own solidified parts, though it is urged forward by the contribution which continues to flowin the under arches. In this state of the movement trifling accidents, or even human interference, may direct the current this way or that. 

Some of the most interesting chapters in the history of Ætna relate tothe efforts of the people to turn these slow-moving streams so thattheir torrents might flow into wilderness places rather than over thefields and towns. In the great flow of 1669, which menaced the city ofCatania, a large place on the seashore to the southeast of the cone, apublic-spirited citizen, Señor Papallardo, protecting himself and hisservants with clothing made of hides, and with large shields, setforth armed with great hooks with the purpose of diverting the courseof the lava mass. He succeeded in pulling away the stones on theflank of the stream, so that a flow of the molten rock was turned inanother direction. The expedient would probably have been successfulif he had been allowed to continue his labours; but the inhabitants ofa neighbouring village, which was threatened by the off-shootingcurrent which Papallardo had created, took up arms and drove him andhis retainers away. The flow continued until it reached Catania. Thepeople made haste to build the city walls on the side of danger higherthan it was before, but the tide mounted over its summit. 

Although the lavas which come forth from the volcano evidently have ahigh temperature, their capacity for melting other rocks is relativelysmall. They scour these rocks, because of their weight, even moreenergetically than do powerful torrents of water, but they arerelatively ineffective in melting stone. On Ætna and elsewhere we mayoften observe lavas which have flowed through forests. When the tideof molten rock has passed by, the trees may be found charred but notentirely burned away; even stems a few inches in diameter retainstrength enough to uphold considerable fringes and clots of the lavawhich has clung to them. These facts bear out the conclusion that thefluidity of the heated stone depends in considerable measure on thewater which is contained, either in its fluid or vaporous state, between the particles of the material. 

If we consider the Italian volcanoes as a whole, we find that they liein a long, discontinuous line extending from the northern part of thevalley of the Po, within sight of the Alps, to Ætna, and insubterranean cones perhaps to the northern coast of Africa. At thenorthern end of the line we have a beautiful group of extinctvolcanoes, known as the Eugean Mountains. Thence southward to southernTuscany craters are wanting, but there is evidence of fissures in theearth which give forth thermal waters. From southern Tuscany southwardthrough Rome to Naples there are many extinct craters, none of whichhave been active in the historic period. From Naples southward thecones of this system, about a dozen in number, are on islands or closeto the margin of the sea. It is a noteworthy fact that the greaterpart of these shore or insular vents have been active since the dawnof history; several of them frequently and furiously so, while none ofthose occupying an inland position have been the seat of explosions. This is a striking instance going to show the relation of theseprocesses to conditions which are brought about on the sea bottom. 

Ætna is, as we have noticed, a much more powerful volcano thanVesuvius. Its outbreaks are more vigorous, its emanations vastlygreater in volume, and the mass of its constructions many times asgreat as those accumulated in any other European cone. There are, however, a number of volcanoes in the world which in certain featuressurpass Ætna as much as that crater does Vesuvius. Of these we shallconsider but two--Skaptar Jokul, of Iceland, remarkable for the volumeof its lava flow, and Krakatoa, an island volcano between Java andSumatra, which was the seat of the greatest explosion of which we haveany record. 

The whole of Iceland may be regarded as a volcanic mass composedmainly of lavas and ashes which have been thrown up by a group ofvolcanoes lying near the northern end of the long igneous axis whichextends through the centre of the Atlantic. The island has been theseat of numerous eruptions; in fact, since its settlement by theNorthmen in 1070 its sturdy inhabitants have been almost as muchdistressed by the calamities which have come from the internal heat asthey have been by the enduring external cold. They have, indeed, beenbetween frost and fire. The greatest recorded eruption of Icelandoccurred in 1783, when the volcano of Skaptar, near the southernborder of the island, poured forth, first, a vast discharge of dustand ashes, and afterward in the languid state of eruption inundated aseries of valleys with the greatest lava flow of which we have anywritten record. The dust poured forth into the upper air, being finelydivided and in enormous quantity, floated in the air for months, giving a dusky hue to the skies of Europe, which led the common peopleand many of the learned to fear that the wrath of God was upon them, and that the day of judgment was at hand. Even the poet Cowper, a manof high culture and education, shared in this unreasonable view. 

The lava flow in this eruption filled one of the considerable valleysof the island, drying up the river, and inundating the plains oneither side. Estimates which have been made as to the volume of thisflow appear to indicate that it may have amounted to more than thebulk of the Mont Blanc. 

This great eruption, by the direct effect of the calamity, and by thefamine due to the ravaging of the fields and the frightening of thefish from the shores which it induced, destroyed nearly one fifth ofthe Icelandic people. It is, in fact, to be remembered as one of thethree or four most calamitous eruptions of which we have any account, and, from the point of view of lava flow, the greatest in history. 

Just a hundred years after the great Skaptar eruption, which darkenedthe skies of Europe, the island of Krakatoa, an isle formed by a smallvolcano in the straits of Java, was the seat of a vapour explosionwhich from its intensity is not only unparalleled, but almostunapproached in all accounts of such disturbances. Krakatoa had longbeen recognised as a volcanic isle; it is doubtful, however, if it hadever been seen in eruption during the three centuries or more sinceEuropean ships began to sail by it until the month of May of the yearabove mentioned. Then an outbreak of what may be called ordinaryviolence took place, which after a few days so far ceased thatobservers landed and took account of the changes which the convulsionhad brought about. For about three months there were no further signsof activity, but on the 29th of August a succession of vast explosionstook place, which blew away a great part of the island, forming in itsplace a submarine crater two or three miles in diameter, creatingworld-wide disturbances of sea and air. The sounds of the outbreakwere heard at a distance of sixteen hundred miles away. The waves ofthe air attendant on the explosion ran round the earth at least once, as was distinctly indicated by the self-recording barometers; it ispossible, indeed, that, crossing each other in their east and westcourses, these atmospheric tides twice girdled the sphere. In effect, the air over the crater was heaved up to the height of some tens ofthousands of feet, and thence rolled off in great circular waves, suchas may be observed in a pan of milk when a sharp blow pushes thebottom upward. 

The violent stroke delivered to the waters of the sea created a vastwave, which in the region where it originated rolled upon the shoreswith a surf wall fifty or more feet high. In a few minutes aboutthirty thousand people were overwhelmed. The wave rolled on beyond itsdestructive limits much in the manner of the tide; its influence wasfelt in a sharp rise and fall of the waters as far as the Pacificcoast of North America, and was indicated by the tide gauges in theAtlantic as far north as the coast of Europe. 

Owing to the violence of the eruption, Krakatoa poured forth no lava, but the dust and ashes which ascended into the air--or, inother words, the finely divided lava which escaped into theatmosphere--probably amounted in bulk to more than twenty cubic miles. The coarser part of this material, including much pumice, fell uponthe seas in the vicinity, where, owing to its lightness, it was freeto drift in the marine currents far and wide throughout the oceanicrealm. The finer particles, thrown high into the air, perhaps to theheight of nearly a hundred thousand feet--certainly to the elevationof more than half this amount--drifted far and wide in theatmosphere, so that for years the air of all regions was clouded byit, the sunrise and sunset having a peculiar red glow, which the dustparticles produce by the light which they reflect. In this period, atall times when the day was clear, the sun appeared to be surrounded bya dusky halo. In time the greater part of this dust was drawn down bygravity, some portion of it probably falling on every square foot ofthe earth. Since the disappearance of the characteristic phenomenawhich it produced in the atmosphere, European observers have noted theexistence of faint clouds lying in the upper part of the air at theheight of a hundred miles or more above the surface. These clouds, which were at first distinctly visible in the earliest stage of dawnand in the latest period of the sunset glow, seemed to be in rapidmotion to the eastward, and to be mounting higher above the earth. Ithas been not unreasonably supposed that these shining clouds representportions of the finest dust from Krakatoa, which has been thrown sofar above the earth's attraction that it is separating itself from thesphere. If this view be correct, it seems likely that we may look togreat volcanic explosions as a source whence the dustlike particleswhich people the celestial spaces may have come. They may, in a word, be due to volcanic explosions occurring on this and other celestialspheres. 

The question suggested above as to the possibility of volcanicejections throwing matter from the earth beyond the control of itsgravitative energy is one of great scientific interest. Computations(not altogether trustworthy) show that a body leaving the earth'ssurface under the conditions of a cannon ball fired vertically upwardwould have to possess a velocity at the start of at least seven milesa second in order to go free into space. It would at first sight seemthat we should be able to reckon whether volcanoes can propel earthmatter upward with this speed. In fact, however, sufficient data arenot obtainable; we only know in a general way that the column ofvapour rises to the height of thirty or forty thousand feet, and thisin eruptions of no great magnitude. In an accident such as that atKrakatoa, even if an observer were near enough to see clearly what wasgoing on, the chance of his surviving the disturbance would be small. Moreover, the ascending vapours, owing to their expansion of the steamin the column, begin to fly out sideways on its periphery, so that theupper part of the central section in the discharge is not visible fromthe earth. 

It is in the central section of the uprushing mass, if anywhere, thatthe dust might attain the height necessary to put it beyond theearth's attraction, bringing it fairly into the realm of the solarsystem, or to the position where its own motion and the attraction ofthe other spheres would give it an independent orbital movement aboutthe sun, or perhaps about the earth. We can only say that observationson the height of volcanic ejections are extremely desirable; they canprobably only be made from a balloon. An ascension thus made beyondthe cloud disk which the eruption produces might bring the observerwhere he could discern enough to determine the matter. Although themovements of the rocky particles could not be observed, the colourwhich they would give to the heavens might tell the story which wewish to know. There is evidence that large masses of stone hurled upby volcanic eruption have fallen seven miles from the base of thecone. Assuming that the masses went straight upward at the beginningof their ascent, and that they were afterward borne outwardly by theexpansion of the column, computations which have a general but noabsolute value appear to indicate that the masses attained a height offrom thirty to fifty miles, and had an initial velocity which, ifdoubled, might have carried them into space. 

Last of all, we shall note the conditions which attend the eruptionsof submarine volcanoes. Such explosions have been observed in but afew instances, and only in those cases where there is reason tobelieve that the crater at the time of its explosion had attained towithin a few hundred feet of the sea level. In these cases theejections, never as yet observed in the state of lava, but in thecondition of dust and pumice, have occasionally formed a low island, which has shortly been washed away by the waves. Knowing as we do thatvolcanoes abound on the sea floor, the question why we do not oftenersee their explosions disturbing the surface of the waters is veryinteresting, but not as yet clearly explicable. It is possible, however, that a volcanic discharge taking place at the depth ofseveral thousand feet below the surface of the water would not be ableto blow the fluid aside so as to open a pipe to the surface, but wouldexpend its energy in a hidden manner near the ocean floor. The vapourswould have to expand gradually, as they do in passing up through therock pipe of a volcano, and in their slow upward passage might beabsorbed by the water. The solid materials thrown forth would in thiscase necessarily fall close about the vent, and create a very steepcone, such, indeed, as we find indicated by the soundings off certainvolcanic islands which appear only recently to have overtopped thelevel of the waters. 

As will be seen, though inadequately from the diagrams of Vesuvius, volcanic cones have a regularity and symmetry of form far exceedingthat afforded by the outlines of any other of the earth's features. Where, as is generally the case, the shape of the cone is determinedby the distribution of the falling cinders or divided lava whichconstitutes the mass of most cones, the slope is in general that knownas a catenary curve--i. E. , the line formed by a chain hanging betweentwo points at some distance from the vertical. It is interesting tonote that this graceful outline is a reflection or consequence of thecurve described by the uprushing vapour. The expansion in theascending column causes it to enlarge at a somewhat steadfast rate, while the speed of the ascent is ever diminishing. Precisely the sameaction can be seen in the like rush of steam and other gases andvapours from the cannon's mouth; only in the case of the gun, even ofthe greatest size, we can not trace the movement for more than a fewhundred feet. In this column of ejection the outward movement from thecentre carries the bits of lava outwardly from the centre of theshaft, so that when they lose their ascending velocity they are drawndownward upon the flanks of the cone, the amount falling upon eachpart of that surface being in a general way proportional to thethickness of the vaporous mass from which they descend. The result is, that the thickest part of the ash heap is formed on the upper part ofthe crater, from which point the deposit fades away in depth in everydirection. In a certain measure the concentration toward the centre ofthe cone is brought about by the draught of air which moves in towardthe ascending column. 

Although, in general, ejections of volcanic matter take place throughcones, that being the inevitable form produced by the escaping steam, very extensive outpourings of lava, ejections which in mass probablyfar exceed those thrown forth through ordinary craters, areoccasionally poured out through fissures in the earth's crust. Thus inOregon, Idaho, and Washington, in eastern Europe, in southern India, and at some other points, vast flows, which apparently took place fromfissures, have inundated great realms with lava ejections. Theconditions which appear to bring about these fissure eruptions of lavaare not yet well understood. A provisional and very probable accountof the action can be had in the hypothesis which will now be setforth. 

Where any region has been for a long time the seat of volcanic action, it is probable that a large amount of rock in a more or less fluidcondition exists beneath its surface. Although the outrushing steamejects much of this molten material, there are reasons to suppose thata yet greater part lies dormant in the underground spaces. Thus in thecase of Ætna we have seen that, though some thousands of miles ofrock matter have come forth, the base of the cone has been uplifted, probably by the moving to that region of more or less fluid rock. Ifnow a region thus underlaid by what we may call incipient lavas issubjected to the peculiar compressive actions which lead tomountain-building, we should naturally expect that such soft materialwould be poured forth, possibly in vast quantities through faultfissures, which are so readily formed in all kinds of rock whensubject to irregular and powerful strains, such as are necessarilybrought about when rocks are moved in mountain-making. The greateruptions which formed the volcanic table-lands on the west coast ofNorth America appear to have owed the extrusion of their materials tomountain-building actions. This seems to have been the case also insome of those smaller areas where fissure flows occur in Europe. It islikely that this action will explain the greater part of these massiveeruptions. 

It need not be supposed that the rock beneath these countries, whichwhen forced out became lava, was necessarily in the state of perfectfluidity before it was forced through the fissures. Situated at greatdepth in the earth, it was under a pressure so great that itsparticles may have been so brought together that the material wasessentially solid, though free to move under the great strains whichaffected it, and acquiring temperature along with the fluidity whichheat induces as it was forced along by the mountain-building pressure. As an illustration of how materials may become highly heated whenforced to move particle on particle, it may be well to cite the casein which the iron stringpiece on top of a wooden dam near Holyoke, Mass. , was affected when the barrier went away in a flood. The ironstringer, being very well put together, was, it is said, drawn out bythe strain until it became sensibly reddened by the motion of itsparticles, and finally fell hissing into the waters below. A likeheating is observable when metal is drawn out in making wire. Thus amass of imperfectly fluid rock might in a forced journey of a fewmiles acquire a decided increase of temperature. 

Although the most striking volcanic action--all such phenomena, indeed, as commonly receives the name--is exhibited finally on theearth's surface, a great deal of work which belongs in the same groupof geological actions is altogether confined to the deep-lying rock, and leads to the formation of dikes which penetrate the strata, but donot rise to the open air. We have already noted the fact that dikesabound in the deeper parts of volcanic cones, though the fissures intowhich they find their way are seldom riven up to the surface. In thesame way beneath the ground in non-volcanic countries we may discoverat a great depth in the older, much-changed rock a vast number ofthese crevices, varying from a few inches to a hundred feet or more inwidth, which have been filled with lavas, the rock once molten havingafterward cooled. In most cases these dikes are disclosed to usthrough the down-wearing of the earth that has removed the beds intowhich the dikes did not penetrate, thus disclosing the realm in whichthe disturbances took place. 

Where, as is occasionally the case in deep mines, or on some barerocky cliff of great height, we can trace a dike in its upward coursethrough a long distance, we find that we can never distinctly discoverthe lower point of its extension. No one has ever seen in a clear waythe point of origin of such an injection. We can, however, oftenfollow it upward to the place where there was no longer a rift intowhich it could enter. In its upward path the molten matter appearsgenerally to have followed some previously existing fracture, a jointplane or a fault, which generally runs through the rocks on thoseplanes. We can observe evidence that the material was in the state ofigneous fluidity by the fact that it has baked the country rocks oneither side of the fissure, the amount of baking being in proportionto the width of the dike, and thus to the amount of heat which itcould give forth. A dike six inches in diameter will sometimes barelysear its walls, while one a hundred feet in width will often alter thestrata for a great distance on either side. In some instances, as inthe coal beds near Richmond, Va. , dikes occasionally cut through bedsof bituminous coal. In these cases we find that the coal has beenconverted into coke for many feet either side of a considerableinjection. The fact that the dike material was molten is still furthershown by the occurrence in it of fragments which it has taken up fromthe walls, and which may have been partly melted, and in most caseshave clearly been much heated. 

Where dikes extend up through stratified beds which are separated fromeach other by distinct layers, along which the rock is not firmlybound together, it now and then happens, as noted by Mr. G. K. Gilbert, of the United States Geological Survey, that the lava has forced itsway horizontally between these layers, gradually uplifting theoverlying mass, which it did not break through, into a dome-shapedelevation. These side flows from dikes are termed laccolites, a wordwhich signifies the pool-like nature of the stony mass which they formbetween the strata. 

In many regions, where the earth has worn down so as to reveal thezone of dikes which was formed at a great depth, the surface of thecountry is fairly laced with these intrusions. Thus on Cape Ann, arocky isle on the east coast of Massachusetts, having an area of abouttwenty square miles, the writer, with the assistance of his colleague, Prof. R. S. Tarr, found about four hundred distinct dikes exhibited onthe shore line where the rocks had been swept bare by the waves. Ifthe census of these intrusions could have been extended over the wholeisland, it would probably have appeared that the total number exceededfive thousand. In other regions square miles can be found where thedikes intercepted by the surface occupy an aggregate area greater thanthat of the rocks into which they have been intruded. 

Now and then, but rarely, the student of dikes finds one where thebordering walls, in place of having the clean-cut appearance whichthey usually exhibit, has its sides greatly worn away and much melted, as if by the long-continued passage of the igneous fluid through thecrevice. Such dikes are usually very wide, and are probably the pathsthrough which lavas found their way to the surface of the earth, pouring forth in a volcanic eruption. In some cases we can trace theirrelation to ancient volcanic cones which have worn down in all theirpart which were made up of incoherent materials, so that there remainsonly the central pipe, which has been preserved from decay by thecoherent character of the lava which filled it. 

The hypothesis that dikes are driven upward into strata by thepressure of the beds which overlie materials hot and soft enough to beput in motion when a fissure enters them, and that their movementupward through the crevice is accounted for by this pressure, makescertain features of these intrusions comprehensible. Seeing that verylong, slender dikes are found penetrating the rock, which could nothave had a high temperature, it becomes difficult to understand howthe lava could have maintained its fluidity; but on the suppositionthat it was impelled forward by a strong pressure, and that the energythus transmitted through it was converted into heat, we discover ameans whereby it could have been retained in the liquid condition, even when forced for long distances through very narrow channels. Moreover, this explanation accounts for the fact which has longremained unexplained that dikes, except those formed about volcaniccraters, rarely, if ever, rise to the surface. 

The materials contained in dikes differ exceedingly in their chemicaland mineral character. These variations are due to the differences inNature of the deposits whence they come, and also in a measure toexchanges which take place between their own substance and that of therocks between which they are deposited. This process often hasimportance of an economic kind, for it not infrequently leads to theformation of metalliferous veins or other aggregations of ores, eitherin the dike itself or in the country rock. The way in which this isbrought about may be easily understood by a familiar example. If fleshbe placed in water which has the same temperature, no exchange ofmaterials will take place; but if the water be heated, a circulationwill be set up, which in time will bring a large part of the solublematter into the surrounding water. This movement is primarilydependent on differences of temperature, and consequently differencesin the quantity of soluble substances which the water seeks to takeup. When a dike is injected into cooler rocks, such a slow circulationis induced. The water contained in the interstices of the stonebecomes charged with mineral materials, if such exist in positionswhere it can obtain possession of them, and as cooling goes on, thesedissolved materials are deposited in the manner of veins. These veinsare generally laid down on the planes of contact between the two kindsof stone, but they may be formed in any other cavities which exist inthe neighbourhood. The formation of such veins is often aided by theconsiderable shrinkage of the lava in the dike, which, when it cools, tends to lose about fifteen per cent of its volume, and is thus likelyto leave a crevice next the boundary walls. Ores thus formed affordsome of the commonest and often the richest mineral deposits. AtLeadville, in Colorado, the great silver-bearing lodes probably wereproduced in this manner, wherein lavas, either those of dikes or thosewhich flowed in the open air, have come in contact with limestones. The mineral materials originally in the once molten rock or in thelimy beds was, we believe, laid down on ancient sea floors in theremains of organic forms, which for their particular uses took thematerials from the old sea water. The vein-making action has served toassemble these scattered bits of metal into the aggregation whichconstitutes a workable deposit. In time, as the rocks wear down, thematerials of the veins are again taken into solution and returned tothe sea, thence perhaps to tread again the cycle of change. 

In certain dikes, and sometimes also, perhaps, in lavas known asbasalts, which have flowed on the surface, the rock when cooling, fromthe shrinkage which then occurs, has broken in a very regular way, forming hexagonal columns which are more or less divided on theirlength by joints. When worn away by the agencies of decay, especiallywhere the material forms steep cliffs, a highly artificial effect isproduced, which is often compared, where cut at right angles to thecolumns, to pavements, or, where the division is parallel to thecolumns, to the pipes of an organ. 

What we know of dikes inclines us to the opinion that as a whole theyrepresent movements of softened rock where the motion-compelling agentis not mainly the expansion of the contained water which gives rise tovolcanic ejection, but rather in large part due to the weight ofsuperincumbent strata setting in motion materials which were somewhatsoftened, and which tended to creep, as do the clays in deep coalmines. It is evident, however; it is, moreover, quite natural, thatdike work is somewhat mingled with that produced by the volcanicforces; but while the line between the two actions is not sharp, thediscrimination is important, and occurs with a distinctness ratherunusual on the boundary line between two adjacent fields of phenomena. 

 * * * * *

We have now to consider the general effects of the earth's interiorheat so far as that body of temperature tends to drive materials fromthe depths of the earth to the surface. This group of influences isone of the most important which operates on our sphere; as we shallshortly see, without such action the earth would in time become anunfit theatre for the development of organic life. To perceive theeffect of these movements, we must first note that in the greatrock-constructing realm of the seas organic life is constantlyextracting from the water substances, such as lime, potash, soda, anda host of other substances necessary for the maintenance ofhigh-grade organisms, depositing these materials in the growingstrata. Into these beds, which are buried as fast as they form, goesnot only these earthy materials, but a great store of the sea water aswell. The result would be in course of time a complete withdrawal intothe depths of the earth of those substances which play a necessarypart in organic development. The earth would become more or lesscompletely waterless on its surface, and the rocks exposed to viewwould be composed mainly of silica, the material which to a greatextent resists solution, and therefore avoids the dissolving whichovertakes most other kinds of rocks. Here comes in the machinery ofthe hot springs, the dikes, and the volcanoes. These agents, operatingunder the influence of the internal heat of the earth, are constantlyengaged in bearing the earthy matter, particularly its precious moresolvent parts, back to the surface. The hot springs and volcanoes workswiftly and directly, and return the water, the carbon dioxide, and ahost of other vaporizable and soluble and fusible substances to therealm of solar activity, to the living surface zone of the earth. Thedikes operate less immediately, but in the end to the same effect. They lift their materials miles above the level where they wereoriginally laid, probably from a zone which is rarely if ever exposedto view, placing them near the surface, where the erosive agents canreadily find access to them. 

Of the three agents which serve to export earth materials from itsdepths, volcanoes are doubtless the most important. They send forththe greater part of the water which is expelled from the rocks. Various computations which the writer has made indicate that anordinary volcano, such as Ætna, in times of most intense explosion, may send forth in the form of steam one fourth of a cubic mile ormore of water during each day of its discharge, and in a single greateruption may pour forth several times this quantity. In its historyÆtna has probably returned to the atmosphere some hundred cubic milesof water which but for the process would have remained permanentlylocked up in its rock prison. 

The ejection of rock material, though probably on the average less inquantity than the water which escapes, is also of noteworthyimportance. The volcanoes of Java and the adjacent isles have, duringthe last hundred and twenty years, delivered to the seas more earthmaterial than has been carried into those basins by the great rivers. If we could take account of all the volcanic ejections which haveoccurred in this time, we should doubtless find that the sum of thematerials thus cast forth into the oceans was several times as greatas that which was delivered from the lands by all the superficialagents which wear them away. Moreover, while the material from theland, except the small part which is in a state of complete solution, all falls close to the shore, the volcanic waste, because of its finedivision or because of the blebs of air which its masses contain, mayfloat for many years before it finds its way to the bottom, it may beat the antipodes of the point at which it came from the earth. Whilethus journeying through the sea the rock matter from the volcanoes isapt to become dissolved in water; it is, indeed, doubtful if anyconsiderable part of that which enters the ocean goes by gravitationto its floor. The greater portion probably enters the state ofsolution and makes its way thence through the bodies of plants andanimals again into the ponderable state. 

If an observer could view the earth from the surface of the moon, hewould probably each day behold one of these storms which the volcanoessend forth. In the fortnight of darkness, even with the naked eye, itwould probably be possible to discern at any time several eruptions, some of which would indicate that the earth's surface was ravaged bygreat catastrophes. The nearer view of these actions shows us thatalthough locally and in small measure they are harmful to the life ofthe earth, they are in a large way beneficent. 

 CHAPTER VIII. 

 THE SOIL. 

The frequent mention which it has been necessary to make of soilphenomena in the preceding chapters shows how intimately this featurein the structure of the earth is blended with all the elements of itsphysical history. It is now necessary for us to take up the phenomenaof soils in a consecutive manner. 

The study of any considerable river basin enables us to trace the moreimportant steps which lead to the destructure and renovation of theearth's detrital coating. In such an interpretation we note thateverywhere the rocks which were built on the sea bottom, and more orless made over in the great laboratory of the earth's interior, are atthe surface, when exposed to the conditions of the atmosphere, inprocess of being taken to pieces and returned to the sea. This actiongoes on everywhere; every drop of rain helps it. It is aided by frost, or even by the changes of expansion and contraction which occur in therocks from variations of heat. The result is that, except where theslopes are steep, the surface is quickly covered with a layer offragments, all of which are in the process of decay, and ready toafford some food to plants. Even where the rock appears bare, it isgenerally covered with lichens, which, adhering to it, obtain a shareof nutriment from the decayed material which they help to hold on theslope. When they have retained a thin sheet of the _débris_, mossesand small flowering plants help the work of retaining the detritus. Soon the strong-rooted bushes and trees win a foothold, and by sendingtheir rootlets, which are at first small but rapidly enlarge, into thecrevices, they hasten the disruption of the stones. 

If the construction of soil goes on upon a steep cliff, the quantityretained on the slope may be small, but at the base we find a talus, composed of the fragments not held by the vegetation, which graduallyincreases as the cliff wears down, until the original precipice may bequite obliterated beneath a soil slope. At first this process israpid; it becomes gradually slower and slower as the talus mounts upthe cliff and as the cliff loses its steepness, until finally a gentleslope takes the place of the steep. 

From the highest points in any river valley to the sea level thebroken-up rock, which we term soil, is in process of continuousmotion. Everywhere the rain water, flowing over the surface or soakingthrough the porous mass, is conveying portions of the material whichis taken into solution in a speedy manner to the sea. Everywhere theexpansion of the soil in freezing, or the movements imposed on it bythe growth of roots, by the overturning of trees, or by theinnumerable borings and burrowings which animals make in the mass, isthrough the action of gravitation slowly working down the slope. Everylittle disturbance of the grains or fragments of the soil which liftsthem up causes them when they fall to descend a little way farthertoward the sea level. Working toward the streams, the materials of thesoil are in time delivered to those flowing waters, and by them urgedspeedily, though in most cases interruptedly, toward the ocean. 

There is another element in the movement of the soils which, thoughless appreciable, is still of great importance. The agents of decaywhich produce and remove the detritus, the chemical changes of the bedrock, and the mechanical action which roots apply to them, along withthe solutional processes, are constantly lowering the surface of themass. In this way we can often prove that a soil continuouslyexisting has worked downward through many thousand feet of strata. Inthis process of downgoing the country on which the layer rests mayhave greatly changed its form, but the deposit, under favourableconditions, may continue to retain some trace of the materials whichit derived from beds which have long since disappeared, their positionhaving been far up in the spaces now occupied by the air. Where theslopes are steep and streams abound, we rarely find detritus whichbelonged in rock more than a hundred feet above the present surface ofthe soil. Where, however, as on those isolated table-lands or butteswhich abound in certain portions of the Mississippi Valley, as well asin many other countries, we find a patch of soil lying on a nearlylevel surface, which for geologic ages has not felt the effect ofstreams, we may discover, commingled in the _débris_, the harderwreckage derived from the decay of a thousand feet or more of vanishedstrata. 

When we consider the effect of organic life on the processes which goon in the soil, we first note the large fact that the development ofall land vegetation depends upon the existence of this detritus--in aword, on the slow movement of the decaying rocky matter from the pointwhere it is disrupted to its field of rest in the depths of the sea. The plants take their food from the portion of this rocky waste whichis brought into solution by the waters which penetrate the mass. Onthe plants the animals feed, and so this vast assemblage of organismsis maintained. Not only does the land life maintain itself on thesoil, and give much to the sea, but it serves in various ways toprotect this detrital coating from too rapid destruction, and toimprove its quality. To see the nature of this work we should visit aregion where primeval forests still lie upon the slopes of a hillyregion. In the body of such a wood we find next the surface a coatingof decayed vegetable matter, made up of the falling leaves, bark, branches, and trunks which are constantly descending to the earth. Ordinarily, this layer is a foot or more in thickness; at the top itis almost altogether composed of vegetable matter; at the bottom itverges into the true soil. An important effect of this decayedvegetation is to restrain the movement of the surface water. Even inthe heaviest rains, provided the mass be not frozen, the water istaken into it and delivered in the manner of springs to the largerstreams. We can better note the measure of this effect by observingthe difference in the ground covered by this primeval forest and thatwhich we find near by which has been converted into tilled fields. With the same degree of rapidity in the flow, the distinct streamchannels on the tilled ground are likely to be from twenty to ahundred times in length what they are on the forest bed. The result isthat while the brook which drains the forested area maintains atolerably constant flow of clean water, the other from the tilledground courses only in times of heavy rain, and then is heavilycharged with mud. In the virgin conditions of the soil the downwear isvery slow; in its artificial state this wearing goes on so rapidlythat the sloping fields are likely to be worn to below the soil levelin a few score years. 

Not only does the natural coating of vegetation, such as our forestsimpose upon the country, protect the soil from washing away, but theroots of the larger plants are continually at work in various ways toincrease the fertility and depth of the stratum. In the form ofslender fibrils these underground branches enter the joints and bedplanes of the rock, and there growing they disrupt the materials, giving them a larger surface on which decay may operate. These bits, at first of considerable size, are in turn broken up by the sameaction. Where the underlying rocks afford nutritious materials, thebranches of our tap-rooted trees sometimes find their way ten feet ormore below the base of the true soil. Not only do they thus break upthe stones, but the nutrition which they obtain in the depths isbrought up and deposited in the parts above the ground, as well as inthe roots which lie in the true soil, so that when the tree dies itbecomes available for other plants. Thus in the forest condition of acountry the amount of rock material contributed to the deposit ingeneral so far exceeds that which is taken away to the rivers by theunderground water as to insure the deepening of the soil bed to thepoint where only the strongest roots--those belonging to ourtap-rooted trees--can penetrate through it to the bed rocks. 

Almost all forests are from time to time visited by winds which uprootthe trees. When they are thus rent from the earth, the undergroundbranches often form a disk containing a thick tangle of stones andearth, and having a diameter of ten or fifteen feet. The writer hasfrequently observed a hundred cubic feet of soil matter, some of ittaken from the depth of a yard or more, thus uplifted into the air. Inthe path of a hurricane or tornado we may sometimes find thousands ofacres which have been subjected to this rude overturning--a naturalploughing. As the roots rot away, the _débris_ which they held fallsoutside of the pit, thus forming a little hillock along the side ofthe cavity. After a time the thrusting action of other roots and theslow motion of the soil down the slope restore the surface from itshillocky character to its original smoothness; but in many cases thenaturalist who has learned to discern with his feet may note theseirregularities long after it has been recovered with the forest. 

Great as is the effect of plants on the soil, that influence is almostequalled by the action of the animals which have the habit of enteringthe earth, finding there a temporary abiding place. The number ofthese ground forms is surprisingly great. It includes, indeed, a hostof creatures which are efficient agents in enriching the earth. Thespecies of earthworms, some of which occupy forested districts as wellas the fields, have the habit of passing the soil material throughtheir bodies, extracting from the mass such nutriment as it maycontain. In this manner the particles of mineral matter becomepulverized, and in a measure affected by chemical changes in thebodies of the creatures, and are thus better fitted to afford plantfood. Sometimes the amount of the earth which the creatures take in inmoving through their burrows and void upon the surface is sufficientto form annually a layer on the surface of the ground having a depthof one twentieth of an inch or more. It thus may well happen that thesoil to the depth of two or three feet is completely overturned in thecourse of a few hundred years. As the particles which the creaturesdevour are rather small, the tendency is to accumulate the finerportions of the soil near the surface of the earth, where by solutionthey may contribute to the needs of the lowly plants. It is probablydue to the action of these creatures that small relics of ancient men, such as stone tools, are commonly found buried at a considerable depthbeneath the earth, and rarely appear upon the surface except where ithas been subjected to deep ploughing or to the action of runningstreams. 

Along with the earthworms, the ants labour to overturn the soil;frequently they are the more effective of the two agents. The commonspecies, though they make no permanent hillocks, have been observed bythe writer to lay upon the surface each year as much as a quarter ofan inch of sand and other fine materials which they have brought upfrom a considerable depth. In many regions, particularly in thoseoccupied by glacial drift, and pebbly alluvium along the rivers, theeffect of this action, like that of earthworms, is to bring to thesurface the finer materials, leaving the coarser pebbles in thedepths. In this way they have changed the superficial character of thesoil over great areas; we may say, indeed, over a large part of theearth, and this in a way which fits it better to serve the needs ofthe wild plants as well as the uses of the farmer. 

Many thousand species of insects, particularly the larger beetles, have the habit of passing their larval state in the under earth. Herethey generally excavate burrows, and thus in a way delve the soil. Asmany of them die before reaching maturity, their store of organicmatter is contributed to the mass, and serves to nourish the plants. If the student will carefully examine a section of the earth either inits natural or in its tilled state, he will be surprised to find hownumerous the grubs are. They may often be found to the number of ascore or more of each cubic foot of material. Many of the specieswhich develop underground come from eggs which have carefully beenencased in organic matter before their deposition in the earth. Thussome of the carrion beetles are in the habit of laying their eggs inthe bodies of dead birds or field mice, which they then bury to thedepth of some inches in the earth. In this way nearly all the smallbirds and mammals of our woods disappear from view in a few hoursafter they are dead. Other species make balls from the dung of cattlein which they lay their eggs, afterward rolling the little spheres, itmay be for hundreds of feet, to the chambers in the soil which theyhave previously prepared. In this way a great deal of animal matter isintroduced into the earth, and contributes to its fertility. 

Many of our small mammals have the habit of making their dwellingplaces in the soil. Some of them, such as the moles, normally abide inthe subterranean realm for all their lives. Others use the excavationsas places of retreat. In any case, these excavations serve to move theparticles of the soil about, and the materials which the animals draginto the earth, as well as the excrement of the creatures, act toenrich it. This habit of taking food underground is not limited to themammals; it is common with the ants, and even the earthworms, as notedby Charles Darwin in his wonderful essay on these creatures, areaccustomed to drag into their burrows bits of grass and the slenderleaves of pines. It is not known what purpose they attain by theseactions, but it is sufficiently common somewhat to affect theconditions of the soil. 

The result of these complicated works done by animals and plants onthe soil is that the material to a considerable depth are constantlybeing supplied with organic matter, which, along with the mineralmaterial, constitutes that part of the earth which can supportvegetation. Experiment will readily show that neither crushed rock norpure vegetable mould will of itself serve to maintain any but thelowliest vegetation. It requires that the two materials be mixed inorder that the earth may yield food for ordinary plants, particularlyfor those which are of use to man, as crops. On this account all theprocesses above noted whereby the waste of plant and animal life iscarried below the surface are of the utmost importance in the creationand preservation of the soil. It has been found, indeed, in almost allcases, necessary for the farmer to maintain the fertility of hisfields to plough-in quantities of such organic waste. By so doing heimitates the work which is effected in virgin soil by natural action. As the process is costly in time and material, it is often neglectedor imperfectly done, with the result that the fields rapidly diminishin fertility. 

The way in which the buried organic matter acts upon the soil is notyet thoroughly understood. In part it accomplishes the results by thematerials which on its decay it contributes to the soil in a state inwhich they may readily be dissolved and taken up by the roots intotheir sap; in part, however, it is believed that they better theconditions by affording dwelling places for a host of lowly species, such as the forms which are known as bacteria. The organisms probablyaid in the decomposition of the mineral matter, and in the conversionof nitrogen, which abounds in the air or the soil, into nitrates ofpotash and soda--substances which have a very great value asfertilizers. Some effect is produced by the decay of the foreignmatter brought into the soil, which as it passes away leaves channelsthrough which the soil water can more readily pass. 

By far the most general and important effect arising from the decayof organic matter in the earth is to be found in the carbon dioxidewhich is formed as the oxygen of the air combines with the carbonwhich all organic material contains. As before noted, water thuscharged has its capacity for taking other substances into solutionvastly increased, and on this solvent action depends in large part thedecay of the bed rocks and the solution of materials which are to beappropriated by the plants. 

Having now sketched the general conditions which lead to the formationof soils, we must take account of certain important variations intheir conditions due to differences in the ways in which they areformed and preserved. These matters are not only of interest to thegeologist, but are of the utmost importance to the life of mankind, aswell as all the lower creatures which dwell upon the lands. First, weshould note that soils are divisible into three great groups, which, though not sharply parted from each other, are sufficiently peculiarfor the purposes of classification. Where the earth material has beenderived from the rocks which nearly or immediately underlie it, wehave a group of soils which may be entitled those of immediatederivation--that is, derived from rocks near by, or from beds whichonce overlaid the level and have since been decayed away. Next, wehave alluvial soils, those composed of materials which have beentransported by streams, commonly from a great distance, and laid downon their flood plains. Third, the soils the mineral matters of whichhave been brought into their position by the action of glaciers; thesein a way resemble those formed by rivers, but the materials aregenerally imperfectly sorted, coarse and fine being mingled together. Last of all, we have the soils due to the accumulation of blown dustor blown sand, which, unlike the others, occupy but a small part ofthe land surface. It would be possible, indeed, to make yet anotherdivision, including those areas which when emerging from the sea werecovered with fine, uncemented detritus ready at once to serve thepurposes of a soil. Only here and there, and but seldom, do we findsoils of this nature. 

It is characteristic of soils belonging to the group to which we havegiven the title of immediate derivation that they have accumulatedslowly, that they move very gradually down the slopes on which theylie, and that in all cases they represent, with a part of their massat least, levels of rock which have disappeared from the region whichthey occupied. The additions made to their mass are from below, andthat mass is constantly shrinking, generally at a pretty rapid rate, by the mineral matter which is dissolved and goes away with the springwater. They also are characteristically thin on steep slopes, thickening toward the base of the incline, where the diminished gradepermits the soil to move slowly, and therefore to accumulate. 

In alluvial soils we find accumulations which are characterized bygrowth on their upper surfaces, and by the distant transportation ofthe materials of which they are composed. In these deposits theoutleaching removes vast amounts of the materials, but so long as thefloods from time to time visit their surfaces the growth of thedeposits is continued. This growth rarely takes place from the wasteof the bed rocks on which the alluvium lies. It is characteristic ofalluvial soils that they are generally made up of _débris_ derivedfrom fields where the materials have undergone the change which wehave noted in the last paragraph; therefore these latter deposits havethroughout the character which renders the mineral materials easilydissolved. Moreover, the mass as it is constructed is commonly mingledwith a great deal of organic waste, which serves to promote itsfertility. On these accounts alluvial grounds, though they varyconsiderably in fertility, commonly afford the most fruitful fields ofany region. They have, moreover, the signal advantage that they oftenmay be refreshed by allowing the flood waters to visit them, anaction which but for the interference of man commonly takes place onceeach year. Thus in the valley of the Nile there are fields which havebeen giving rich grain harvests probably for more than four thousandyears, without any other effective fertilizing than that derived fromthe mud of the great river. 

The group of glaciated soils differs in many ways from either of thosementioned. In it we find the mineral matter to have been broken up, transported, and accumulated without the influence of those conditionswhich ordinarily serve to mix rock _débris_ with organic matter duringthe process by which it is broken into bits. When vegetation came topreoccupy the fields made desolate by glacial action, it found in mostplaces more than sufficient material to form soils, but the greaterpart of the matter was in the condition of pebbles of very hard rockand sand grains, fragments of silex. Fortunately, the broken-up stateof this material, by exposing a great surface of the rocky matter todecay, has enabled the plants to convert a portion of the mass intoearth fit for the uses of their roots. But as the time which haselapsed since the disappearance of the glaciers is much less than thatoccupied in the formation of ordinary soil, this decay has in mostcases not yet gone very far, so that in a cubic foot of glaciatedwaste the amount of material available for plants is often only afraction of that held in the soils of immediate derivation. 

In the greater portion of the fields occupied by glacial waste theprocesses which lead to the introduction of organic matter into theearth have not gone far enough to set in effective work the greatlaboratory which has to operate in order to give fertile soil. Thepebbles hinder the penetration of the roots as well as the movement ofinsects and other animals. There has not been time enough for theoverturning of trees to bring about a certain admixture of vegetablematter with the soil--in a word, the process of soil-making, thoughthe first condition, that of broken-up rock, has been accomplished, is as yet very incomplete. It needs, indeed, care in the introductionof organic matter for its completion. 

It is characteristic of glacial soils that they are indefinitely deep. This often is a disadvantageous feature, for the reason that the soilwater may pass so far down into the earth that the roots are oftendeprived of the moisture which they need, and which in ordinary soilsis retained near the surface by the hard underlayer. On the otherhand, where the glacial waste is made up of pebbles formed from rocksof varied chemical composition, which contain a considerable share oflime, potash, soda, and other substances which are required by plants, the very large surface which they expose to decay provides the soilwith a continuous enrichment. In a cubic foot of pebbly glacial earthwe often find that the mass offers several hundred times as muchsurface to the action of decay as is afforded by the underlying solidbed rock from which a soil of immediate derivation has to win itsmineral supply. Where the pebbly glacial waste is provided with amixture of vegetable matter, the process of decay commonly goesforward with considerable rapidity. If the supply of such matter islarge, such as may be produced by ploughing in barnyard manure orgreen crops, the nutritive value of the earth may be brought to a veryhigh point. 

It is a familiar experience in regions where glacial soils exist thatthe earth beneath the swamps when drained is found to beextraordinarily well suited for farming purposes. On inspecting thepebbles from such places, we observe that they are remarkably decayed. Where the masses contain large quantities of feldspar, as is the casein the greater part of our granitic and other crystalline rocks, thismaterial in its decomposition is converted into kaolin or feldsparclay, and gives the stones a peculiar white appearance, which marksthe decomposition, and indicates the process by which a great varietyof valuable soil ingredients are brought into a state where they maybe available for plants. 

In certain parts of the glacial areas, particularly in the region nearthe margin of the ice sheet, where the glacier remained in oneposition for a considerable time, we find extensive deposits ofsilicious sand, formed of the materials which settled from theunder-ice stream, near where they escaped from the glacial cavern. These kames and sand plains, because of the silicious nature of theirmaterials and the very porous nature of the soil which they afford, are commonly sterile, or at most render a profit to the tiller by dintof exceeding care. Thus in Massachusetts, although the first settlersseized upon these grounds, and planted their villages upon thembecause the forests there were scanty and the ground free fromencumbering boulders, were soon driven to betake themselves to thoseareas where the drift was less silicious, and where the pebblesafforded a share of clay. Very extensive fields of this sandy naturein southeastern New England have never been brought under tillage. Thus on the island of Martha's Vineyard there is a connected areacontaining about thirty thousand acres which lies in a very favourableposition for tillage, but has been found substantially worthless forsuch use. The farmers have found it more advantageous to clear awaythe boulders from the coarser drift in order to win soil which wouldgive them fair returns. 

Those areas which are occupied by soil materials which have beenbrought into their position by the action of the wind may, as regardstheir character, be divided into two very distinct groups--the dunesand loess deposits. In the former group, where, as we have noted (seepage 123), the coarse sea sands or those from the shores of lakes aredriven forward as a marching hillock, the grains of the material arealmost always silicious. The fragments in the motion are not taken upinto the air, but are blown along the surface. Such dune accumulationsafford an earth which is even more sterile than that of the glacialsand plains, where there is generally a certain admixture of pebblesfrom rocks which by their decomposition may afford some elements offertility. Fortunately for the interests of man, these wind-bornesands occupy but a small area; in North America, in the aggregate, there probably are not more than one thousand square miles of suchdeposits. 

Where the rock material drifted by the winds is so fine that it mayrise into the air in the form of dust, the accumulations made of itgenerally afford a fertile soil, and this for the reason that they arecomposed of various kinds of rock, and not, as in the case of dunes, of nearly pure silica. In some very rare cases, where the seashore isbordered by coral reefs, as it is in parts of southern Florida, andthe strand is made up of limestone bits derived from the hard partswhich the polyps secrete, small dunes are made of limy material. Owing, however, in part to the relatively heavy nature of thissubstance, as well as to the rapid manner in which its grains becomecemented together, such limestone dunes never attain great size nortravel any distance from their point of origin. 

As before noted, dust accumulations form the soil in extended areaswhich lie to the leeward of great deserts. Thus a considerable part ofwestern China and much of the United States to the west of theMississippi is covered by these wind-blown earths. Wherever therainfall is considerable these loess deposits have proved to have ahigh agricultural value. 

Where a region has an earth which has recently passed from beneath thesea or a great lake, the surface is commonly covered by incoherentdetritus which has escaped consolidation into hard rock by the factthat it has not been buried and thus brought into the laboratory ofthe earth's crust. When such a region becomes dry land, the materialsare immediately ready to enter into the state of soil. They commonlycontain a good deal of waste derived from the organic life whichdwelt upon the sea bottom and was embedded in the strata as they wereformed. Where these accumulations are made in a lake, the landvegetation at once possesses the field, even a single year beingsufficient for it to effect its establishment. Where the lands emergefrom the sea, it requires a few years for the salt water to drain awayso that the earth can be fit for the uses of plants. In a general waythese sea-bottom soils resemble those formed in the alluvial plains. They are, however, commonly more sandy, and their substances lesspenetrated by that decay which goes on very freely in the atmospherebecause of the abundant supply of oxygen, and but slowly on the seafloor. Moreover, the marine deposits are generally made up in largepart of silicious sand, a material which is produced in largequantities by the disruption of the rocks along the sea coast. Thelargest single field of these ocean-bottom soils of North America isfound in the lowland region of the southern United States, a wide beltof country extending along the coast from the Rio Grande to New York. Although the streams have channelled shallow valleys in the beds ofthis region, the larger part of its surface still has the peculiarfeatures of form and composition which were impressed upon it when itlay below the surface of the sea. 

Local variations in the character of the soil covering are exceedinglynumerous, and these differences of condition profoundly affect theestate of man. We shall therefore consider some of the more importantof these conditions, with special reference to their origin. 

The most important and distinctly marked variation in the fertility ofsoils is that which is produced by differences in the rainfall. Noparts of the earth are entirely lacking in rain, but over considerableareas the precipitation does not exceed half a foot a year. In suchrealms the soil is sterile, and the natural coating of vegetationlimited to those plants which can subsist on dew or which can take onan occasional growth at such times as moisture may come upon them. With a slight increase in precipitation, the soil rapidly increases inproductivity, so that we may say that where as much as about teninches of water enters the earth during the summer half of the year, it becomes in a considerable measure fit for agriculture. Observationsindicate that the conditions of fertility are not satisfied where therainfall is just sufficient to fill the pores of the soil; there mustbe enough water entering the earth to bring about a certain amount ofoutflow in the form of springs. The reason of this need becomesapparent when we study the evident features of those soils which, though from season to season charged with water, do not yield springs, but send the moisture away through the atmosphere. Wherever theseconditions occur we observe that the soil in dry seasons becomescoated with a deposit of mineral matter, which, because of its taste, has received the name of alkali. The origin of this coating is asfollows: The pores of the soil, charged from year to year withsufficient water to fill them, become stored with a fluid whichcontains a very large amount of dissolved mineral matter--too much, indeed, to permit the roots of plants, save a few species which havebecome accustomed to the conditions, to do their appointed work. Infact, this water is much like that of the sea, which the roots of onlya few of our higher plants can tolerate. When the dry season comes on, the heat of the sun evaporates the water at the surface, leavingbehind a coating composed of the substances which the water contains. The soil below acts in the manner of a lamp-wick to draw up fluid asrapidly as the heat burns it away. When the soil water is as far aspossible exhausted, the alkali coating may represent a considerablepart of the soluble matter of the soil, and in the next rainy seasonit may return in whole or in part to the under-earth, again to bedrawn in the manner before described to the upper level. It istherefore only when a considerable share of the ground water goesforth to the streams in each year that the alkaline materials are inquantity kept down to the point where the roots of our crop-givingplants can make due use of the soil. Where, in an arid region, theground can be watered from the enduring streams or from artificialreservoirs, the main advantage arising from the process is commonlyfound in the control which it gives the farmer in the amount of thesoil water. He can add to the rainfall sufficient to take away theexcess of mineral matter. When such soils are first brought undertillage it is necessary to use a large amount of water from thecanals, in order to wash away the old store of alkali. After that acomparatively small contribution will often keep the soil in excellentcondition for agriculture. It has been found, however, in theirrigated lands beside the Nile that where too much saving ispractised in the irrigation, the alkaline coating will appear where ithas been unknown before, and with it an unfitness of the earth to bearcrops. 

Although the crust of mineral matters formed in the manner abovedescribed is characteristic of arid countries, and in general peculiarto them, a similar deposit may under peculiar conditions be formed inregions of great rainfall. Thus on the eastern coast of New England, where the tidal marshes have here and there been diked from the seaand brought under tillage, the dissolved mineral matters of the soil, which are excessive in quantity, are drawn to the surface, forming acoating essentially like that which is so common in arid regions. Thewriter has observed this crust on such diked lands, having a thicknessof an eighth of an inch. In fact, this alkali coating representsmerely the extreme operation of a process which is going on in allsoils, and which contributes much to their fertility. When rain fallsand passes downward into the earth, it conveys the soluble matter to adepth below the surface, often to beyond the point where our ordinarycrop plants, such as the small grains, can have access to it, andthis for the reason that their roots do not penetrate deeply. When dryweather comes and evaporation takes place from the surface, the fluidis drawn up to the upper soil layer, and there, in process ofevaporation, deposits the dissolved materials which it contains. Thusthe mineral matter which is fit for plant food is constantly set inmotion, and in its movement passes the rootlets of the plants. It isprobably on this account--at least in part--that very wet weather isalmost as unfavourable to the farmer as exceedingly dry, the normalalternation in the conditions being, as is well known, best suited tohis needs. 

So long as the earth is subjected to conditions in which the rainfallmay bring about a variable amount of water in the superficial detritallayer, we find normal fruitful soils, though in their more aridconditions they may be fit for but few species of plants. When, byincreasing aridity, we pass to conditions where there is no tolerablypermanent store of water in the _débris_, the material ceases to havethe qualities of a soil, and becomes mere rock waste. At the otherextreme of the scale we pass to conditions where the water issteadfastly maintained in the interstices of the detritus, and thereagain the characteristic of the soil and its fitness for the uses ofland vegetation likewise disappear. In a word, true soil conditionsdemand the presence of moisture, but that in insufficient quantities, to keep the pores of the earth continually filled; where they are thusfilled, we have the condition of swamps. Between these extremes thelevel at which the water stands in the soil in average seasons iscontinually varying. In rainy weather it may rise quite to thesurface; in a dry season it may sink far down. As this water rises andfalls, it not only moves, as before noted, the soluble mineralmaterials, but it draws the air into and expels it from the earth witheach movement. This atmospheric circulation of the soil, as has beenproved by experiment, is of great importance in maintaining itsfertility; the successive charges of air supply the needs of themicroscopic underground creatures which play a large part in enrichingthe soil, and the direct effect of the oxygen in promoting decay islikewise considerable. A part of the work which is accomplished byoverturning the earth in tillage consists in this introduction of theair into the pores of the soil, where it serves to advance the actionswhich bring mineral matters into solution. 

[Illustration: _Mountain gorge, Himalayas, India. Note the differencein the slope of the eroded rocks and the effect of erosion upon them;also the talus slopes at the base of the cliffs which the torrent iscutting away. On the left of the foreground there is a little benchshowing a recent higher line of the water. _]

In the original conditions of any country which is the seat ofconsiderable rainfall, and where the river system is not so fardeveloped as to provide channels for the ready exit of the waters, wecommonly find very extensive swamps; these conditions of bad drainagealmost invariably exist where a region has recently been elevatedabove the level of the sea, and still retains the form of an irregularrolling plain common to sea floors, and also in regions where the workdone by glaciers has confused the drainage which the antecedentstreams may have developed. In an old, well-elaborated river systemswamps are commonly absent, or, if they occur, are due to localaccidents of an unimportant nature. 

For our purpose swamps may be divided into three groups--climbingbogs, lake bogs, and marine marshes. The first two of these groupsdepend on the movements of the rain water over the land; the third onthe action of the tides. Beginning our account with the first and mostexceptional of these groups, we note the following features in theirinteresting history:

Wherever in a humid region, on a gentle slope--say with an inclinationnot exceeding ten feet to the mile--the soil is possessed by anyspecies of plants whose stems grow closely together, so that fromtheir decayed parts a spongelike mass is produced, we have theconditions which favour the development of climbing bogs. Beginningusually in the shores of a pool, these plants, necessarily of awater-loving species, retain so much moisture in the spongy masswhich they form that they gradually extend up the slope. Thusextending the margin of their field, and at the same time thickeningthe deposit which they form, these plants may build a climbing bogover the surface until steeps are attained where the inclination is sogreat that the necessary amount of water can not be held in the spongymass, or where, even if so held, the whole coating will in time slipdown in the manner of an avalanche. 

The greater part of the climbing bogs of the world are limited to themoist and cool regions of high latitudes, where species of mossbelonging to the genus _Sphagnum_ plentifully flourish. These plantscan only grow where they are continuously supplied with a bath ofwater about their roots. They develop in lake bogs as far south asMexico, but in the climbing form they are hardly traceable south ofNew England, and are nowhere extensively developed within the limitsof the United States. In more northern parts of this continent, and innorthwestern Europe, particularly in the moist climate of Ireland, climbing bogs occupy great areas, and hold up their lakes ofinterstitially contained water over the slopes of hills, where thesurface rises at the rate of thirty feet or more to the mile. So longas the deposit of decayed vegetable matter which has accumulated inthis manner is thin, therefore everywhere penetrated by the fibrousroots of the moss, it may continue to cling to its sloping bed; butwhen it attains a considerable thickness, and the roots in the lowerpart decay, the pulpy mass, water-laden in some time of heavy rain, break away in a vast torrent of thick, black mud, which may inundatethe lower lands, causing widespread destruction. 

In more southern countries, other water-loving plants lead to theformation of climbing bogs. Of these, the commonest and most effectiveare the species of reeds, of which our Indian cane is a familiarexample. Brakes of this vegetation, plentifully mingled with otherspecies of aquatic growth, form those remarkable climbing bogs knownas the Dismal and other swamps, which numerously occur along the coastline of the United States from southern Maryland to eastern Texas. Climbing bogs are particularly interesting, not only from the factthat they are eminently peculiar effects of plant growth, but becausethey give us a vivid picture of those ancient morasses in which grewthe plants that formed the beds of vegetable matter now appearing inthe state of coal. Each such bed of buried swamp material was, withrare exceptions, where the accumulation took place in lakes, gatheredin climbing bogs such as we have described. 

Lake bogs occur in all parts of the world, but in their bestdevelopment are limited to relatively high latitudes, and this for thereason that the plants which form vegetable matter grow mostluxuriantly in cool climates and in regions where the level of thebasin is subject to less variation than occurs in the alternating wetand dry seasons which exist in nearly all tropical regions. Thefittest conditions are found in glaciated regions, where, as beforenoted, small lakes are usually very abundant. On the shores of one ofthese pools, of size not so great that the waves may attain aconsiderable height, or in the sheltered bay of a larger lake, variousaquatic plants, especially the species of pond lilies, take root uponthe bottom, and spread their expanded leaves on the surface of thewater. These flexible-leaved and elastic-stemmed plants can endurewaves which attain no more than a foot or two of height, and by thefriction which they afford make the swash on the shore very slight. Inthe quiet water, rushes take root, and still further protect thestrand, so that the very delicate vegetation of the mosses, such asthe _Sphagnum_, can fix itself on the shore. 

As soon as the _Sphagnum_ mat has begun its growth, the strength givenby its interlaced fibres enables it to extend off from the shore andfloat upon the water. In this way it may rapidly enlarge, if notbroken up by the waves, so that its front advances into the lake atthe rate of several inches each year. While growing outwardly itthickens, so that the bottom of the mass gradually works down towardthe floor of the basin. At the same time the lower part of the sheet, decaying, contributes a shower of soft peat mud to the floor of thelake. In this way, growing at its edge, deepening, and contributing toan upgrowth from the bottom, a few centuries may serve entirely tofill a deep basin with peaty accumulation. In general, however, thesurface of the bog closes over the lake before the accumulation hascompletely filled the shoreward portions of the area. In theseconditions we have what is familiarly known as a quaking bog, whichcan be swayed up and down by a person who quickly stoops and riseswhile standing on the surface. In this state the tough and thick sheetof growing plants is sufficient to uphold a considerable weight, butso elastic that the underlying water can be thrown into waves. Longbefore the bog has completely filled the lake with the peatyaccumulations the growth of trees is apt to take place on its surface, which often reduces the area to the appearance of a very level wetwood. 

[Illustration: Fig. 17. --Diagram showing beginning of peat bog: A, lake; B, lilies and rushes; C, lake bog; D, climbing bog. ]

Climbing and lake bogs in the United States occupy a total area ofmore than fifty thousand square miles. In all North America the totalarea is probably more than twice as great. Similar deposits areexceedingly common in the Eurasian continent and in southernPatagonia. It is probable that the total amount of these fields indifferent parts of the world exceeds half a million square miles. These two groups of fresh-water swamps have an interest, for thereason that when reduced to cultivation by drainage and by subsequentremoval of the excess of peaty matter, by burning or by natural decay, afford very rich soil. The fairest fields of northern Europe, particularly in Great Britain and Ireland, have been thus won totillage. In the first centuries of our era a large part ofEngland--perhaps as much as one tenth of the ground now tilled in thatcountry--was occupied by these lands, which retained water in suchmeasure as to make them unfit for tillage, the greater portion of thisarea being in the condition of thin climbing bog. For many centuriesmuch of the energy of the people was devoted to the reclamation ofthese valuable lands. This task of winning the swamp lands toagriculture has been more completely accomplished in England thanelsewhere, but it has gone far on the continent of Europe, particularly in Germany. In the United States, owing to the fact thatlands have been cheap, little of this work of swamp-draining has asyet been accomplished. It is likely that the next great field ofimprovement to be cultivated by the enterprising people will be foundin these excessively humid lands, from which the food-giving resourcesfor the support of many million people can be won. 

[Illustration: Fig. 18. --Diagram showing development of swamp: A, remains of lake; B, surface growth; c, peat. ]

The group of marine marshes differs in many important regards fromthose which are formed in fresh water. Where the tide visits anycoast line, and in sheltered positions along that shore, a number ofplants, mostly belonging to the group of grasses, species which havebecome accustomed to having their roots bathed by salt water, beginthe formation of a spongy mat, which resembles that composed of_Sphagnum_, only it is much more solid. This mat of the marine marshessoon attains a thickness of a foot or more, the upper or growingsurface lying in a position where it is covered for two or three hoursat each visit of the tide. Growing rapidly outward from the shore, andhaving a strength which enables it to resist in a tolerably effectivemanner waves not more than two or three feet high, this accumulationmakes head against the sea. To a certain extent the waves underminethe front of the sheet and break up masses of it, which theydistribute over the shallow bottom below the level at which theseplants can grow. In this deeper water, also, other marine animals andplants are continually developing, and their remains are added to theaccumulations which are ever shallowing the water, thus permitting afurther extension of the level, higher-lying marsh. This processcontinues until the growth has gone as far as the scouring action ofthe tidal currents will permit. In the end the bay, originally ofwide-open water, is only such at high tide. For the greater part ofthe time it appears as broad savannas, whose brilliant green givesthem the aspect of rare fertility. 

Owing to the conditions of their growth, the deposits formed in marinemarshes contain no distinct peat, the nearest approach to thatsubstance being the tangle of wirelike roots which covers the upperfoot or so of the accumulation. The greater part of the mass iscomposed of fine silt, brought in by the streams of land water whichdischarge into the basin, and by the remains of animals which dweltupon the bottom or between the stalks of the plants that occupy thesurface of the marshes. These interspaces afford admirable shelter toa host of small marine forms. The result is, that the tidal marshes, as well as the lower-lying mud flats, which have been occupied by themat of vegetation, afford admirable earth for tillage. Unfortunately, however, there are two disadvantages connected with the redemption ofsuch lands. In the first place, it is necessary to exclude the seafrom the area, which can only be accomplished by considerableengineering work; in the second place, the exclusion of the tideinevitably results in the silting up of the passage by which the waterfound its way to the sea. As these openings are often used forharbours, the effect arising from their destruction is often ratherserious. Nevertheless, in some parts of the world very extensive andmost fertile tracts of land have thus been won from the sea; a largepart of Holland and shore-land districts in northern Europe are madeup of fields which were originally covered by the tide. Near the mouthof the Rhine, indeed, the people have found these sea-bottom soils soprofitable that they have gone beyond the zone of the marshes, andhave drained considerable seas which of old were permanently covered, even at the lowest level of the waters. 

On the coast of North America marine marshes have an extensivedevelopment, and vary much in character. In the Bay of Fundy, wherethe tides have an altitude of fifty feet or more, the energy of theircurrents is such that the marsh mat rarely forms. Its place, however, is taken by vast and ever-changing mud flats, the materials of whichare swept to and fro by the moving waters. The people of this regionhave learned an art of a peculiar nature, by which they win broadfields of excellent land from the sea. Selecting an area of the flats, the surface of which has been brought to within a few feet of hightide, they inclose it with a stout barrier or dike, which has openingsfor the free admission of the tidal waters. Entering this basin, thetide, moving with considerable velocity, bears in quantities ofsediment. In the basin, the motion being arrested, this sedimentfalls to the bottom, and serves to raise its level. In a few monthsthe sheet of sediment is brought near the plane of the tidal movement, then the gates are closed at times when the tide has attained half ofits height, so that the ground within the dike is not visited by thesea water, and can be cultivated. 

[Illustration: Fig. 19. --Map of Ipswich marshes, Massachusetts, formedbehind a barrier beach. ]

Along the coast of New England the ordinary marine marshes attain anextensive development in the form of broad-grassed savannas. With thisaspect, though with a considerable change in the plants which theybear, the fringe of savannas continues southward along the coast tonorthern Florida. In the region about the mouth of the Savannah River, so named from the vast extent of the tidal marshes, these fieldsattain their greatest development. In central and southern Florida, however, where the seacoast is admirably suited for their development, these coastal marshes of the grassy type disappear, their place beingtaken by the peculiar morasses formed by the growth of the mangrovetree. 

In the mangrove marshes the tree which gives the areas their namecovers all the field which is visited by the tide. This tree growswith its crown supported on stiltlike roots, at a level above hightide. From its horizontal branches there grow off roots, which reachdownward into the water, and thence to the bottom. The seeds of themangrove are admirably devised so as to enable the plant to obtain afoothold on the mud flats, even where they are covered at low tidewith a depth of two or three feet of water. They are several inches inlength, and arranged with booklets at their lower ends; floating nearthe bottom, they thus catch upon it, and in a few weeks' growth pushthe shoot to the level of the water, thus affording a foundation for anew plantation. In this manner, extending the old forests out into theshallow water of the bays, and forming new colonies wherever the wateris not too deep, these plants rapidly occupy all the region whichelsewhere would appear in the form of savannas. 

[Illustration: Fig. 20. --Diagram showing mode of growth of mangroves. ]

The tidal marshes of North America, which may be in time converted tothe uses of man, probably occupy an area exceeding twenty thousandsquare miles. If the work of reclaiming such lands from the sea everattains the advance in this country that it has done in Holland, thearea added to the dry land by engineering devices may amount to asmuch as fifty thousand square miles--a territory rather greater thanthe surface of Kentucky, and with a food-yielding power at least fivetimes as great as is afforded by that fertile State. In fact, theseconquests from the sea are hereafter to be among the great works whichwill attract the energies of mankind. In the arid region of theCordilleras, as well as in many other countries, the soil, thoughdestitute of those qualities which make it fit for the uses of man, because of the absence of water in sufficient amount, is, as regardsits structure and depth, as well as its mineral contents, admirablysuited to the needs of agriculture. The development of soils in desertregions is in almost all cases to be accounted for by the formerexistence in the realms they occupy of a much greater rainfall thannow exists. Thus in the Rocky Mountain country, when the deep soilsof the ample valleys were formed, the lakes, as we have before noted, were no longer dead seas, as is at present so generally the case, butpoured forth great streams to the sea. Here, as elsewhere, we findevidence that certain portions of the earth which recently had anabundant rainfall have now become starved for the lack of that supply. All the soils of arid regions where the trial has been made haveproved very fertile when subjected to irrigation, which can often beaccomplished by storing the waters of the brief rainy season or bydiverting those of rivers which enter the deserts from well-wateredmountain fields. In fact, the soil of these arid realms yieldspeculiarly ample returns to the husbandman, because of certainconditions due to the exceeding dryness of the air. This leads to anabsence of cloudy weather, so that from the time the seed is plantedthe growth is stimulated by uninterrupted and intense sunshine. Thesame dryness of the air leads, as we have seen, to a rapid evaporationfrom the surface, by which, in a manner before noted, the dissolvedmineral matter is brought near the top of the soil, where it can bestserve the greater part of our crop plants. On these accounts an acreof irrigated soil can be made to yield a far greater return than canbe obtained from land of like chemical composition in humid regions. 

In many parts of the world, particularly in the northern and westernportions of the Mississippi Valley, there are widespread areas, which, though moderately well watered, were in their virgin state almostwithout forests. In the prairie region the early settlers found thecountry unwooded, except along the margins of the streams. On theborders of the true prairies, however, they found considerable areasof a prevailingly forested land, with here and there a tract ofprairie. There were several of these open fields south of the Ohio, though the country there is in general forested; one of these prairieareas, in the Green River district of Kentucky, was several thousandsquare miles in extent. At first it was supposed that the absence oftrees in the open country of the Mississippi Valley was due to somepeculiarity of the soil, but experience shows that plantationsluxuriantly develop, and that the timber will spread rapidly in thenatural way. In fact, if the seeds of the trees which have beenplanted since the settlement of the country were allowed to develop asthey seek to do, it would only be a few centuries before the regionwould be forest-clad as far west as the rainfall would permit theplants to develop. Probably the woods would attain to near thehundredth meridian. 

In the opinion of the writer, the treeless character of the Westernplains is mainly to be accounted for by the habit which our Indianshad of burning the herbage of a lowly sort each year, so that thelarge game might obtain better pasturage. It is a well-known fact toall those who have had to deal with cattle on fields which are in thenatural state that fire betters the pasturage. Beginning this methodof burning in the arid regions to the west of the original forests, the natural action of the fire has been gradually to destroy thesewoods. Although the older and larger trees, on account of their thickbark and the height of their foliage above the ground, escapeddestruction, all the smaller and younger members of the species wereconstantly swept away. Thus when the old trees died they left nosuccession, and the country assumed its prairie character. That theprairies were formed in this manner seems to be proved by thetestimony which we have concerning the open area before mentioned ashaving existed in western Kentucky. It is said that around thetimberless fields there was a wide fringe of old fire-scarred trees, with no undergrowth beneath their branches, and that as they died nokind of large vegetation took their place. When the Indians who setthese fires were driven away, as was the case in the last decade ofthe last century, the country at once began to resume its timberedcondition. From the margin and from every interior point where thetrees survived, their seeds spread so that before the open land wasall subjugated to the plough it was necessary in many places to clearaway a thick growth of the young forest-building trees. 

The soils which develop on the lavas and ashes about an active volcanoafford interesting subjects for study, for the reason that they showhow far the development of the layer which supports vegetation maydepend upon the character of the rocks from which it is derived. Wherethe materials ejected from a volcano lie in a rainy district, theprocess of decay which converts the rock into soil is commonly veryrapid, a few years of exposure to the weather being sufficient tobring about the formation of a fertile soil. This is due to the factthat most lavas, as well as the so-called volcanic ashes, which are ofthe same material as the lavas, only blown to pieces, are composed ofvaried minerals, the most of which are readily attacked by the agentsof decay. Now and then, however, we find the materials ejected from aparticular volcano, or even the lavas and ashes of a single eruption, in such a chemical state that soils form upon them with exceedingslowness. 

 * * * * *

The foregoing incomplete considerations make it plain that thesoil-covering of the earth is the result of very delicate adjustments, which determine the rate at which the broken-down rocks find theirpath from their original bed places to the sea. The admirable way inwhich this movement is controlled is indicated by the fact that almosteverywhere we find a soil-covering deep enough for the use of a variedvegetation, but rarely averaging more than a dozen feet in depth. Onlyhere and there are the rocks bare or the earth swathed in a profoundmass of detritus. This indicates how steadfast and measured is themarch of the rock waste from the hills to the sea. Unhappily, man, when by his needs he is forced to till the soil, is compelled to breakup this ancient and perfect order. He has to strip the living mantlefrom the earth, replacing it with growth of those species which servehis needs. Those plants which are most serviceable--which are, indeed, indispensable in the higher civilization, the grains--require fortheir cultivation that the earth be stripped bare and deeply stirredduring the rainy season, and thus subjected to the most destructiveeffect of the rainfall. The result is, that in almost all grain fieldsthe rate of soil destruction vastly surpasses that at which theaccumulation is being made. We may say, indeed, that, except inalluvial plains, where the soil grows by flood-made additions to itsupper surface, no field tilled in grain can without exceeding careremain usable for a century. Even though the agriculturist returns tothe earth all the chemical substances which he takes away in hiscrops, the loss of the soil by the washing away of its substance tothe stream will inevitably reduce the region to sterility. 

It is not fanciful to say that the greatest misfortune which in alarge way man has had to meet in his agriculture arises from thispeculiar stress which grain crops put upon the soil. If these grainsgrew upon perennial plants, in the manner of our larger fruits, theproblem of man's relation to the soil would be much simpler than it isat present. He might then manage to till the earth without bringingupon it the inevitable destruction which he now inflicts. As it is, heshould recognise that his needs imperil this ancient and preciouselement in the earth's structure, and he should endeavour in everypossible way to minimize the damage which he brings about. This resulthe may accomplish in certain simple ways. 

First, as regards the fertility of the soil, as distinguished from thethickness of the coating, it may be said that modern discoveriesenable us to see the ways whereby we may for an indefinite periodavoid the debasement of our great heritage, the food-giving earth. Wenow know in various parts of the world extensive and practicallyinexhaustible deposits, whence may be obtained the phosphates, potash, soda, etc. , which we take from the soil in our crops. We alsohave learned ways in which the materials contained in our sewage maybe kept from the sea and restored to the fields. In fact, the recentdevelopments of agriculture have made it not only easy, but in mostcases profitable, to avoid this waste of materials which has reducedso many regions to poverty. We may fairly look forward to the time, not long distant, when the old progressive degradation in thefertility of the soil coating will no longer occur. It is otherwisewith the mass of the soil, that body of commingled decayed rock andvegetable matter which must possess a certain thickness in order toserve its needs. As yet no considerable arrest has been made in theprocesses which lead to the destruction of this earthy mass. In allcountries where tillage is general the rivers are flowing charged withall they can bear away of soil material. Thus in the valley of the Po, a region where, if the soil were forest-clad, the down-wearing of thesurface would probably be at no greater rate than one foot in fivethousand years, the river bears away the soil detritus so rapidly thatat the present time the downgoing is at the rate of one foot in eighthundred years, and each decade sees the soil disappear from hillsideswhich were once fertile, but are now reduced to bare rocks. All aboutthe Mediterranean the traveller notes extensive regions which wereonce covered with luxuriant forests, and were afterward the seats ofprosperous agriculture, where the soil has utterly disappeared, leaving only the bare rocks, which could not recover its naturalcovering in thousands of years of the enforced fallow. 

Within the limits of the United States the degradation of the soil, owing to the peculiar conditions of the country, is in many districtsgoing forward with startling rapidity. It has been the habit of ourpeople--a habit favoured by the wide extent of fertile and easilyacquired frontier ground--recklessly to till their farms until thefields were exhausted, and then to abandon them for new ground. Byshallow ploughing on steep hillsides, by neglect in the beginning ofthose gulches which form in such places, it is easy in the hillcountry of the eastern United States to have the soil washed awaywithin twenty years after the protecting forests have been destroyed. The writer has estimated that in the States south of the Ohio andJames Rivers more than eight thousand square miles of originallyfertile ground have by neglect been brought into a condition where itwill no longer bear crops of any kind, and over fifteen hundred milesof the area have been so worn down to the subsoil or the bed rock thatit may never be profitable to win it again to agricultural uses. 

Hitherto, in our American agriculture, our people have been to a greatextent pioneers; they have been compelled to win what they could inthe cheapest possible way and with the rudest implements, and withoutmuch regard to the future of those who were in subsequent generationsto occupy the fields which they were conquering from the wildernessand the savages. The danger is now that this reckless tillage, in away justified of old, may be continued and become habitual with ourpeople. It is, indeed, already a fixed habit in many parts of thecountry, particularly in the South, where a small farmer expects towear out two or three plantations in the course of his natural life. Many of them manage to ruin from one to two hundred acres of land inthe course of half a century of uninterrupted labour. This systemdeserves the reprobation of all good citizens; it would be well, indeed, if it were possible to do so, to stamp it out by the law. Thesame principle which makes it illegal for a man to burn his owndwelling house may fairly be applied in restraining him fromdestroying the land which he tills. 

There are a few simple principles which, if properly applied, mayserve to correct this misuse of our American soil. The careful tillershould note that all soils whatever which lie on declivities having aslope of more than one foot in thirty inevitably and rapidly wastewhen subject to plough tillage. This instrument tends to smear andconsolidate the layer of earth over which its heel runs, so that at adepth of a few inches below the surface a layer tolerably imperviousto water is formed. The result is that the porous portion of thedeposit becomes excessively charged with water in times of heavy rain, and moves down the hillside in a rapid manner. All such steep slopesshould be left in their wooded state, or, if brought into use, shouldbe retained as pasture lands. 

Where, as is often the case with the farms in hilly countries, all thefields are steeply inclined, it is an excellent precaution to leavethe upper part of the slope with a forest covering. In this conditionnot only is the excessive flow of surface water diminished, but themoisture which creeps down the slope from the wooded area tends tokeep the lower-lying fields in a better state for tillage, andpromotes the decay of the underlying rocks, and thus adds to the bodyand richness of the earth. 

On those soils which must be tilled, even where they tend to washaway, the aim should be to keep the detritus open to such a depth thatit may take in as much as possible of the rainfall, yielding the waterto the streams through the springs. This end can generally beaccomplished by deep ploughing; it can, in almost all cases, beattained by under-drainage. The effect of allowing the water topenetrate is not only to diminish the superficial wearing, but tomaintain the process of subsoil and bed-rock decay by which thedetrital covering is naturally renewed. Where, as in many parts of thecountry, the washing away of the soil can not otherwise be arrested, the progress of the destruction can be delayed by forming with theskilful use of the plough ditches of slight declivity leading alongthe hillsides to the natural waterways. One of the most satisfactorymarks of the improvement which is now taking place in the agricultureof the cotton-yielding States of this country is to be found in therapid increase in the use of the ditch system here mentioned. Thissystem, combined with ploughing in the manner where the earth is witheach overturning thrown uphill, will greatly reduce the destructiveeffect of rainfall on steep-lying fields. But the only effectiveprotection, however, is accomplished by carefully terracing theslopes, so that the tilled ground lies in level benches. This systemis extensively followed in the thickly settled portions of Europe, butit may be a century before it will be much used in this country. 

The duty of the soil-tiller by the earth with which he deals may bebriefly summed up: He should look upon himself as an agent necessarilyinterfering with the operations which naturally form and preserve thesoil. He should see that his work brings two risks; he may impoverishthe accumulation of detrital material by taking out the plant foodmore rapidly than it is prepared for use. This injurious result may beat any time reparable by a proper use of manures. Not so, however, with the other form of destruction, which results in the actualremoval of the soil materials. Where neglect has brought about thisdisaster, it can only be repaired by leaving the area to recoverbeneath the slowly formed forest coating. This process in almost allcases requires many thousands of years for its accomplishment. The manwho has wrought such destruction has harmed the inheritance of life. 

 CHAPTER IX. 

 THE ROCKS AND THEIR ORDER. 

In the preceding chapters of this book the attention of the studenthas been directed mainly to the operations of those natural forceswhich act upon the surface of the earth. Incidentally the consequencesarising from the applications of energy to the outer part of theplanet have been attended to, but the main aim has been to set forththe work which solar energy, operating in the form of heat, accomplishes upon the lands. We have now to consider one of the greatresults of these actions, which is exhibited in the successive stratathat make up the earth's crust. 

The most noteworthy effect arising from the action of the solar forceson the earth and their co-operation with those which originate in oursphere is found in the destruction of beds or other deposits of rock, and the removal of the materials to the floors of water basins, wherethey are again aggregated in strata, and gradually brought once moreinto a stable condition within the earth. This work is accomplished bywater in its various states, the action being directly affected bygravitation. In the form of steam, water which has been built intorocks and volcanically expelled by tensions, due to the heat which ithas acquired at great depths below the surface, blows forth greatquantities of lava, which is contributed to the formation of strata, either directly in the solid form or indirectly, after having beendissolved in the sea. Acting as waves, water impelled by solar energytransmitted to it by the winds beats against the shores, wearing awaygreat quantities of rock, which is dragged off to the neighbouring seabottoms, there to resume the bedded form. Moving ice in glaciers, water again applying solar energy given to it by its elevation abovethe sea, most effectively grinds away the elevated parts of the crust, the _débris_ being delivered to the ocean. In the rain the same workis done, and even in the wind the power of the sun serves to abradethe high-lying rocks, making new strata of their fragments. 

As gravity enters as an element in all the movements of divided rock, the tendency of the waste worn from the land is to gather on to thebottoms of basins which contain water. Rarely, and only in a smallway, this process results in the accumulation of lake deposits; thegreater part of the work is done upon the sea floor. When the beds areformed in lake basins, they may be accumulated in either of two verydiverse conditions. They may be formed in what are called dead seas, in which case the detrital materials are commonly small in amount, forthe reason that the inflowing streams are inconsiderable; in suchbasins there is normally a large share of saline materials, which arelaid down by the evaporation of the water. In ordinary lakes thedeposits which are formed are mostly due to the sediment that therivers import. These materials are usually fine-grained, and the sandor pebbles which they contain are plentifully mingled with clay. Hencelake deposits are usually of an argillaceous nature. As organic life, such as secretes limestone, is rarely developed to any extent in lakebasins, limy beds are very rarely formed beneath those areas of water. Where they occur, they are generally due to the fact that riverscharged with limy matter import such quantities of the substance thatit is precipitated on the bottom. 

As lake deposits are normally formed in basins above the level of thesea, and as the drainage channels of the basins are always cuttingdown, the effect is to leave such strata at a considerable heightabove the sea level, where the erosive agents may readily attack them. In consequence of this condition, lacustrine beds are rarely found ofgreat antiquity; they generally disappear soon after they are formed. Where preserved, their endurance is generally to be attributed to thefact that the region they occupy has been lowered beneath the sea andcovered by marine strata. 

The great laboratory in which the sedimentary deposits areaccumulated, the realm in which at least ninety-nine of the hundredparts of these materials are laid down, is the oceanic part of theearth. On the floors of the seas and oceans we have not only theregion where the greater part of the sedimentation is effected, butthat in which the work assumes the greatest variety. The sea bottoms, as regards the deposits formed upon them, are naturally divided intotwo regions--the one in which the _débris_ from the land forms animportant part of the sediment, and the other, where the remotenessof the shores deprives the sediment of land waste, or at least ofenough of that material in any such share as can affect the characterof the deposits. 

What we may term the littoral or shore zone of the sea occupies a beltof prevailingly shallow water, varying in width from a few score to afew hundred miles. Where the bottom descends steeply from the coast, where there are no strong off-shore setting currents, and where theregion is not near the mouth of a large river which bears a great tideof sediment to the sea, the land waste may not affect the bottom formore than a mile or two from the shore. Where these conditions arereversed, the _débris_ from the air-covered region may be found threeor four hundred miles from the coast line. It should also be notedthat the incessant up-and-down goings of the land result in a constantchange in the position of the coast line, and consequently in theextension of the land sediment, in the course of a few geologicalperiods over a far wider field of sea bottom than that to which theywould attain if the shores remained steadfast. 

It is characteristic of the sediments deposited within the influenceof the continental detritus that they vary very much in their action, and that this variation takes place not only horizontally along theshores in the same stratum, but vertically, in the succession of thebeds. It also may be traced down the slope from the coast line to deepwater. Thus where all the _débris_ comes from the action of the waves, the deposits formed from the shore outwardly will consist of coarsematerials, such as pebbles near the coast, of sand in the deeper andremoter section, and of finer silt in the part of the deposit which isfarthest out. With each change in the level of the coast line theposition of these belts will necessarily be altered. Where a greatriver enters the sea, the changes in the volume of sediment which itfrom time to time sends forth, together with the alternations in theposition of its point of discharge, led to great local complexities inthe strata. Moreover, the turbid water sent forth by the stream may, as in the case of the tide from the Amazon, be drifted for hundreds ofmiles along the coast line or into the open sea. 

The most important variations which occur in the deposits of thelittoral zone are brought about by the formations of rocks more orless composed of limestone. Everywhere the sea is, as compared withlake waters, remarkably rich in organic life. Next the shore, partlybecause the water is there shallow, but also because of its relativewarmth and the extent to which it is in motion, organic life, boththat of animals and plants, commonly develops in a very luxuriant way. Only where the bottom is composed of drifting sands, which do notafford a foothold for those species which need to rest upon the shore, do we fail to find that surface thickly tenanted with varied forms. These are arranged according to the depth of the bottom. The speciesof marine plants which are attached to fixed objects are limited tothe depth within which the sunlight effectively penetrates the water;in general, it may be said that they do not extend below a depth ofone hundred feet. The animal forms are distributed, according to theirkinds, over the floor, but few species having the capacity to endureany great range in the pressure of the sea water. Only a few forms, indeed, extend from low tide to the depth of a thousand feet. 

The greatest development of organic life, the realm in which thelargest number of species occur, and where their growth is most rapid, lies within about a hundred feet of the low-tide level. Here sunlight, warmth, and motion in the water combine to favour organic development. It is in this region that coral reefs and other great accumulations oflimestone, formed from the skeletons of polyps and mollusks, mostabundantly occur. These deposits of a limy nature depend upon a verydelicate adjustment of the conditions which favour the growth ofcertain creatures; very slight geographic changes, by inducingmovements of sand or mud, are apt to interrupt their formation, bringing about a great and immediate alteration in the character ofthe deposits. Thus it is that where geologists find considerablefields of rock, where limestones are intercalated with sandstones anddeposits of clay, they are justified in assuming that the strata werelaid down near some ancient shore. In general, these coast depositsbecome more and more limy as we go toward the tropical realms, andthis for the reason that the species which secrete large amounts oflime are in those regions most abundant and attain the most rapidgrowth. The stony polyps, the most vigorous of the limestone makers, grow in large quantities only in the tropical realm, or near to it, where ocean streams of great warmth may provide the creatures with theconditions of temperature and food which they need. 

As we pass from the shore to the deeper sea, the share of landdetritus rapidly diminishes until, as before remarked, at the distanceof five hundred miles from the coast line, very little of that waste, except that from volcanoes, attains the bottom of the sea. By far thelarger part of the contributions which go to the formation of thesedeep-sea strata come from organic remains, which are continuallyfalling upon the sea floor. In part, this waste is derived fromcreatures which dwell upon the bottom; in considerable measure, however, it is from the dead bodies of those forms which live near thesurface of the sea, and which when dying sink slowly through theintermediate realm to the bottom. 

Owing to the absence of sunlight, the prevailingly cold water of thedeeper seas, and the lack of vegetation in those realms, the growth oforganic forms on the deep-sea floor is relatively slow. Thus ithappens that each shell or other contribution to the sediment lies forsome time on the bottom before it is buried. While in this conditionit is apt to be devoured by some of the many species which dwell onthe bottom and subsist from the remains of animals and plants whichthey find there. In all cases the fossilization of any form dependsupon the accumulation of sediment before the processes of destructionhave overtaken them, and among these processes we must give the firstplace to the creatures which subsist on shells, bones, or othersubstances of like nature which find their way to the ocean floor. Inthe absolute darkness, the still water, and the exceeding cold of thedeeper seas, animals find difficult conditions for development. Moreover, in this deep realm there is no native vegetation, and, ingeneral, but little material of this nature descends to the bottomfrom the surface of the sea. The result is, the animals have tosubsist on the remains of other animals which at some step in thesuccession have obtained their provender from the plants which belongon the surface or in the shallow waters of the sea. This limitationof the food supply causes the depths of the sea to be a realm ofcontinual hunger, a region where every particle of organic matter isapt to be seized upon by some needy creature. 

In consequence of the fact that little organic matter on the deepersea floors escapes being devoured, the most of the material of thisnature which goes into strata enters that state in a finely dividedcondition. In the group of worms alone--forms which in a greatdiversity of species inhabit the sea floor--we find creatures whichare specially adapted to digesting the _débris_ which gathers on thesea bottom. Wandering over this surface, much in the manner of ourordinary earthworms, these creatures devour the mud, voiding thematter from their bodies in a yet more perfectly divided form. Henceit comes about that the limestone beds, so commonly formed beneath theopen seas, are generally composed of materials which show but few andvery imperfect fossils. Studying any series of limestone beds, wecommonly find that each layer, in greater or less degree, is made upof rather massive materials, which evidently came to their place inthe form of a limy mud. Very often this lime has crystallized, andthus has lost all trace of its original organic structure. 

One of the conspicuous features which may be observed in anysuccession of limestone beds is the partings or divisions into layerswhich occur with varied frequency. Sometimes at vertical intervals ofnot more than one or two inches, again with spacings of a score offeet, we find divisional planes, which indicate a sudden change in theprocess of rock formation. The lime disappears, and in place of it wehave a thin layer of very fine detritus, which takes on the form of aclay. Examining these partings with care, we observe that on the uppersurface on the limestone the remains of the animal which dwelt on theancient sea floor are remarkably well preserved, they having evidentlyescaped the effect of the process which reduced their ancestors, whose remains constitute the layer, to mud. Furthermore, we note thatthe shaly layer is not only lacking in lime, but commonly contains notrace of animals such as might have dwelt on the bottom. The fossilsit bears are usually of species which swam in the overlying water andcame to the bottom after death. Following up through the layer ofshale, we note that the ordinary bottom life gradually reappears, andshortly becomes so plentiful that the deposit resumes the characterwhich it had before the interruption began. Often, however, we notethat the assemblage of species which dwelt on the given area of seafloor has undergone a considerable change. Forms in existence in thelower layer may be lacking in the upper, their place being taken bynew varieties. 

So far the origin of these divisional planes in marine deposits hasreceived little attention from geologists; they have, indeed, assumedthat each of these alterations indicates some sudden disturbance ofthe life of the sea floors. They have, however, generally assumed thatthe change was due to alterations in the depth of the sea or in therun of ocean currents. It seems to the writer, however, that whilethese divisions may in certain cases be due to the above-mentionedand, indeed, to a great variety of causes, they are in general best tobe explained by the action of earthquakes. Water being an exceedinglyelastic substance, an earthquake passes through it with much greaterspeed than it traverses the rocks which support the ocean floor. Theresult is that, when the fluid and solid oscillate in the repeatedswingings which a shock causes, they do not move together, but rubover each other, the independent movements having the swing of from afew inches to a foot or two in shocks of considerable energy. 

When the sea bottom and the overlying water, vibrating under theimpulse of an earthquake shock, move past each other, the inevitableresult is the formation of muddy water; the very fine silt of thebottom is shaken up into the fluid, which afterward descends as asheet to its original position. It is a well-known fact that suchmuddying of water, in which species accustomed to other conditionsdwell, inevitably leads to their death by covering their breathingorgans and otherwise disturbing the delicately balanced conditionswhich enable them to exist. We find, in fact, that most of the tenantsof the water, particularly the forms which dwell upon the bottom, areprovided with an array of contrivances which enable them to clear awayfrom their bodies such small quantities of silt as may inconveniencethem. Thus, in the case of our common clam, the breathing organs arecovered with vibratory cilia, which, acting like brooms, sweep off anyforeign matter which may come upon their surfaces. Moreover, thecreature has a long, double, spoutlike organ, which it can elevatesome distance above the bottom, through which it draws and dischargesthe water from which it obtains food and air. Other forms, such as thecrinoids, or sea lilies, elevate the breathing parts on top of tallstems of marvellous construction, which brings those vital organs atthe level, it may be, of three or four feet above the zone of mud. Inconsequence of the peculiar method of growth, the crinoids oftenescape the damage done by the disturbance of the bottom, and thus formlimestone beds of remarkable thickness; sometimes, indeed, we findthese layers composed mainly of crinoidal remains, which exhibit onlyslight traces of partings such as we have described, being essentiallyunited for the depth of ten or twenty feet. Where the layers have beenmainly accumulated by shellfish, their average thickness is less thanhalf a foot. 

When we examine the partitions between the layers of limestone, wecommonly find that, however thin, they generally extend for anindefinite distance in every direction. The writer has traced some ofthese for miles; never, indeed, has he been able to find where theydisappeared. This fact makes it clear that the destruction which tookplace at the stage where these partings were formed was widespread; sofar as it was due to earthquake shocks, we may fairly believe that inmany cases it occurred over areas which were to be measured by tens ofthousands of square miles. Indeed, from what we know of earthquakeshocks, it seems likely that the devastation may at times haveaffected millions of square miles. 

Another class of accidents connected with earthquakes may alsosuddenly disturb the mud on the sea bottom. When, as elsewhere noted, a shock originates beneath the sea, the effect is suddenly to elevatethe water over the seat of the jarring and the regions thereabouts tothe height of some feet. This elevation quickly takes the shape of aringlike wave, which rolls off in every direction from its point oforigin. Where the sea is deep, the effect of this wave on the bottommay be but slight; but as the undulation attains shallower water, andin proportion to the shoaling, the front of the surge is retarded inits advance by the friction of the bottom, while the rear part, beingin deeper water, crowds upon the advancing line. The action isprecisely that which has been described as occurring in wind-madewaves as they approach the beach; but in this last-named group ofundulations, because of the great width of the swell, the effect ofthe shallowing is evident in much deeper water. It is likely that atthe depth of a thousand feet the passing of one of these vast surgesborn of earthquakes may so stir the mud of the sea floor as to bringabout a widespread destruction of life, and thus give rise to many ofthe partitions between strata. 

If we examine with the microscope the fine-grained silts which make upthe shaly layers between limestones, we find the materials to bemostly of inorganic origin. It is hard to trace the origin of themineral matter which it contains; some of the fragments are likely toprove of Volcanic origin; others, bits of dust from meteorites; yetothers, dust blown from the land, which may, as we know, be conveyedfor any distance across the seas. Mingled with this sediment of aninorganic origin we almost invariably find a share of organic waste, derived not from creatures which dwelt upon the bottom, but from thosewhich inhabited the higher-lying waters. If, now, we take a portion ofthe limestone layer which lies above or below the shale parting, andcarefully dissolve out with acids the limy matter which it contains, we obtain a residuum which in general character, except so far as theparticles may have been affected by the acid, is exactly like thematerial which forms the claylike partition. We are thus readily ledto the conclusion that on the floors of the deeper seas there isconstantly descending, in the form of a very slow shower, a mass ofmineral detritus. Where organic life belonging to the species whichsecrete hard shells or skeletons is absent, this accumulation, proceeding with exceeding slowness, gradually accumulates layers, which take on a shaly character. Where limestone-making animalsabound, they so increase the rate of deposition that the proportion ofthe mineral material in the growing strata is very much reduced; itmay, indeed, become as small as one per cent of the mass. In this casewe may say that the deposit of limestone grew a hundred times as fastas the intervening beds of shale. 

The foregoing considerations make it tolerably clear that the seafloor is in receipt of two diverse classes of sediment--those of amineral and those of an organic origin. The mineral, or inorganic, materials predominate along the shores. They gradually diminish inquantity toward the open sea, where the supply is mainly dependent onthe substances thrown forth from volcanoes, on pumice in its massiveor its comminuted form--i. E. , volcanic dust, states of lava in whichthe material, because of the vesicles which it contains, can float forages before it comes to rest on the sea bottom. Variations in thevolcanic waste contributed to the sea floor may somewhat affect thequantity of the inorganic sediments, but, as a whole, the downfallingof these fragments is probably at a singularly uniform rate. It isotherwise with the contributions of sediment arising from organicforms. This varies in a surprising measure. On the coral reefs, suchas form in the mid oceans, the proportion of matter which has not comeinto the accumulation through the bodies of animals and plants may beas small as one tenth of one per cent, or less. In the deeper seas, itis doubtful whether the rate of animal growth is such as to permit theformation of any beds which have less than one half of their mass madeup of materials which fell through the water. 

In certain areas of the open seas the upper part of the water is dweltin by a host of creatures, mostly foraminifera, which extractlimestone from the water, and, on dying, send their shells to thebottom. Thus in the North Atlantic, even where the sea floor is ofgreat depth beneath the surface, there is constantly accumulating amass of limy matter, which is forming very massive limestone strata, somewhat resembling chalk deposits, such as abundantly occur in GreatBritain, in the neighbouring parts of Europe, in Texas, and elsewhere. Accumulations such as this, where the supply is derived from thesurface of the water, are not affected by the accidents which dividebeds made on the bottom in the manner before described. They may, therefore, have the singularly continuous character which we note inthe English chalk, where, for the thickness of hundreds of feet, wemay have no evident partitions, except certain divisions, which haveevidently originated long after the beds were formed. 

We have already noted the fact that, while the floors of the deeperseas appear to lack mountainous elevations, those arising from thefolding of strata, they are plentifully scattered over with volcaniccones. We may therefore suppose that, in general, the deposits formedon the sea floor are to a great extent affected by the materials whichthese vents cast forth. Lava streams and showers represent only apart of the contributions from volcanoes, which finally find their wayto the bottom. In larger part, the materials thrown forth are probablyfirst dissolved in the water and then taken up by the organic species;only after the death of these creatures does the waste go to thebottom. As hosts of these creatures have no solid skeleton tocontribute to the sea floor, such mineral matter as they may obtain isafter their death at once restored to the sea. 

Not only does the contribution of organic sediment diminish inquantity with the depth which is attained, but the deeper parts of theocean bed appear to be in a condition where no accumulations of thisnature are made, and this for the reason that the water dissolves theorganic matter more rapidly than it is laid down. Thus in place oflimestone, which would otherwise form, we have only a claylikeresiduum, such as is obtained when we dissolve lime rocks in acids. This process of solution, by which the limy matter deposited on thebottom is taken back into the water, goes on everywhere, but at a ratewhich increases with the depth. This increase is due in part to theaugmentation of pressure, and in part to the larger share of carbonicdioxide which the water at great depths holds. The result is, thatexplorations with the dredge seem to indicate that on certain parts ofthe deeper sea floors the rocks are undergoing a process ofdissolution comparable to that which takes place in limestone caverns. So considerable is the solvent work that a large part of the inorganicwaste appears to be taken up by the waters, so as to leave the bottomessentially without sedimentary accumulations. The sea, in a word, appears to be eating into rocks which it laid down before thedepression attained its present great depth. 

We should here note something of the conditions which determine thesupply of food which the marine animals obtain. First of all, we mayrecur to the point that the ocean waters appear to contain somethingof all the earth materials which do not readily decompose when theyare taken into the state of solution. These mineral substances, including the metals, are obtained in part from the lands, through theaction of the rain water and the waves, but perhaps in larger sharefrom the volcanic matter which, in the form of floating lava, pumice, or dust, is plentifully delivered to the sea. Except doubtfully, andat most in a very small way, this chemical store of the sea water cannot be directly taken into the structures of animals; it can only beimmediately appropriated by the marine plants. These forms can onlydevelop in that superficial realm of the seas which is penetrated bythe sunlight, or say within the depth of five hundred feet, mostlywithin one hundred feet of the surface, about one thirtieth of theaverage, and about one fiftieth of the maximum ocean depth. On thismarine plant life, and in a small measure on the vegetable matterderived from the land, the marine animals primarily depend for theirprovender. Through the conditions which bring about the formation of_Sargassum_ seas, those areas of the ocean where seaweeds grow afloat, as well as by the water-logging and weighting down of other vegetablematter, some part of the plant remains is carried to the sea floor, even to great depths; but the main dependence of the deep-sea forms ofanimals is upon other animal forms, which themselves may have obtainedtheir store from yet others. In fact, in any deep-sea form we mightfind it necessary to trace back the food by thousands of steps beforewe found the creature which had access to the vegetable matter. It iseasy to see how such conditions profoundly limit the development oforganic being in the abysm of the ocean. 

The sedentary animals, or those which are fixed to the sea bottom--agroup which includes the larger part of the marine species--have todepend for their sustenance on the movement of the water which passestheir station. If the seas were perfectly still, none of thesecreatures except the most minute could be fed; therefore the currentsof the ocean go far by their speed to determine the rate at which lifemay flourish. At great depths, as we have seen, these movements arepractically limited to that which is caused by the slow movement whichthe tide brings about. The amount of this motion is proportional tothe depth of the sea; in the deeper parts, it carries the water to andfro twice each day for the distance of about two hundred and fiftyfeet. In the shallower water this motion increases in proportion tothe shoaling, and in the regions near the shores the currents of thesea which, except the massive drift from the poles, do not usuallytouch the bottom, begin to have their influence. Where the water isless than a hundred feet in depth, each wave contributes to themovement, which attains its maximum near the shore, where every surgesweeps the water rapidly to and fro. It is in this surge belt, wherethe waves are broken, that marine animals are best provided with food, and it is here that their growth is most rapid. If the student willobtain a pint of water from the surf, he will find that it is cloudedby fragments of organic matter, the quantity in a pound of the fluidoften amounting to the fiftieth part of its weight. He will thusperceive that along the shore line, though the provision of victualsis most abundant, the store is made from the animals and plants whichare ground up in the mill. In a word, while the coast is a place ofrapid growth, it is also a region of rapid destruction; only in thecase of the coral animals, which associate their bodies with a numberof myriads in large and elaborately organized communities, do we findanimals which can make such head against the action of the waves thatthey can build great deposits in their realm. 

It should be noted that a part of the advantage which is afforded toorganic life by the shore belt is due to the fact that the waters arethere subjected to a constant process of aëration by the whipping intofoam and spray which occurs where the waves overturn. 

It will be interesting to the student to note the great number ofmechanical contrivances which have been devised to give security toanimals and plants which face these difficult conditions arising fromsuccessive violent blows of falling water. Among these may be brieflynoted those of the limpets--mollusks which dwell in a conical shell, which faces the water with a domelike outside, and which at the momentof the stroke is drawn down upon the rock by the strong muscle whichfastens the creature to its foundation. The barnacles, which withtheir wedge-shaped prows cut the water at the moment of the stroke, but open in the pauses between the waves, so that the creature maywith its branching arms grasp at the food which floats about it; thenullipores, forms of seaweed which are framed of limestone and clingfirmly to the rock--afford yet other instances of protectiveadaptations contrived to insure the safety of creatures which dwell inthe field of abundant food supply. 

 * * * * *

The facts above presented will show the reader that the marinesediments are formed under conditions which permit a great variety inthe nature of the materials of which they are composed. As soon as thedeposits are built into rocks and covered by later accumulations, their materials enter the laboratory of the under earth, where theyare subjected to progressive changes. Even before they have attained agreat depth, through the laying down of later deposits upon them, changes begin which serve to alter their structure. The fragments of asoluble kind begin to be dissolved, and are redeposited, so that themass commonly becomes much more solid, passing from the state ofdetritus to that of more or less solid rock. When yet more deeplyburied, and thereby brought into a realm of greater warmth, or perhapswhen penetrated by dikes and thereby heated, these changes go yetfurther. More of the material is commonly rearranged by solution andredeposition, so that limestone may be converted into crystallinemarble, granular sandstones into firm masses, known as quartzites, andclays into the harder form of slate. Where the changes go to theextreme point, rocks originally distinctly bedded probably may be sotaken to pieces and made over that all traces of their stratificationmay be destroyed, all fossils obliterated, and the stone transformedinto mica schist, or granite or other crystalline rock. It may beinjected into the overlying strata in the form of dikes, or it may beblown forth into the air through volcanoes. Involved inmountain-folding, after being more or less changed in the mannerdescribed, the beds may become tangled together like the rumpledleaves of a book, or even with the complexity of snarled thread. Allthese changes of condition makes it difficult for the geologist tounravel the succession of strata so that he may know the true order ofthe rocks, and read from them the story of the successive geologicalperiods. This task, though incomplete, has by the labours of manythousand men been so far advanced that we are now able to divide therecord into chapters, the divisions of the geologic ages, and to givesome account of the succession of events, organic and geographic, which have occurred since life began to write its records. 

 EARTHQUAKES. 

In ordinary experience we seem to behold the greater part of the earthwhich meets our eyes as fixed in its position. A better understandingshows us that nothing in this world is immovable. In the realm of theinorganic world the atoms and molecules even in solid bodies have tobe conceived as endowed with ceaseless though ordered motions. Evenwhen matter is built into the solid rock, it is doubtful whether anygrain of it ever comes really to rest. Under the strains which arisefrom the contraction of the earth's interior and the chemical changeswhich the rocks undergo, each bit is subject to ever-changingthrusts, which somewhat affect its position. If we in any way couldbring a grain of sand from any stratum under a microscope, so that wecould perceive its changes of place, we should probably find that itwas endlessly swaying this way and that, with reference to an ideallyfixed point, such as the centre of the earth. But even that centre, whether of gravity or of figure, is probably never at rest. 

Earth movements may be divided into two groups--those which arise fromthe bodily shifting of matter, which conveys the particles this way orthat, or, as we say, change their place, and those which merelyproduce vibration, in which the particles, after their vibratorymovement, return to their original place. For purposes of illustrationthe first, or translatory motion, may be compared to that which takesplace when a bell is carried along upon a locomotive or a ship; andthe second, or vibratory movement, to what takes place when the bellis by a blow made to ring. It is with these ringing movements, as wemay term them, that we find ourselves concerned when we undertake thestudy of earthquakes. 

It is desirable that the reader should preface his study ofearthquakes by noting the great and, at the same time, variableelasticity of rocks. In the extreme form this elasticity is very wellshown when a toy marble, which is made of a close-textured rock, suchas that from which it derives its name, is thrown upon a pavementcomposed of like dense material. Experiment will show that the littlesphere can often be made to bounce to the height of twenty feetwithout breaking. If, then, with the same energy the marble is thrownupon a brick floor, the rebound will be very much diminished. It iswell to consider what happens to produce the rebound. When the spherestrikes the floor it changes its shape, becoming shorter in the axisat right angles to the point which was struck, and at the same instantexpanded along the equator of that axis. The flattening remains foronly a small fraction of a second; the sphere vibrates so that itstretches along the line on which it previously shortened, and, asthis movement takes place with great swiftness, it may be said topropel itself away from the floor. At the same time a similar movementgoes on in the rock of the floor, and, where the rate of vibration isthe same, the two kicks are coincident, and so the sphere is impelledviolently away from the point of contact. Where the marble comes incontact with brick, in part because of the lesser elasticity of thatmaterial, due to its rather porous structure, and partly because itdoes not vibrate at the same rate as the marble, the expelling blow ismuch less strong. 

All rocks whatever, even those which appear as incoherent sands, aremore or less set into vibratory motion whenever they are struck by ablow. In the crust of the earth various accidents occur which mayproduce that sudden motion which we term a blow. When we have examinedinto the origin of these impulses, and the way in which they aretransmitted through the rocks, we obtain a basis for understandingearthquake shocks. The commonest cause of the jarrings in the earth isfound in the formation of fractures, known as faults. If the readerhas ever been upon a frozen lake at a time when the weather wasgrowing colder, and the ice, therefore, was shrinking, he may havenoted the rending sound and the slight vibration which comes with theformation of a crack traversing the sheet of ice. At such a time hefeels a movement which is an earthquake, and which represents thesimpler form of those tremors arising from the sudden rupture of faultplanes. If he has a mind to make the experiment, he may hang a bulletby a thread from a small frame which rests upon the ice, and note thatas the vibration occurs the little pendulum sways to and fro, thusindicating the oscillations of the ice. The same instrument will movein an identical manner when affected by a quaking in the rocks. 

Where the rocks are set in vibration by a rent which is formed inthem, the phenomena are more complicated, and often on a vastly largerscale than in the simple conditions afforded by a sheet of ice. Therocks on either side of the rupture generally slide over each other, and the opposing masses are rent in their friction upon one another;the result is, not only the first jar formed by the initial fracture, but a great many successive movements from the other breakages whichoccur. Again, in the deeper parts of the crust, the fault fissures areoften at the moment of their formation filled by a violent inrush ofliquid rock. This, as it swiftly moves along, tears away masses fromthe walls, and when it strikes the end of the opening delivers a blowwhich may be of great violence. The nature of this stroke may bejudged by the familiar instance where the relatively slow-flowingstream from a hydrant pipe is suddenly choked by closing the stopcock. Unless the plumber provides a cushion of air to diminish the energy ofthe blow, it is often strong enough to shake the house. Again, whensteam or other gases are by a sudden diminution of pressure enabled toexpand, they may deliver a blow which is exactly like that caused bythe explosion of gunpowder, which, even when it rushes against thesoft cushion of the air, may cause a jarring that may be felt as wellas heard to a great distance. Such movements very frequently occur inthe eruptions of volcanoes; they cause a quivering of the earth, whichmay be felt for a great distance from the immediate seat of thedisturbance. 

When by any of the sudden movements which have been above described ajar is applied to the rocks, the wave flies through the more or lesselastic mass until the energy involved in it is exhausted. This maynot be brought about until the motion has travelled for the distanceof hundreds of miles. In the great earthquake of 1755, known as theLisbon shock, the records make it seem probable that the movement wasfelt over one eighth part of the earth's surface. Such greatdisturbances probably bring about a motion of the rocks near the pointof origin, which may be expressed in oscillations having an amplitudeof one to two feet; but in the greater number of earthquakes themaximum swing probably does not exceed the tenth of that amount. Verysensible shaking, even such as may produce considerable damage tobuildings, are caused by shocks in which the earth vibrates with lessthan an inch of swing. 

When a shock originates, the wave in the rocks due to the compressionwhich the blow inflicts runs at a speed varying with the elasticity ofthe substance, but at the rate of about fifteen hundred feet a second. The movements of this wave are at right angles to the seat of theoriginating disturbance, so that the shock may come to the surface ina line forming any angle between the vertical and the nearlyhorizontal. Where, as in a volcanic eruption, the shock originateswith an explosion, these waves go off in circles. Where, however, asis generally the case, the shock originates in a fault plane, whichmay have a length and depth of many miles, the movement has anelliptical form. 

If the earthquake wave ran through a uniform and highly elasticsubstance, such as glass, it would move everywhere with equal speed, and, in the case of the greater disturbances, the motion might be feltover the whole surface of the earth. But as the motion takes placethrough rocks of varying elasticity, the rate at which it journeys isvery irregular. Moving through materials of one density, and with arate of vibration determined by those conditions, the impulse is withdifficulty communicated to strata which naturally vibrate at anotherspeed. In many cases, as where a shock passing through densecrystalline strata encounters a mass of soft sandstone, the wave, inplace of going on, is reflected back toward its point of origin. Theseearthquake echoes sometimes give rise to very destructive movements. It often happens that before the original tremors of a shock havepassed away from a point on the surface the reflex movements rush in, making a very irregular motion, which may be compared to that of thewaves in a cross-sea. 

The foregoing account of earthquake action will serve to prepare thereader for an understanding of those very curious and importanteffects which these accidents produce in and on the earth. Below thesurface the sensible action of earthquake shocks is limited. It hasoften been observed that people in mines hardly note a swaying whichmay be very conspicuous to those on the surface, the reason for thisbeing that underground, where the rocks are firmly bound together, allthose swingings which are due to the unsupported position of suchobjects as buildings, columnar rocks, trees, and the waters of theearth, are absent. The effect of the movements which earthquakesimpress on the under earth is mainly due to the fact that in almostevery part of the crust tensions or strains of other kinds arecontinually forming. These may for ages prove without effect until theearth is jarred, when motions will suddenly take place which in amoment may alter the conditions of the rocks throughout a wide field. In a word, a great earthquake caused by the formation of an extensivefault is likely to produce any number of slight dislocations, each ofwhich is in turn shock-making, sending its little wave to complicatethe great oscillation. Nor does the perturbing effect of these jarringmovements cease with the fractures which they set up and the newstrains which are in turn developed by the motions which they induce. The alterations of the rocks which are involved in chemical changesare favoured by such motions. It is a familiar experience that avessel of water, if kept in the state of repose, may have itstemperature lowered three or four degrees below the freezing pointwithout becoming frozen. If the side of the vessel is then tapped withthe finger, so as to send a slight quake through the mass, it willinstantly congeal. Molecular rearrangements are thus favoured byshocks, and the consequences of those which run through the earth are, from a chemical point of view, probably important. 

The reader may help himself to understand something of the complicatedproblem of earth tensions, and the corresponding movements of therocks, by considering certain homely illustrations. He may observe howthe soil cracks as it shrinks in times of drought, the openingsclosing when it rains. In a similar way the frozen earth breaks open, sometimes with a shock which is often counted as an earthquake. Again, the ashes in a sifter or the gravel on a sieve show how each shakingmay relieve certain tensions established by gravity, while they createothers which are in turn to be released by the next shock. An ordinarydwelling house sways and strains with the alternations of temperatureand moisture to which it is subjected in the round of climatalalterations. Now and then we note the movements in a cracking sound, but by far the greater part of them escape observation. 

With this sketch of the mechanism of earthquake shocks we now turn toconsider their effects upon the surface of the earth. From ageological point of view, the most important effect of earthquakeshocks is found in the movement of rock masses down steep slopes, which is induced by the shaking. Everywhere on the land the agents ofdecay and erosion tend to bring heavy masses into position wheregravitation naturally leads to their downfall, but where they mayremain long suspended, provided they are not disturbed. Thus, whereverthere are high and steep cliffs, great falls of rock are likely tooccur when the earthquake movements traverse the under earth. In morethan one instance observers, so placed that they commanded a view ofdistant mountains, have noticed the downfall of precipices in the pathof the shock before the trembling affected the ground on which theystood. In the famous earthquake of 1783, which devastated southernItaly, the Prince of Scylla persuaded his people to take refuge intheir boats, hoping that they might thereby escape the destructionwhich threatened them on the land. No sooner were the unhappy folk onthe water than the fall of neighbouring cliffs near the sea produced agreat wave, which overwhelmed the vessels. 

Where the soil lies upon steep slopes, in positions in which it hasaccumulated during ages of tranquillity, a great shock is likely tosend it down into the valleys in vast landslides. Thus, in theearthquake of 1692, the Blue Mountains of Jamaica were so violentlyshaken that the soil and the forests which stood on it wereprecipitated into the river beds, so that many tree-clad summitsbecame fields of bare rock. The effect of this action is immensely toincrease the amount of detritus which the streams convey to the sea. After the great Jamaica shock, above noted, the rivers for a whileceased to flow, their waters being stored in the masses of loosematerial. Then for weeks they poured forth torrents of mud and the_débris_ of vegetation--materials which had to be swept away as thestreams formed new channels. 

In all regions where earthquake movements are frequent, and the shockof considerable violence, the trained observer notes that the surfacesof bare rock are singularly extensive, the fact being that many ofthese areas, where the slope lies at angles of from ten to thirtydegrees, which in an unshaken region would be thickly soil-covered, are deprived of the coating by the downward movement of the wastewhich the disturbances bring about. A familiar example of this actionmay be had by watching the workmen engaged in sifting sand, by castingthe material on a sloping grating. The work could not be done but foran occasional blow applied to the sifter. An arrangement for such ajarring motion is commonly found in various ore-dressing machines, where the object is to move fragments of matter over a slopingsurface. 

Even where the earth is so level that an earthquake shock does notcause a sliding motion of the materials, such as above described, other consequences of the shaking may readily be noted. As the motionruns through the mass, provided the movement be one of considerableviolence, crevices several feet in width, and sometimes having thelength of miles, are often formed. In most cases these fissures, opened by one pulsation of the shock, are likely to be closed by thereturn movement, which occurs the instant thereafter. The consequencesof this action are often singular, and in cases constitute the mostfrightful elements of a shock which the sufferer beholds. In the greatearthquake of 1811, which ravaged the section of the MississippiValley between the mouth of the Ohio and Vicksburg, these creviceswere so numerously formed that the pioneers protected themselves fromthe danger of being caught in their jaws by felling trees so that theylay at right angles to the direction in which the rents extended, building on these timbers platforms to support their temporarydwelling places. The records of earthquakes supply many instances inwhich people have been caught in these earth fissures, and in a singlecase it is recorded that a man who disappeared into the cavity was ina moment cast forth in the rush of waters which in this, as in manyother cases, spouts forth as the walls of the opening come together. 

Sometimes these rents are attended by a dislocation, which brings theearth on one side much higher than on the other. The step thusproduced may be many miles in length, and may have a height of twentyfeet or more. It needs no argument to show that we have here the topof a fault such as produced the shock, or it may be one of a secondarynature, such as any earthquake is likely to bring about in the stratawhich it traverses. In certain cases two faults conjoin their action, so that a portion of the surface disappears beneath the earth, entombing whatever may have stood on the vanished site. Thus in thegreat shock known as that of Lisbon, which occurred in 1755, the stonequay along the harbour, where many thousand people had sought refugefrom the falling buildings of the city, suddenly sank down with themultitude, and the waters closed over it; no trace of the people or ofthe structure was to be found after the shock was over. There is astory to the effect that during the same earthquake an Arab village innorthern Africa sank down, the earth on either side closing over it, so that no trace of the habitations remained. In both these instancesthe catastrophes are best explained by the diagram. 

[Illustration: Fig. 21. --Diagram showing how a portion of the earth'ssurface may be sunk by faulting. Fig. A shows the original position;B, the position after faulting; b b' and c c' the planes of thefaults; the arrows the direction of the movement. ]

In the earthquake of 1811 the alluvial plains on either side of theMississippi at many points sank down so that arable land was convertedinto lakes; the area of these depressions probably amounted to somehundred square miles. The writer, on examining these sunken lands, found that the subsidences had occurred where the old moats orabandoned channels of the great river had been filled in with amixture of decaying timber and river silt. When violently shaken, thisloose-textured _débris_ naturally settled down, so that it formed abasin occupied by a crescent-shaped lake. The same process of settlingplentifully goes on wherever the rocks are still in an uncementedstate. The result is often the production of changes which lead to theexpulsion of gases. Thus, in the Charleston earthquake of 1883, thesurface over an area of many hundred square miles was pitted withsmall craters, formed by the uprush of water impelled by its containedgases. These little water volcanoes--for such we may callthem--sometimes occur to the number of a dozen or more on each acre ofground in the violently shaken district. They indicate one result ofthe physical and chemical alterations which earthquake shocks bringabout. As earthquakes increase in violence their effect upon the soilbecomes continually greater, until in the most violent shocks all theloose materials on the surface of the earth may be so shaken about asto destroy even the boundaries of fields. After the famous earthquakeof Riobamba, which occurred on the west coast of South America in1797, the people of the district in which the town of that name wassituated were forced to redivide their land, the original boundarieshaving disappeared. Fortunately, shocks of this description areexceedingly rare. They occur in only a few parts of the world. 

Certain effects of earthquakes where the shock emerges beneath the seahave been stated in the account of volcanic eruptions (see page 299). We may therefore note here only certain of the more general facts. While passing through the deep seas, this wave may have a height ofnot more than two or three feet and a width of some score miles. As itrolls in upon the shore the front of the undulation is retarded by thefriction of the bottom in such a measure that its speed is diminished, while the following part of the waves, being less checked, crowds uptoward this forward part. The result is, that the surge mounts everhigher and higher as it draws near the shore, upon which it may rollas a vast wave having the height of fifty feet or more and a widthquite unparalleled by any wave produced from wind action. Waves ofthis description are most common in the Pacific Ocean. Although butoccasional, the damage which they may inflict is very great. As themovement approaches the shore, vessels, however well anchored, aredragged away to seaward by the great back lash of the wave, aphenomenon which may be perceived even in the case of the ordinarysurf. Thus forced to seaward, the crews of the ships may find theirvessels drawn out for the distance of some miles, until they come nearthe face of the advancing billow. This, as it approaches the shore, straightens up to the wall-fronted form, and then topples upon theland. Those vessels which are not at once crushed down by the blow aregenerally hurled far inland by the rush of waters. In the greatJamaica earthquake of 1692 a British man-of-war was borne over thetops of certain warehouses and deposited at a distance from the shore. 

Owing to the fact that water is a highly elastic material, the shockstransmitted to it from the bottom are sent onward with their energybut little diminished. While the impulse is very violent, theseoscillations may prove damaging to shipping. The log-books of marinersabound in stories of how vessels were dismasted or otherwise badlyshaken by a sudden blow received in the midst of a quiet sea. Theimpression commonly conveyed to the sailors is that the craft hasstruck upon a rock. The explanation is that an earthquake jar, intraversing the water, has delivered its blow to the ship. As the speedof this jarring movement is very much greater than that of anyordinary wave, the blow which it may strike may be most destructive. There seems, indeed, little reason to doubt that a portion of thevessels which are ever disappearing in the wilderness of the ocean arelost by the crushing effect of these quakings which pass through thewaters of the deep. 

We have already spoken of the earthquake shock as an oscillation. Itis a quality of all bodies which oscillate under the influence of ablow, such as originates in earthquake shocks, to swing to and fro, after the manner of the metal in a bell or a tuning fork, in asuccession of movements, each less than the preceding, until theimpulse is worn out, or rather, we should in strict sense say, changed to other forms of energy. The result is, that even in theslightest earthquake shock the earth moves not once to and fro, butvery many times. In a considerable shock the successive diminishingswingings amount to dozens before they become so slight as to eludeperception. Although the first swaying is the strongest, and generallythe most destructive, the quick to-and-fro motions are apt to continueand to complete the devastation which the first brings about. Thevibrations due to any one shock take place with great rapidity. Theymay, indeed, be compared to those movements which we perceive in themargin of a large bell when it has received a heavy blow from theclapper. The reader has perhaps seen that for a moment the rim of thebell vibrates with such rapidity that it has a misty look--that is, the motions elude the sight. It is easy to see that a shaking of thiskind is particularly calculated to disrupt any bodies which stand freein the air and are supported only at their base. 

In what we may call the natural architecture of the earth, thepinnacles and obelisks, such as are formed in many high countries, theeffect of these shakings is destructive, and, as we have seen, eventhe firmer-placed objects, such as the strong-walled cliffs and steepslopes of earth, break down under the assaults. It is therefore nomatter of surprise that the buildings which man erects, where they arecomposed of masonry, suffer greatly from these tremblings. In almostall cases human edifices are constructed without regard to otherproblems of strength than those which may be measured by their weightand the resistance to fracture from gravitation alone. They are notbuilt with expectation of a quaking, but of a firm-set earth. 

The damage which earthquakes do to buildings is in most cases due tothe fact that they sway their walls out of plumb, so that they are nolonger in position to support the weight which they have to bear. Theamount of this swaying is naturally very much greater than that whichthe earth itself experiences in the movement. A building of any heightwith its walls unsupported by neighbouring structures may find itsroof rocked to and fro through an arc which has a length of feet, while its base moves only through a length of inches. The reader maysee an example of this nature if he will poise a thin book or a bit ofplank a foot long on top of a small table; then jarring the table sothat it swings through a distance of say a quarter of an inch, he willsee that the columnar object swings at its top through a much greaterdistance, and is pretty sure to be overturned. 

Where a building carries a load in its upper parts, such as may beafforded by its heavy roof or the stores which it contains, the effectof an earthquake shock such as carries the earth to and fro becomesmuch more destructive than it might otherwise be. This weight lagsbehind when the earth slips forward in the first movement of theoscillation, with the effect that the walls of the building are prettysure to be thrust so far beyond the perpendicular that they give wayand are carried down by the weight which they bore. It has often beenremarked in earthquake shocks that tall columns, even where composedof many blocks, survive a shock which overturns lower buildings wherethin walls support several floors, on each of which is accumulated aconsiderable amount of weight. In the case of the column, the strainsare even, and the whole structure may rock to and fro without topplingover. As the energy of the undulations diminish, it gradually regainsthe quiet state without damage. In the ordinary edifice the irregulardisposition of the weight does not permit the uniform movement whichmay insure safety. Thus, if the city of Washington should ever beviolently shaken, the great obelisk, notwithstanding that it is fivehundred feet high, may survive a disturbance which would wreck thelower and more massive edifices which lie about it. 

Where, as is fortunately rarely the case, the great shock comes tothe earth in a vertical direction, the effect upon all movable objectsis in the highest measure disastrous. In such a case buildings arecrushed as if by the stroke of a giant's hand. The roofs and floorsare at one stroke thrown to the foundations, and all the parts of thewalls which are not supported by strong masonry continuous from top tobottom are broken to pieces. In such cases it has been remarked thatthe bodies of men are often thrown considerable distances. It isasserted, indeed, that in the Riobamba shock they were cast upward tothe height of more than ninety feet. It is related that the solosurvivor of a congregation which had hastened at the outset of thedisturbance into a church was thrown by the greatest and mostdestructive shock upward and through a window the base of which was atthe height of more than twenty feet from the ground. 

It is readily understood that an earthquake shock may enter a buildingin any direction between the vertical and the horizontal. As themovement exhausts itself in passing from the place of its origin, thehorizontal shocks are usually of least energy. Those which areaccurately vertical are only experienced where the edifices are placedimmediately over the point where the motion originates. It follows, therefore, that the destructive work of earthquakes is mainlyperformed in that part of the field where the motion is, as regardsits direction, between the vertical and the horizontal--a position inwhich the edifice is likely to receive at once the destructive effectarising from the sharp upward thrust of the vertical movement and theoscillating action of that which is in a horizontal direction. Againststrains of this description, where the movements have an amplitude ofmore than a few inches, no ordinary masonry edifice can be madeperfectly safe; the only tolerable security is attained where thebuilding is of well-framed timber, which by its elasticity permits agood deal of motion without destructive consequences. Even suchbuildings, however, those of the strongest type, may be ruined by thegreater earthquakes. Thus, in the Mississippi Valley earthquake of1811, the log huts of the frontiersmen, which are about as strong asany buildings can be made, were shaken to pieces by the sharp andreiterated shocks. 

It is by no means surprising to find that the style of architectureadopted in earthquake countries differs from that which is developedin regions where the earth is firm-set. The people generally learnthat where their buildings must meet the trials of earthquakes theyhave to be low and strong, framed in the manner of fortifications, towithstand the assault of this enemy. We observe that Gothicarchitecture, where a great weight of masonry is carried upon slendercolumns and walls divided by tall windows, though it became thedominant style in the relatively stable lands of northern Europe, never gained a firm foothold in those regions about the Mediterraneanwhich are frequently visited by severe convulsions of the earth. Therethe Grecian or the Romanesque styles, which are of a much more massivetype, retain their places and are the fashions to the present day. Even this manner of building, though affording a certain securityagainst slight tremblings, is not safe in the greater shocks. Againand again large areas in southern Italy have been almost swept oftheir buildings by the destructive movements which occur in thatrealm. The only people who have systematically adapted theirarchitectural methods to earthquake strains are the Japanese, who incertain districts where such risks are to be encountered constructtheir dwellings of wood, and place them upon rollers, so that they mayreadily move to and fro as the shock passes beneath them. In a measurethe people of San Francisco have also provided against this danger byavoiding dangerous weights in the upper parts of their buildings, aswell as the excessive height to which these structures are lifted insome of our American towns. 

Earthquakes of sensible energy appear to be limited to particularparts of the earth's crust. The regions, indeed, where within theperiod of human history shocks of devastating energy have occurred donot include more than one fifteenth part of the earth's surface. Thereis a common notion that these movements are most apt to happen involcanic regions. It is, indeed, true that sensible shocks commonlyattend the explosions from great craters, but the records clearly showthat these movements are very rarely of destructive energy. Thus inthe regions about the base of Vesuvius and of Ætna, the two volcanoesof which most is known, the shocks have never been productive ofextensive disaster. In fact, the reiterated slight jarrings whichattend volcanic action appear to prevent the formation of those greatand slowly accumulated strains which in their discharge produce themost violent tremblings of the earth. The greatest and most continuousearthquake disturbances of history--that before noted in the earlydays of this century, in the Mississippi Valley, where shocks ofconsiderable violence continued for two years--came about in a fieldvery far removed from active volcanoes. So, too, the disturbancesbeneath the Atlantic floor which originated the shocks that led to thedestruction of Lisbon, and many other similar though less violentmovements, are developed in a field apparently remote from livingvolcanoes. Eastern New England, which has been the seat of severalconsiderable earthquakes, is about as far away from active vents asany place on the habitable globe. We may therefore conclude that, while volcanoes necessarily produce shocks resulting from thedischarge of their gases and the intrusion of lava into the dikeswhich are formed about them, the greater part of the important shocksare in no wise connected with volcanic explosions. 

With the exception of the earthquake in the Mississippi Valley, allthe great shocks of which we have a record have occurred in or nearregions where the rocks have been extensively disturbed bymountain-building forces, and where the indications lead us tobelieve that dislocations of strata, such as are competent to rive thebeds asunder, may still be in progress. This, taken in connection withthe fact that many of these shocks are attended by the formation offault planes, which appear on the surface, lead us to the conclusionthat earthquakes of the stronger kind are generally formed by theriving of fissures, which may or may not be developed upward to thesurface. This view is supported by many careful observations on theeffect which certain great earthquakes have exercised on the buildingswhich they have ravaged. The distinguished observer, Mr. CharlesMallet, who visited the seat of the earthquake which, in 1854, occurred in the province of Calabria in Italy, with great labour andskill determined the direction in which the shock moved through somehundreds of edifices on which it left the marks of its passage. Platting these lines of motion, he found that they were all referredto a vertical plane lying at the depth of some miles beneath thesurface, and extending for a great distance in a north and southdirection. This method of inquiry has been applied to other fields, with the result that in the case of all the instances which have beensubjected to this inquiry the seat of the shock has been traced tosuch a plane, which can best be accounted for by the supposition of afault. 

The method pursued by Mr. Mallet in his studies of the origin ofearthquakes, and by those who have continued his inquiry, may bebriefly indicated as follows: Examining disrupted buildings, it iseasy to determine those which have been wrecked by a shock thatemerged from the earth in a vertical direction. In these cases, thoughtall walls may remain standing, the roofs and floors are thrown intothe cellars. With a dozen such instances the plane of what is calledthe seismic vertical is established (_seismos_ is the Greek forearthquake). Then on either side of this plane, which indicates theline but not the depth of the disturbance, other observations may bemade which give the clew to the depth. Thus a building may be foundwhere the northwest corner at its upper part has been thrown off. Sucha rupture was clearly caused by an upward but oblique movement, whichin the first half of the oscillation heaved the structure upwardlyinto the northwest, and then in the second half, or rebound, drew themass of the building away from the unsupported corner, allowing thatpart of the masonry to fly off and fall to the ground. Constructing aline at right angles to the plane of the fracture, it will be found tointersect the plane, the position of which has been in part determinedby finding the line where it intersects the earth, or the seismicvertical before noted. Multiplying such observations on either side ofthe last-mentioned line, the attitude of the underground parts of theplane, as well as the depth to which it attained, can be approximatelydetermined. 

It is worth while to consider the extent to which earthquake shocksmay affect the general quality of the people who dwell in countrieswhere these disturbances occur with such frequency and violence as toinfluence their lives. There can be no question that whereverearthquakes occur in such a measure as to produce widespread terror, where, recurring from time to time, they develop in men a sense ofabiding insecurity, they become potent agents of degradation. All thebest which men do in creating a civilization rests upon a sense ofconfidence that their efforts may be accumulated from year to year, and that even after death the work of each man may remain as aheritage to his kind. It is likely, indeed, that in certain realms, asin southern Italy, a part of the failure of the people to advance inculture is due to their long experience of such calamities, and thenatural expectation that they will from time to time recur. In asimilar way the Spanish settlements in Central and South America, which lie mostly in lands that are subject to disastrous shocks, mayhave been retarded by the despair, as well as the loss of propertyand life, which these accidents have so frequently inflicted uponthem. It will not do, however, to attribute too much to suchterrestrial influences. By far the most important element indetermining the destiny of a people is to be found in their nativequality, that which they owe to their ancestors of distantgenerations. In this connection it is well to consider the history ofthe Icelandic people, where a small folk has for a thousand years beenexposed to a range and severity of trials, such as earthquakes, volcanic explosions, and dearth of harvests may produce, and all thesein a measure that few if any other countries experience. Notwithstanding these misfortunes, the Icelanders have developed andmaintained a civilization which in all else, except its materialresults, on the average transcends that which has been won by anyother folk in modern times. If a people have the determining spiritwhich leads to high living, they can successfully face calamities fargreater than those which earthquakes inflict. 

It was long supposed that the regions where earthquakes are notnoticeable by the unaided senses were exempt from all suchdisturbances. The observations which seismologists have made in recentyears point to the conclusion that no part of the earth's surface isquite exempt from movements which, though not readily perceived, canbe made visible by the use of appropriate instruments. With anapparatus known as the horizontal pendulum it is possible to observevibrations which do not exceed in amplitude the hundredth part of aninch. This mechanism consists essentially of a slender bar supportednear one end by two wires, one from above, the other from below. Itmay readily be conceived that any measurable movement will cause thelonger end of the rod to sway through a considerable arc. Whereversuch a pendulum has been carefully observed in any district, it hasbeen found that it indicates the occurrence of slight tremors. Evencertain changes of the barometer, which alter the weight of theatmosphere that rests upon the earth to the amount indicated by aninch in the height of the mercury column, appears in all cases tocreate such tremors. Many of these slight shocks may be due to theeffect of more violent quakings, which have run perhaps for thousandsof miles from their point of origin, and have thus been reduced in theamplitude of their movement. Others are probably due to the slightmotion brought about through the chemical changes of the rocks, whichare continuously going on. The ease with which even small motions arecarried to a great distance may be judged by the fact that when theground is frozen the horizontal pendulum will indicate the jarring dueto a railway train at the distance of a mile or more from the track. 

In connection with the earth jarring, it would be well to note theoccurrence of another, though physically different, kind of movement, which we may term earth swayings, or massive movements, which slowlydislocate the vertical, and doubtless also the horizontal, position ofpoints upon its surface. It has more than once been remarked that inmountain countries, where accurate sights have been taken, the heightsof points between the extremities of a long line appear somewhat tovary in the course of a term of years. Thus at a place in theApennines, where two buildings separated by some miles of distance arecommonly intervisible over the crest of a neighbouring peak, it hashappened that a change of level of some one of the points has made itimpossible to see the one edifice from the other. Knowing as we dothat the line of the seacoast is ever-changing, uprising taking placeat some points and down-sinking at others, it seems not unlikely thatthese irregular swayings are of very common occurrence. Moreover, astronomers are beginning to remark the fact that their observatoriesappear not to remain permanently in the same position--that is, theydo not have exactly the same latitude and longitude. Certain of thesechanges have recently been explained by the discovery of a new andhitherto unnoted movement of the polar axis. It is not improbable, however, that the irregular swaying of the earth's crust, due to thefolding of strata and to the alterations in the volume of rocks whichare continually going on, may have some share in bringing about thesedislocations. 

Measured by the destruction which was wrought to the interests of man, earthquakes deserve to be reckoned among the direst calamities ofNature. Since the dawn of history the records show us that thedestruction of life which is to be attributed to them is to be countedby the millions. A catalogue of the loss of life in the accidents ofthis description which have occurred during the Christian era has ledthe writer to suppose that probably over two million persons haveperished from these shocks in the last nineteen centuries. Nevertheless, as compared with other agents of destruction, such aspreventable disease, war, or famine, the loss which has been inflictedby earth movements is really trifling, and almost all of it is due toan obstinate carelessness in the construction of buildings withoutreference to the risks which are known to exist in earthquake-riddencountries. 

Although all our exact knowledge concerning the distribution ofearthquakes is limited to the imperfect records of two or threethousand years, it is commonly possible to measure in a general waythe liability to such accidents which may exist in any country by acareful study of the details of its topography. In almost every largearea the process of erosion naturally leaves quantities of rock, either in the form of detached columns or as detrital accumulationsdeposited on steep slopes. These features are of relatively slowformation, and it is often possible to determine that they have beenin their positions for a time which is to be measured by thousands ofyears. Thus, on inspecting a country such as North America, where thehistoric records cover but a brief time, we may on inquiry determinewhich portions of its area have long been exempt from powerful shocks. Where natural obelisks and steep taluses abound--features which wouldhave disappeared if the region had been moved by great shocks--we maybe sure that the field under inspection has for a great period beenexempt from powerful shaking. Judged by this standard, we may safelysay that the region occupied by the Appalachian Mountains has beenexempt from serious trouble. So, too, the section of the Cordilleraslying to the east of what is commonly called the Great Basin, betweenthe Rocky Mountains and the Sierra Nevada, has also enjoyed a longreign of peace. In glaciated countries the record is naturally lessclear than in those parts of the world which have been subjected tolong-continued, slow decay of the rocks. Nevertheless, in those fieldsboulders are often found poised in position which they could not havemaintained if subjected to violent shaking. Judged by this evidence, we may say that a large part of the northern section of thiscontinent, particularly the area about the Great Lakes, has beenexempt from considerable shocks since the glacier passed away. 

The shores which are subject to the visitations of the great marinewaves, caused by earthquake shocks occurring beneath the bottom of theneighbouring ocean, are so swept by those violent inundations thatthey lose many features which are often found along coasts that havebeen exempted from such visitations. Thus wherever we find extensiveand delicately moulded dunes, poised stones, or slender pinnacledrocks along a coast, we may be sure that since these features wereformed the district has not been swept by these great waves. 

[Illustration: Fig. 22. --Poised rocks indicating a long exemption fromstrong earthquakes in the places where such features occur. ]

Around the northern Atlantic we almost everywhere find the glacialwaste here and there accumulated near the margin of the sea in thecomplicated sculptured outlines which are assumed by kame sands andgravels. From a study of these features just above the level of hightide, the writer has become convinced that the North Atlantic districthas long been exempt from the assaults of other waves than those whichare produced during heavy storms. At the present time the wavesformed by earthquakes appear to be of destructive violence only on thewest coast of South America, where they roll in from a region of thePacific lying to the south of the equator and a few hundred miles fromthe shore of the continent, which appears to be the seat ofexceedingly violent shocks. A similar field occurs in the Atlanticbetween the Lesser Antilles and the Spanish peninsula, but no greatwaves have come thence since the time of the Lisbon earthquake. Thebasin of the Caribbean and the region about Java appear to be alsofields where these disturbances may be expected, though in each butone wave of this nature has been recorded. Therefore we may regardthese secondary results of a submarine earthquake as seldom phenomena. 

 DURATION OF GEOLOGICAL TIME. 

Although it is beyond the power of man to conceive any such lapses oftime as have taken place in the history of this earth, it isinteresting, and in certain ways profitable, to determine as near aspossible in the measure of years the duration of the events which arerecorded in the rocks. Some astronomers, basing their conclusions onthe heat-containing power of matter, and on the rate at which energyin this form flows from the sun, have come to the conclusion that ourplanet could not have been in independent existence for more thanabout twenty million years. The geologist, however, resting hisconclusions on the records which are the subject of his inquiry, comeson many different lines to an opinion which traverses that entertainedby some distinguished astronomers. The ways in which the student ofthe earth arrives at this opinion will now be set forth. 

By noting the amount of sediment carried forth to the sea by therivers, the geologist finds that the lands of the earth--those, atleast, which are protected by their natural envelopes ofvegetation--are wearing down at a rate which pretty certainly doesnot exceed one foot in about five thousand years, or two hundred feetin a million years. Discovering at many places on the earth's surfacedeposits which originally had a thickness of five thousand feet ormore, which have been worn down to the depths of thousands of feet ina single rather brief section of geological time, the student readilyfinds himself prepared to claim that a period of from five to tenmillion years has often been required for the accomplishment of but avery small part of the changes which he knows to have occurred on thisearth. 

As the geologist follows down through the sections of the stratifiedrocks, and from the remains of strata determines the erosion which hasborne away the greater part of the thick deposits which have beenexposed to erosion, he comes upon one of those breaks in thesuccession, or encounters what is called an unconformity, as whenhorizontal strata lie against those which are tilted. In many cases hemay observe that at this time there was a great interval unrepresentedby deposits at the place where his observations are made, yet a greatlapse of time is indicated by the fact that a large amount of erosiontook place in the interval between the two sets of beds. 

Putting together the bits of record, and assuming that the rate oferosion accomplished by the agents which operate on the land hasalways been about the same, the geologist comes to the conclusion thatthe section of the rocks from the present day to the lowest strata ofthe Laurentian represents in the time required for their formation notless than a hundred million years; more likely twice that duration. Tothis argument objection is made by some naturalists that the agents oferosion may have been more active in the past than they are atpresent. They suggest that the rainfall may have been much greater orthe tides higher than they now are. Granting all that can be claimedon this score, we note the fact that the rate of erosion evidentlydoes not increase in anything like a proportionate way with theamount of rainfall. Where a country is protected by its naturalcoating of vegetation, the rain is delivered to the streams withoutmaking any considerable assault upon the surface of the earth, howeverlarge the fall may be. Moreover, the tides have little direct cuttingpower; they can only remove detritus which other agents have broughtinto a condition to be borne away. The direct cutting power of thetidal movement does not seem to be much greater in the Bay of Fundy, where the maximum height of the waves amounts to fifty feet, than onthe southern coast of Massachusetts, where the range is not more thanfive. So far as the observer can judge, the climatal conditions andthe other influences which affect the wear of rocks have not greatlyvaried in the past from what they are at the present day. Now and thenthere have been periods of excessive erosion; again, ages in whichlarge fields were in the conditions of exceeding drought. It is, however, a fair presumption that these periods in a way balance eachother, and that the average state was much like that which we find atpresent. 

If after studying the erosive phenomena exhibited in the structure ofthe earth the student takes up the study of the accumulations ofstrata, and endeavours to determine the time required for the layingdown of the sediments, he finds similar evidence of the earth's greatantiquity. Although the process of deposition, which has given us therocks visible in the land masses, has been very much interrupted, thesection which is made by grouping the observations made in variousfields shows that something like a maximum thickness of a hundred andfifty thousand feet of beds has been accumulated in that part ofgeologic time during which strata were being laid down in the fieldsthat are subjected to our study. Although in these rocks there aremany sets of beds which were rapidly formed, the greater part of themhave been accumulated with exceeding slowness. Many fine shales, suchas those which plentifully occur in the Devonian beds of this country, must have required a thousand years or more for the deposition of thematerials that now occupy an inch in depth. In those sections a singlefoot of the rock may well represent a period of ten thousand years. Inmany of the limestones the rate of accumulation could hardly have beenmore speedy. The reckoning has to be rough, but the impression whichsuch studies make upon the mind of the unprejudiced observer is to theeffect that the thirty miles or so of sedimentary deposits could nothave been formed in less than a hundred million years. In thisreckoning it should be noted that no account is taken of those greatintervals of unrecorded time, such as elapsed between the close of theLaurentian and the beginning of the Cambrian periods. 

There is a third way in which we may seek an interpretation ofduration from the rocks. In each successive stage of the earth'shistory, in different measure in the various ages, mountains wereformed which in time, during their exposure to the conditions of theland, were worn down to their roots and covered by depositsaccumulated during the succeeding ages. A score or more of thesesuccessively constructed series of elevations may readily be observed. Of old, it was believed that mountain ranges were suddenly formed, butthere is, however, ample evidence to prove that these disturbedportions of the strata were very gradually dislocated, the rate of themountainous growth having been, in general, no greater in the pastthan it is at the present day, when, as we know full well, themovements are going on so slowly that they escape observation. Onlyhere and there, as an attendant on earthquake shocks or other relatedmovements of the crust, do we find any trace of the upward march whichproduces these elevations. Although not a subject for exactmeasurements, these features of mountain growth indicate a vast lapseof time, during which the elevations were formed and worn away. 

Yet another and very different method by which we may obtain somegauge of the depths of the past is to be found in the steps which haveled organic life from its lowest and earliest known forms to thepresent state of advancement. Taking the changes of species which haveoccurred since the beginning of the last ice epoch, we find that thechanges which have been made in the organic life have been very small;no naturalist who has obtained a clear idea of the facts will questionthe statement that they are not a thousandth part of the alterationswhich have occurred since the Laurentian time. The writer is of theopinion that they do not represent the ten thousandth part of thosevast changes. These changes are limited in the main to thedisappearance of a few forms, and to slight modifications in thosepreviously in existence which have survived to the present day. So faras we can judge, no considerable step in the organic series has takenplace in this last great period of the earth's history, although ithas been a period when, as before noted, all the conditions havecombined to induce rapid modifications in both animals and plants. If, then, we can determine the duration of this period, we may obtain agauge of some general value. 

Although we can not measure in any accurate way the duration of theevents which have taken place since the last Glacial period began towane, a study of the facts seems to show that less than a hundredthousand years can not well be assumed for this interval. Some of thestudents who have approached the subject are disposed to allow aperiod of at least twice this length as necessary for the perspectivewhich the train of events exhibits. Reckoning on the lowest estimate, and counting the organic changes which take place during the age asamounting to the thousandth part of the organic changes since theLaurentian age, we find ourselves in face once again of thatinconceivable sum which was indicated by the physical record. 

Here, again, the critics assert that there may have been periods inthe history of the earth when the changes of organic life occurred ina far swifter manner than in this last section of the earth's history. This supposition is inadmissible, for it rests on no kind of proof; itis, moreover, contraindicated by the evident fact that the advance inthe organic series has been more rapid in recent time than at anystage of the past. In a word, all the facts with which the geologistdeals are decidedly against the assumption that terrestrial changes inthe organic or the inorganic world ever proceed in a spasmodic manner. Here and there, and from time to time, local revolutions of a violentnature undoubtedly occur, but, so far as we may judge from the aspectof the present or the records of the past, these accidents arestrictly local; the earth has gone forward in its changes much as itis now advancing. Its revolutions have been those of order rather thanthose of accident. 

The first duty of the naturalist is to take Nature as he finds it. Hemust avoid supposing any methods of action which are not clearlyindicated in the facts that he observes. The history of his own and ofall other sciences clearly shows that danger is always incurred wheresuppositions as to peculiar methods of action are introduced into theinterpretation. It required many centuries of labour before thestudents of the earth came to adopt the principle of explaining theproblems with which they had to deal by the evidence that the earthsubmitted to them. Wherever they trusted to their imaginations forguidance, they fell into error. Those who endeavour to abbreviate ourconception of geologic time by supposing that in the olden days theorder of events was other than that we now behold are going counter tothe best traditions of the science. 

Although the aspect of the record of life since the beginning of theCambrian time indicates a period of at least a hundred million years, it must not be supposed that this is the limit of the time requiredfor the development of the organic series. All the important types ofanimals were already in existence in that ancient period with theexception of the vertebrates, the remains of which have apparently nowbeen traced down to near the Cambrian level. In other words, at thestage where we first find evidence of living beings the series towhich they belong had already climbed very far above the level oflifeless matter. Few naturalists will question the statement that halfthe work of organic advance had been accomplished at the beginning ofthe Cambrian rocks. The writer is of the opinion that the developmentwhich took place before that age must have required a much longerperiod than has elapsed from that epoch to the present day. We thuscome to the conclusion that the measurement of duration afforded byorganic life indicates a yet more lengthened claim of events, anddemands more time than appears to be required for the formation of thestratified rocks. 

The index of duration afforded by the organic series is probably moretrustworthy than that which is found in the sedimentary strata, andthis for the reason that the records of those strata have beensubjected to numerous and immeasurable breaks, while the developmentof organic life has of necessity been perfectly continuous. The onerecord can at any point be broken without interrupting the sequences;the other does not admit of any breaches in the continuity. 

 THE MOON. 

Set over against the earth--related to, yet contrasted with it in manyways--the moon offers a most profitable object to the student ofgeology. He should often turn to it for those lessons which will bebriefly noted. 

In the beginning of their mutual history the materials of earth andmoon doubtless formed one vaporous body which had been parted from theconcentrating mass of the sun in the manner noted in the sketch ofthe history of the solar system. After the earth-moon body hadgathered into a nebulous sphere, it is most likely that a ringresembling that still existing about Saturn was formed about theearth, which in time consolidated into the satellite. Thenceforth thetwo bodies were parted, except for the gravitative attraction whichimpelled them to revolve about their common centre of gravity, andexcept for the light and heat they might exchange with one another. 

The first stages after the parting of the spheres of earth and moonappear to have been essentially the same in each body. Concentratingupon their centres, they became in time fluid by heat; further on, they entered the rigid state--in a word, they froze--at least in theirouter parts. At this point in their existence their histories utterlydiverge; or rather, we may say, the development of the earth continuedin a vast unfolding, while that of the moon appears to have beenabsolutely arrested in ways which we will now describe. 

With the naked eye we see on the moon a considerable variation in thelight of different parts of its surface; we discern that the darkerpatches appear to be rudely circular, and that they run together ontheir margins. Seeing this little, the ancients fancied that oursatellite had seas and lands like the earth. The first telescopes didnot dispel their fancies; even down to the early part of this centurythere were astronomers who believed the moon to be habitable; indeed, they thought to find evidence that it was the dwelling place ofintelligent beings who built cities, and who tried to signal theirintellectual kindred of this planet. When, however, strong glasseswere applied to the exploration, these pleasing fancies were rudelydispelled. 

Seen with a telescope of the better sort, the moon reveals itself tobe in large part made up of circular depressions, each surrounded by aringlike wall, with nearly level but rough places between. Thelargest of these walled areas is some four hundred miles in diameter;thence they grade down to the smallest pits which the glass candisclose, which are probably not over as many feet across. The writer, from a careful study of these pits, has come to the conclusion thatthe wider are the older and the smaller the last formed. The rudeelevations about these pits--some of which rise to the height of tenthousand feet or more--constitute the principal topographic reliefs ofthe lunar surface. Besides the pits above mentioned, there arenumerous fractures in the surface of the plains and ringlike ridges;on the most of these the walls have separated, forming trenches notunlike what we find in the case of some terrestrial breaks such ashave been noted about volcanoes and elsewhere. It may be that theso-called canals of Mars are of the same nature. 

[Illustration: Fig. 23. --Lunar mountains near the Gulf of Iris. ]

The most curious feature on the moon's surface are the bands oflighter colour, which, radiating from certain of the volcanolikepits--those of lesser size and probably of latest origin--extend insome cases for five hundred miles or more across the surface. Theselight bands have never been adequately explained. It seems most likelythat they are stains along the sides of cracks, such as are sometimesobserved about volcanoes. 

The eminent peculiarity of the moon is that it is destitute of anykind of gaseous or aqueous envelope. That there is no distinctatmosphere is clearly shown by the perfectly sharp and sudden way inwhich the light of a star disappears when it goes behind the moon andthe clear lines of the edge of the satellite in a solar eclipse. Thesame evidence shows that there is no vapour of water; moreover, acareful search which the writer has made shows that the surface hasnone of those continuous down grades which mark the work of waterflowing over the land. Nearly all of the surface consists of shallowor deep pits, such as could not have been formed by water action. Wetherefore have not only to conclude that the moon is waterless, butthat it has been in this condition ever since the part that is turnedtoward us was shaped. 

As the moon, except for the slight movement termed its "libration, "always turns the same face to us, so that we see in all only aboutfour sevenths of its surface, it has naturally been conjectured thatthe unseen side, which is probably some miles lower than that turnedtoward us, might have a different character from that which we behold. There are reasons why this is improbable. In the first place, we seeon the extreme border of the moon, when the libration turns one sidethe farthest around toward the earth, the edge of a number of thegreat walled pits such as are so plenty on the visible area; it isfair to assume that these rings are completed in the invisible realm. On this basis we can partly map about a third of the hidden side. Furthermore, there are certain bands of light which, though appearingon the visible side, evidently converge to some points on the other. It is reasonable to suppose that, as all other bands radiate fromwalled pits, these also start from such topographic features. In thisway certain likenesses of the hidden area to that which is visible isestablished, thus making it probable that the whole surface of thesatellite has the same character. 

Clearly as the greater part of the moon is revealed to us--so clearly, indeed, that it is possible to map any elevation of its surface thatattains the height of five hundred feet--the interpretation of itsfeatures in the light of geology is a matter of very greatdifficulty. The main points seem to be tolerably clear; they are asfollows: The surface of the moon as we see it is that which was formedwhen that body, passing from the state of fluidity from heat, formed asolid crust. The pits which we observe on its surface are thedepressions which were formed as the mass gradually ceased to boil. The later formed of these openings are the smaller, as would be thecase in such a slowing down of a boiling process. 

As the diameter of the moon is only about one fourth of that of theearth, its bulk is only about one sixteenth of that of its planet;consequently, it must have cooled to the point of solidification agesbefore the larger sphere attained that state. It is probable that thesame changeless face that we see looked down for millions of years onan earth which was still a seething, fiery mass. In a word, all thatvast history which is traceable in the rocks beneath our feet--whichis in progress in the seas and lands and is to endure for aninconceivable time to come--has been denied our satellite, for thereason that it had no air with which to entrap the solar heat and nowater to apply the solar energy to evolutionary processes. The heatwhich comes upon the moon as large a share for each equal area as itcomes upon the earth flies at once away from the airless surface, atmost giving it a temporary warmth, but instituting no geological workunless it be a little movement from the expansion and contraction ofthe rocks. During the ages in which the moon has remained thuslifeless the earth, owing to its air and water, has applied a vastamount of solar energy to geological work in the development andredevelopment of its geological features and to the processes oforganic life. We thus see the fundamental importance of the volatileenvelopes of our sphere, how absolutely they have determined itshistory. 

It would be interesting to consider the causes which led to theabsence of air and water on the moon, but this matter is one of themost debatable of all that relates to that sphere; we shall thereforehave to content ourselves with the above brief statements as to thevast and far-acting effects which have arisen from the non-existenceof those envelopes on our nearest neighbour of the heavens. 

 METHODS IN STUDYING GEOLOGY. 

So far as possible the preceding pages, by the method adopted in thepresentation of facts, will serve to show the student the ways inwhich he may best undertake to trace the order of events exhibited inthe phenomena of the earth. Following the plan pursued, we shall nowconsider certain special points which need to be noted by those whowould adopt the methods of the geologist. 

At the outset of his studies it may be well for the inquirer to notethe fact that familiarity with the world about him leads the man inall cases to a certain neglect and contempt of all the familiarpresentations of Nature. We inevitably forget that those points oflight in the firmament are vast suns, and we overlook the fact thatthe soil beneath our feet is not mere dirt, but a marvellousstructure, more complicated in its processes than the chemist'slaboratory, from which the sustenance of our own and all other livesis drawn. We feel our own bodies as dear but commonplace possessions, though we should understand them as inheritances from theinconceivable past, which have come to us through tens of thousands ofdifferent species and hundreds of millions of individual ancestors. Wemust overlook these things in our common life. If we could take theminto account, each soul would carry the universe as an intellectualburden. 

It is, however, well from time to time to contemplate the truth, andto force ourselves to see that all this apparently simple and ordinarymedley of the world about us is a part of a vast procession of events, coming forth from the darkness of the past and moving on beyond thelight of the present day. Even in his professional work thenaturalist of necessity falls into the commonplace way of regardingthe facts with which he deals. If he be an astronomer, he cataloguesthe stars with little more sense of the immensities than the man whokeeps a shop takes account of his wares. Nevertheless, the real profitof all learning is in the largeness of the understanding which itdevelops in man. The periods of growth in knowledge are those in whichthe mind, enriched by its store, enlarges its conception while itescapes from commonplace ways of thought. With this brief mention ofwhat is by far the most important principle of guidance which thestudent can follow, we will turn to the questions of method that thestudent need follow in his ordinary work. 

With almost all students a difficulty is encountered which hindersthem in acquiring any large views as to the world about them. This isdue to the fact that they can not make and retain in memory clearpictures of the things they see. They remember words rather thanthings--in fact, the training in language, which is so large a part ofan education, tends ever to diminish the element of visual memory. Thefirst task of the student who would become a naturalist is to take hisknowledge from the thing, and to remember it by the mental picture ofthe thing. In all education in Nature, whether the student is guidedby his own understanding or that of the teacher, a first and verycontinuous aim should be to enforce the habit of recalling verydistinct images of all objects which it is desired to remember. Tothis end the student should practise himself by looking intently upona landscape or any other object; then, turning away, he should try torecall what he has beheld. After a moment the impression by the sightshould be repeated, and the study of the memory renewed. The writerknows by his own experience that even in middle-aged people, where itis hard to breed new habits, such deliberate training can greatlyincrease the capacity of the memory for taking in and reproducingimages which are deemed of importance. Practice of this kind shouldform a part of every naturalist's daily routine. After a certain time, it need not be consciously done. The movements of thought and actionwill, indeed, become as automatic as those which the trained fencermakes with his foil. 

Along with the habit of visualizing memories, and of storing themwithout the use of words, the student should undertake to enlarge hispowers of conceiving spaces and directions as they exist in the fieldabout him. Among savages and animals below the grade of man, thisunderstanding of spacial relations is very clear and strong. Itenables the primitive man to find his way through the tracklessforest, and the carrier pigeon to recover his mate and dwelling placefrom the distance of hundreds of miles away. In civilized men, however, the habit of the home and street and the disuse of theancient freedom has dulled, and in some instances almost destroyed, all sense of this shape of the external world. The best training torecover this precious capacity will now be set forth. 

The student should begin by drawing a map on a true scale, howeverroughly the work may be done, of those features of the earth about himwith which he is necessarily most familiar. The task may well be begunwith his own dwelling or his schoolroom. Thence it may be extended soas to include the plan of the neighbouring streets or fields. Atfirst, only directions and distances should be platted. After a timeto these indications should be added on the map lines indicating in ageneral way contours or the lines formed by horizontal planesintersecting the area subject to delineation. After attaining certainrude skill in such work, the student may advantageously makeexcursions to districts which he can see only in a hurried way. As hegoes, he should endeavour to note on a sketch map the positions of thehills and streams and the directions of the roads. A year of holidaypractice in such work will, if the tasks occupy somewhere about ahundred hours of his time, serve greatly to extend or reawaken whatmay be called the topographic sense, and enable him to place in termsof space the observations of Nature which he may make. 

In his more detailed work the student should select some particularfield for his inquiry. If he be specially interested in geologicphenomena, he will best begin by noting two classes of facts--thoseexhibited in the rocks as they actually appear in the state of reposeas shown in the outcrops of his neighbourhood, and those shown in theactive manifestations of geological work, the decay of the rocks andthe transportation of their waste, or, if the conditions favour, thecomplicated phenomena of the seashores. 

As soon as the student begins to observe, he should begin to make arecord of his studies. To the novice in any science written, andparticularly sketched, notes are of the utmost importance. These, whether in words or in drawings, should be made in face of the facts;they should, indeed, be set down at the close of an observation, though not until the observer feels that the object he is studying hasyielded to him all which it can at that time give. It is well toremark that where a record is made at the outset of a study thestudent is apt to feel that he is in some way pledged to shape all hemay see to fit that which he has first written. In his earlyexperience as a teacher, the writer was accustomed to have studentscompare their work of observation and delineation with that done bytrained men on the same ground. It now seems to him best for thebeginner at first to avoid all such reference of his own work to thatof others. So great is the need of developing independent motive thatit is better at the outset to make many blunders than to secureaccuracy by trust in a leader. The skilful teacher can give fittingwords of caution which may help a student to find the true way, butany reference of his undertakings to masterpieces is sure to breed aservile habit. Therefore such comparisons are fitting only after thehabit of free work has been well formed. The student who can affordthe help of a master, or, better, the assistance of many, such as someof our universities offer, should by all means avail himself of thisresource. More than any other science, geology, because of thecomplexity of the considerations with which it has to deal, dependsupon methods of labour which are to a great extent traditional, andwhich can not, indeed, be well transmitted except in the personal way. In the distinctly limited sciences, such as mathematics, physics, oreven those which deal with organic bodies, the methods of work can beso far set forth in printed directions that the student may to a greatextent acquire sound ways of work without the help of a teacher. 

Although there is a vast and important literature concerning geology, the greater part of it is of a very special nature, and will convey tothe beginner no substantial information whatever. It is not until hehas become familiar with the field with which he is enabled to deal inthe actual way that he can transfer experience thus acquired to othergrounds. Therefore beyond the pleasing views which he may obtain byreading certain general works on the science, the student should atthe outset of his inquiry limit his work as far as possible to hisfield of practice, using a good text-book, such as Dana's Manual ofGeology, as a source of suggestions as to the problems which his fieldmay afford. 

The main aim of the student in this, as in other branches of inquiry, is to gain practice in following out the natural series of actions. Tothe primitive man the phenomenal world presents itself as a merephantasmagoria, a vast show in which the things seen are only relatedto each other by the fact that they come at once into view. The end ofscience is to divine the order of this host, and the ways in which itis marshalled in its onward movement and the ends to which its marchappears to be directed. So far as the student observes well, and thusgains a clear notion of separated facts, he is in a fair way togather the data of knowledge which may be useful; but the real valueof these discernments is not gained until the observations gotogether, so as to make something with a perspective. Until the storeof separate facts is thus arranged, it is merely crude material forthought; it is not in the true meaning science, any more than a storeof stone and mortar is architecture. When the student has developed anappetite for the appreciation of order and sources of energy inphenomena, he has passed his novitiate, and becomes one of that happybody of men who not only see what is perceived by the mass of theirfellows, but are enabled to look through those chains of action which, when comprehended, serve to rationalize and ennoble all that thesenses of man, aided by the instruments which he has devised, tell usconcerning the visible world. 

 INDEX. 

 Ætna, Mount, 381. 

 Agriculture, American, 346; in England, winning swamp lands for, 335; recent developments of, 345. 

 Alaska, changes on the coast of, 96. 

 Ants taking food underground, 319; work of the, on the soil, 318. 

 Apsides, revolution of the, 61, 62. 

 Arabians, chemical experiments of the, 13. 

 Arches, natural, in cavern districts, 258. 

 Artesian wells, 258, 259. 

 Arts, advance of Italian fine, 19. 

 Asteroids, 53; motions of, about their centres and about the sun, 53. 

 Astronomers, the solar system and the early, 79. 

 Astronomy, 31-80; growth of, since the time of Galileo, 33, 34; the first science, 10. 

 Atmosphere, 97-206; along the tropical belt, 102; as a medium of communication between different regions, 99; deprived of water, containing little heat, 105; beginning of the science of the, 117; counter-trade movements of the, 105; envelope of the earth, 98; expansion of, in a hollow wall during the passage of a storm, 114; heat-carrying power of the, 105; heights to which it extends, 99; in water, 99; movements no direct influence on the surface of the earth, 122; movements of the, qualified by the condition which it encounters, 118; of mountains, 98; of the seashore, 98; of the earth, 98; of the sun, 73; snow as an evidence of, 65; supplying needs of underground creatures, 331; uprushes of, 101, 102; upward strain of the, next the earth, 107; weight and motion of the, 120, 121. 

 Atmospheric circulation of the soil, 330, 331; envelopes, 97. 

 Aurora borealis, 168. 

 Avalanches, 210-213; dreaded, in the Alpine regions, 212; great, in the Swiss Oberland, 211, 212; rocky, 175-177. 

 Axis, imaginary changes in the earth's, 59; of the earth's rotation, 58; polar, inclined position of, 58; polar, nodding movement of the axes, 54; rotations of the planetary spheres on their axes, 56. 

 Barometer, causes of changes in the, 117, 118. 

 Basalts, 309. 

 Beaches, 93, 142, 144; boulder, 142, 143; pebbly, 142; sand, 144. 

 Beetles, work of, on the soil, 318, 319. 

 Belief of the early astronomers about the solar system, 79. 

 _Bergschrund_, the, 214. 

 Birds and mammals contributing to the fertility of the soil, 319. 

 "Blanketing, " 269. 

 Bogs, climbing, 331-334; lake, 331-333; peat, 334, 335; quaking, 334. 

 Botany, rapid advance in, 14, 15. 

 Boulders, 217, 220. 

 Breakers, 135, 137, 139. 

 Bridges, natural, 257, 258. 

 Canals of Mars, 67. 

 Cañon, newly formed river cutting a, 195. 

 Cataracts, 193. 

 Caves, 253-258, 261; architecture of, 255-258; hot-water, 261; mammoth cave, 258; stalactites and stalagmites on the roof and floor of, 257. 

 Chasms, 140, 141. 

 Chemistry, 6, 12, 14; advance of, 12; modern, evolving from the studies of alchemists, 13, 14. 

 Chromosphere, 73. 

 Civilization of the Icelanders, 384. 

 Cliffs, sea-beaten, 132, 141, 142. 

 Climate, changes of, due to modifications of the ocean streams, 153; effect of the ocean on the, 147; of the Gulf Stream, 149, 150. 

 Clouds, 159; formation of, 162, 163; shape of, 163; water of, usually frozen, 207; cloud-making, laws of, 161, 162. 

 Coast, changes on the Scandinavian, 96; line, effect of tide on the, 145; of Greenland, 226; of New Jersey sinking, 95; marine, changes in, 95. 

 Cold in Siberia, 243. 

 Comets, 47, 50; collisions of, 50; kinship of meteorites and, 48; omens of calamity to the ancients, 50; the great, of 1811, 49, 50. 

 Cones. See under VOLCANOES. 

 Conflict between religion and science, 20, 22; between the Protestant countries and the followers of science, 20. 

 Continental shelves, 125. 

 Continents and oceans, 83; changes in position of, 91; cyclones of the, 111; forms of, 90; proofs that they have endured for many years, 92; shape of, 84, 96. 

 Coral reefs, 153, 353. 

 Corona, realm of the, 73. 

 Craters. See under VOLCANOES. 

 Crevasse, a barrier to the explorer, 218. 

 Crevice water, 250. 

 Curds, 214. 

 Currents, coral reefs in Florida affecting the velocity of, 153; equatorial, 150; of the Gulf Stream, 147-149; hot and cold, of the sea, 102; ocean, 145; oceanic action of trade winds on, 145; effect on migration of, 157; icebergs indicating, 243; surface, history of, 172; uprushing, near the equator, 106. 

 Cyclones, 111; cause of, 111; of North America, 111; secondary storms of, 112. 

 Deltas, 173, 187. 

 Deposits, vein, 260, 261. 

 Deserts, interior, 158. 

 Dew, 159, 160; a concomitant of cloudless skies, 160, and vegetation, 160; formation of, 159-161. 

 Diablerets, 174. 

 Diagram of a vein, 260; showing development of swamp, 335; how a portion of the earth's surface may be sunk by faulting, 374; growth of mangroves, 340; the effect of the position of the fulcrum point in the movement of the land masses, 94. 

 Diameter of our sphere at the equator, 62; of the earth, 82. 

 Dikes, 192, 293; 305-310; abounding in volcanic cones, 305; cutting through coal, 306; driven upward, 307; formation of, 305, 310; material of, 307, 308; representing movements of softened rock, 309; their relation to volcanic cones, 307; variations of the materials of, 307, 308; waterfalls produced by, 192; zone of, 306. 

 Dismal Swamp, 95, 333. 

 Distances, general idea of, 27; good way to study, 27, 28; training soldiers to measure, 28. 

 Doldrums, 104, 109; doldrum of the equator, 109; of the hurricane, 109. 

 Drainage, imperfect, of a country affected by glaciers, 242. 

 Dunes, 123, 124, 325, 326, 387; moulded, 387. 

 Duration of geological time, 389. 

 Dust accumulations from wind, in China, 122. 

 Earth, a flattened sphere, 82; air envelope of the, 98; amount of heat falling from the sun on the, 41; antiquity of the, 391; atmosphere of the, 98; attracting power of the, 127; axis of the rotation of the, 58; composition of the atmosphere of the, 98; crust of the, affected by weight, 93; deviation of the path of the, varied, 61; diameter of the, 82; of the, affected by loss of heat, 131; difference in altitude of the surface of the, 83; discovery that it was globular, 31, 32; effect of imaginary changes in the relations of sun and, 59; effect of the interior heat of the, 309, 310; effect of the sun on the, 60, 61; formerly in a fluid state, 82; imaginary view of the, from the moon, 81; important feature of the surface of the, 83; jarring caused by faults, 367; surface of the, determined by heat and light from the sun, 57; most important feature of the surface of the, 83; motion of the, affecting the direction of trade winds, 103; movements, 366; natural architecture of the, 377; no part of the, exempt from movement, 384; parting of the moon and, 396; path of the, around the sun, 55, 56, 59, 60; revolving from east to west, 103; shrinking of the, from daily escape of heat, 89; soil-covering of the, 343; study of the, 81-96; swaying, 385; tensions, problem of, 371; tremors, caused by chemical changes in the rocks, 385; tropical belt of the, 74; viewed from the surface of the moon, 311, 312; water store of the, 125. 

 Earthquakes, 277, 278, 280, 356, 358, 370-384, 388-390; accidents of, 358; action of, 356; agents of degradation, 383, 384; basis of, 367; certain limitations to, 380, 381; Charleston, of 1883, 374, 375; countries, architecture in, 381; echoes, 369, 370; damages of, 377, 390; effect of, on the soil, 375; the surface of the earth, 371; formed by riving of fissures, 382; great, occurring where rocks have been disturbed by mountain-building, 381, 382; Herculaneum and Pompeii destroyed by an, 277, 280; Italian, in 1783, 371, 372; important, not connected with volcanic explosions, 381; Jamaica, in 1692, 372, 376; Lisbon, in 1755, 368, 369, 373, 374, 381; maximum swing of, 369; measuring the liability to, 386, 387; mechanism of, 370, 371; method of the study of, followed by Mr. Charles Mallet, 382, 383; Mississippi, in 1811, 373, 374, 380, 381; movement of the earth during, 377; originating from a fault plane, 367, 369, 370; originating from the seas, 358, 375; oscillation of, 376; poised rocks indicating a long exemption from strong, 388; Riobamba, in 1797, 375; shocks of, and their effect upon people, 383; the direct calamities of Nature, 386; waves of, 389. 

 Earthworms, 317-319; taking food underground, 319. 

 Eclipses, record of ancient, 130. 

 Electrical action in the formation of rain and snow, 164. 

 Elevations of seas and lands, 83. 

 Energy indestructible, 23. 

 Envelope, lower, of the sun, 74. 

 Equator, diameter of our sphere at the, 62; doldrum of the, 109; updraught under the, 102; uprushing current near the, 106. 

 Equinoxes, precession of the, 61, 62. 

 _Eskers_, 221. 

 Expansion of air contained in a hollow wall during the passage of the storm, 114. 

 Experiment, illustrating consolidation of disseminated materials of the sun and planets, 40. 

 Falls. See WATERFALLS. 

 Fault planes, 382. 

 Feldspar, 324. 

 Floods, 180, 197; rarity of, in New England, 121; river, frequent east of Rocky Mountains, 198. 

 Föhns, 121. 

 Forests, salicified, 124. 

 Fossilization, 354-356. 

 Fulcrum point, 95. 

 Galactic plane, 45. 

 Galongoon, eruption of, 294. 

 Geological work of water, 168-206. 

 Glacial action in the valleys of Switzerland, 224; periods, 63, 243, 246; in the northern hemisphere, 246; waste, 324. 

 Glaciation, effect of, in North America, 241; in Central America, 234; South America, 234. 

 Glaciers, 207-249; action of ice in forming, 230-232; Alaskan, 216; continental, 225, 239, 240; discharge of, 220; exploring, 220; extensive, in Greenland and Scandinavia, 244; former, of North America, 232, 234; map of, and moraines near Mont Blanc, 217; motions of, 213; retreat of the, 228, 230, 235; secrets of the under ice of, 221; speed of a, 224; study of, in the Swiss valleys, 222; testimony of the rocks regarding, 228; when covered with winter snows, 216; valley, 216. 

 Gombridge, 1830, 74. 

 Gravitation, law of, 4. 

 Greeks' idea of the heavens, 31; not mechanically inventive, 22. 

 Gulf Stream, current of the, 147. 

 Heat, amount of, daily escaping from the earth, 89; amount of, falling from the sun on the earth, 41; belief of the ancients regarding, 42; dominating effect on air currents of tropical, 104; energy with which it leaves the sun, 41; internal, of the earth, 88, 89; of the earth's interior, 309, 310; sun, effect on the atmosphere of the, 100; Prof. Newcomb's belief regarding the, of the sun, 52; radiation of the earth's, causing winds, 101; solar, 41; tropical, and air currents, 104. 

 Hills, sand, 123. 

 Horizontal pendulum, 384. 

 Horse latitudes, 104. 

 "Horses, " 261. 

 Hurricanes, 107, 110, 317; commencement of, 107; doldrum of, 109; felt near the sea, 110; in the tropics, 110. 

 Hypothesis, nebular, 34, 35, 39, 52, 56; working, 4, 5. 

 Ice action, effect of intense, 222, 223; in forming glaciers, 230, 232; recent studies in Greenland of, 239; depth of, in Greenland, 227; effect of, on river channels, 196; effect of, on stream beds, 196; expanding when freezing, 237; epoch, 92, 93, 246; floating, 242; made soils rarely fertile, 241; mass, greatest, in Greenland, 226, 227; moulded by pressure, 215; streams, continental, 225, 226; of the mountains, 225; of the Himalayan Mountains, 234. 

 Icebergs, 242, 243; indicating oceanic currents, 243. 

 Iceland, volcanic eruptions in, 297, 298. 

 Instruments, first, astronomical, 10, 11. 

 Inventions, mechanical, aiding science, 22. 

 Islands, 84, 272; continental, 84; in the deeper seas made up of volcanic ejections, 272; volcanic, 272. 

 Jack-o'-lantern, 167. 

 Jupiter, gaseous wraps of, 97; path of the earth affected by, 59, 60; the largest planet of the sun, 69. 

 Kames, 325. 

 Kant, Immanuel, and nebular hypothesis, 34. 

 Kaolin, 324. 

 Klondike district, cold in, 243, 244. 

 Krakatoa, eruption of, 298-300; effect of, on the sea, 299; effect of, on the sun, 300. 

 Lacolites, 306. 

 Lacustrine beds, 351. 

 Lagoons, salt deposits found in, 200. 

 Lake basins, formation of, 200, 201; bogs, 331, 333, 334; deposits, 350, 351. 

 Lakes, 199-206; effect of, on the river system, 205; fresh-water, 145; formed from caverns, 202; great, changing their outlets, 205; of extinct volcanoes, 203; temporary features of the land, 203; volcanic, 203. 

 Lands, great, relatively unchangeable, 96; table, 91; movements resulting in change of coast line, 351, 352; shape of the seas and, 83, 84; accounting for the changes in the attitude of the, 95; and water, divisions of, 84; dry, surface of, 85; general statement as to the division of the, 83, 84; surface, shape of the, 85; triangular forms of great, 90. 

 Latitudes, horse, troublesome to mariners, 104. 

 Laplace and nebular hypothesis, 34. 

 Lava, 266-268, 270, 271, 292, 293, 295, 296, 303, 304; flow of, invading a forest, 268; from Vesuvius, 293; of 1669, 295, 296; temperature of, 295, 296; incipient, 304; outbreaks of, 292, 303; stream eaves, 292, 293. 

 Law, natural, Aristotle and, 3; of gravitation, 4; of the conservation of energy, 23. 

 Leaves, radiation of, 160. 

 Length of days affected by tidal action, 131. 

 Level surfaces, 91. 

 Life, organic, evolution of, 15, 16. 

 Light, belief of the ancients regarding, 42. 

 Lightning, 24, 164-168; noise from, 166; proceeding from the earth to the clouds, 165; protection of buildings from, 165; stroke, wearing-out effect of, 165. 

 Limestones, 353, 357, 358, 360, 364; formation of, 357, 360. 

 Lisbon, earthquake of, 1755, 368, 369. 

 Lowell, Mr. Percival, observations on Venus, 64. 

 Lunar mountains near the Gulf of Iris, 397. 

 Mackerel sky, 35. 

 Mallet, Mr. Charles, and the study of earthquakes, 382, 383. 

 Man as an inventor of tools, 10. 

 Mangroves, 340; diagram showing mode of growth, 340; marshes of, 339. 

 Map of glaciers and moraines near Mont Blanc, 217; of Ipswich marshes, 338. 

 Mapping with contour lines, 27. 

 Maps, desirable, for the study of celestial geography, 77; geographic sketch, 26, 27. 

 Marching sands jeopardizing agriculture, 123. 

 Marine animals, sustenance of, 361-363; deposits, 325-327, 349, 356; marshes, 336-340; waves caused by earthquakes, 387. 

 Mars, 65-67, 84, 97; belief that it has an atmosphere, 65; canals of, 67; gaseous wraps of, 97; more efficient telescopes required for the study of, 67; nearer to the earth than other planets, 65. 

 Marshes, mangrove, 339; map of Ipswich, 338; marine, 336-340; deposits found in, 336; of North America, 337; on the coast of New England, 339; phenomena of, 167, 168; tidal, good earth for tillage, 337; tidal, of North America, 340. 

 Mercury, 55, 63, 78; nearest to the sun, 63; time in which it completes the circle of its year, 55. 

 Meteorites, 47, 48; kinship of comets and, 48. 

 Meteors, 47; falling, 47; composition of, 48; flashing, 39, 40, 47; speed of, 47; inflamed by friction with air, 99. 

 Methods in studying geology, 400. 

 Milky Way, 45; voyage along the path of the, 44, 45. 

 Mineral crusts, 328, 329; deposits, 308. 

 Moon, 38, 395-400; absence of air and water on the, 399; attended by satellites, 57; attraction which it exercises on the earth, 62; curious feature of the, 397; destitute of gaseous or aqueous envelope, 397; diameter of the, 399; imaginary view of the earth from the, 81; "libration" of the, 398; made up of circular depressions, 396, 397; movements of the, 78; no atmosphere in the, 97; parting of the earth and, 396; position of the, in relation to the earth, 62; tidal action and the, 131; tides of the, 126, 127; why does the sun not act in the same manner as the, 78. 

 Moraines, 216, 218, 229, 230; map of glaciers and, near Mont Blanc, 217; movements of the, 216-218; terminal, 228. 

 _Moulin_, 219. 

 Mount Ætna, 288-310; lava yielding, 290, 293, 294; lava stream caves of, 292, 293; more powerful than Vesuvius, 297; peculiarities of, 291, 292; size of, 289-291; turning of the torrents of, 295. 

 Mountain-building, 90-93, 304; folding, 86, 87, 90, 365; attributed to cooling of the earth, 88; growth, 392; Swiss falls, 174; torrents, energy of, 177. 

 Mountains, 85, 86, 89, 90-93; 174-178; form and structure of, 86; partly caused by escape of heat from the earth, 89; sections of, 87. 

 Mount Nuova, formation of, 284. 

 Mount Vesuvius, 263-285, 288, 289, 293, 302, 381; description of the eruption of, in A. D. 79, 277-280; diagrammatic sections through, showing changes in the form of the cone, 283; eruption of, in 1056, 281; in 1882-'83, 264, 266; eruption of, in 1872, 282; eruptions of, increased since 1636, 282; flow of lava from, 285; likely to enter on a period of inaction, 282, 283; outbreak of, in 1882-'83, 264, 266. 

 Naples, prosperity of the city, 289. 

 Nebular hypothesis, 34, 35, 39, 52. 

 Neptune, 70. 

 _Névé_, the, 214; no ice-cutting in the region of the, 224. 

 Newcomb's (Prof. ) belief regarding the heat of the sun, 52. 

 Niagara Falls, 191, 192, 204; cutting back of, 204. 

 North America, changes in the form of, 91, 92; triangular form of, 90. 

 Ocean, average depth of the, 89; climatal effect of the, 147; currents, 145; effect of, on migration, 156; effect of, on organic life, 154; floor, 85, 93; hot and cold currents of the, 102; sinking of the, 93, 94; the laboratory of sedimentary deposits, 351; depth of the, 89, 126. 

 Oceanic circulation, effect of, on the temperature, 152. 

 Oceans and continents, 83. 

 Orbit, alterations of the, and the seasons, 60, 61; changing of the, 59-63; shape of the, 61-63. 

 Organic life, 315, 317, 321, 352, 353, 363; action of, on the soil, 317, 321; advantages of the shore belt to, 363; development of in the sea, 352, 353; effect of ocean currents on, 154; processes of, in the soil, 315; decay of, in the earth, 321. 

 Orion, 46. 

 Oscillations of the shores of the Bay of Naples, 287. 

 Oxbow of a river, 182, 183. 

 Oxbows and cut-off, 182. 

 Pebbles, action of seaweeds on, 143; action of the waves on, 142, 144. 

 Photosphere, 74. 

 Plains, 86; alluvial, 91, 179, 182, 184-186, 325; history of, 91; sand, 325. 

 Planets, 38; attended by satellites, 57; comparative sizes of the, 68; experiments illustrating consolidation of disseminated materials of the sun and, 40; gaseous wraps of, 97; important observations by the ancients of fixed stars and planets, 43; movements of, 57-61; outer, 78; table of relative masses of sun and, 77. 

 Plant life in the Sargassum basins, 156. 

 Plants and animals, protection of, by mechanical contrivances, 364; and trees, work of the roots of, on the soil, 316, 317; water-loving, 181; forming climbing bogs, 332. 

 Polar axes, nodding movement of, 54. 

 Polar snow cap, 66. 

 Polyps, 155, 353. 

 Pools, circular, 203. 

 Prairies, 340, 342. 

 Radiation of heat, 159. 

 Rain, 152, 156, 164, 168, 170, 328, 330; circuit of the, 156-168; drops, force of, 169, 170; spheroidal form of, 170; electrical action in the formation of snow and, 164; work of the, 171. 

 Realm, unseen solar, 75. 

 Reeds, 332. 

 Religion, conflict between science and, 20, 22; struggle between paganism and, 21. 

 Rivers and _débris_, 183; changes in the course of, in alluvial plain, 182; deposition of, accelerated by tree-planting, 181; great, always clear, 205; inundation of the Mississippi, eating away land, 182; muds, 222; newly formed, cutting a cañon, 195; of snow-ice, 211; origin of a normal, 173; oxbow of a, 182, 183; sinking of, 199; swinging movement of, 179-181; river-valleys, 193, 194; diversity in the form of 188-191. 

 Rocks, 145; accidents from falling, 174; cut away by sandstones, 188; divided by crevices, 252; duration of events recorded in, 389, 390, ejection of, material, 311; falling of, 174-176; formation of, 262, 263; from the present day to the strata of the Laurentian, 390; migration of, 291; poised, indicating a long exemption from strong earthquakes, 388; rents in, 252, 253; stratification of, 349, 350, 352, 365, 390; testimony of the, in regard to glaciers, 228; under volcanoes, 303; variable elasticity of, 366; vibration of, 367, 368; rock-waste, march of the, 343; water, 250, 267. 

 Rotation of the earth affected by tides, 130; of the planetary spheres on their axes, 56. 

 Salicified forests, 124. 

 Salt deposits formed in lagoons, 200; found in lakes, 199-200. 

 Sand bars, 183; endurance of, against the waves, 145; hills, travelling of, 123; marching, 123; silicious stones cutting away rooks, 188. 

 Satellites, 53, 54; motions of, about their centres and about the sun, 53, 54. 

 Saturn, 38, 53, 57, 396; cloud bands of, 70; gaseous wraps of, 97; path of the earth affected by, 59, 60. 

 Savages, primitive, students of Nature, 1. 

 Scandinavia, changes on the coasts of, 96. 

 Science, advance of, due to mechanical inventions, 22; astronomy beginning with, 10; chemical, characteristics of, 14; conflict between religion and, 20, 22; conflict between the Roman faith and, 20; mechanical inventions as aids to, 22, 23; modern and ancient, 4; natural, 5, 6; of botany in Aristotle's time, 14; of physiology, 15; of zoölogy in Aristotle's time, 14; resting practically on sight, 10. 

 Scientific development, historic outlines of, 17; tools used in measuring and weighing, as an aid to vision, 12. 

 Sea, battering action of the, 140; coast ever changing, 385, 386; effect of volcanic eruptions on the, 299; floor deposits of the, affected by volcanoes, 360, 361; in receipt of organic and mineral matter, 359; hot and cold currents of the, 102; littoral zone of the, 351, 352; puss, 142; rich in organic life, 352, 353; solvent action of the, 361; strata, formation of, 354; water, minerals in, 185; weeds, 155, 156. 

 Seas, dead, originally living lakes, 200; water of, buoyant, 199; eventually the seat of salt deposits, 199-201; general statement as to division of, 83, 84; shape of the, 83, 84. 

 Seashore, air of the, 98. 

 Seasons, changing the character of the, 61, 62. 

 Sense of hearing, 9, 10; of sight, 10; of smell, 9, 10; of taste, 9, 10; of touch, 9, 10. 

 _Seracs_, 214. 

 Shocks, earthquake. See under EARTHQUAKES. 

 Shore lines, variation of, 83, 84. 

 Shores, cliff, 138-142. 

 Sink holes, 202; in limestone districts, 253, 254. 

 Skaptar, eruption of, 297, 298; lava from the eruption of, 298. 

 Sky, mackerel, 35. 

 Snow, 207-225, 244; as an evidence of atmosphere, 65; blankets, early flowers beginning to blossom under, 208; covering, difference between an annual and perennial, 210; effect of, on plants, 208; electrical action in the formation of rain and, 164; flakes, formation of, 164; red, 210; slides, 210; slides, phenomena of, 210, 211. 

 Soil, alluvial, 321, 322; atmospheric circulation of, 330, 331; conditions leading to formation of, 313, 331; continuous motion of the, 314; covering of the earth, 343; decay of the, 314, 315; degradation of the, 344-348; means for correcting, 346-348; destruction in grain fields greater than the accumulation, 344; developing on lava and ashes an interesting study, 343; development of, in desert regions, 340; effect of animals and plants on the, 317-320; effect of earthquakes on the, 375; fertility of the, distinguished from the coating, 344, 345; fertility of, affected by rain, 327; formation of, 314-321; glacial, characteristics of, 324; glaciated, 323, 324; irrigation of the, 328-330; local variation of, 327; mineral, 321; of arid regions fertile when subjected to irrigation, 341; of dust or blown sand, 321; of immediate derivation, 321, 322; phenomena, 313; processes of organic life in the, 315; variation in, 321-331; vegetation protecting the, 316, 317; washing away of the, 346, 347; winning, from the sea, 337; work of ants on the, 318; tiller, duty of the, 348. 

 Solar bodies, general conditions of the, 63-71; forces, action of, on the earth, 349; system, 52, 56; independent from the fixed stars system, 43; original vapour of, 52, 53; singular features of our, 68; tide, 127. 

 Spheres, difference in magnitude of, 51; motions of the, 50, 51; planetary, rotation of, on their axes, 56. 

 Spots, sun, 72. 

 Spouting horn, 141. 

 Springs, formation of small, 252. 

 Stalactitization, 256. 

 Stalagmites and stalactites on the roof and floor of a cavern, 257. 

 Stars as dark bodies in the heavens, 47; discovery of Fraunhofer and others on, 23, 38; double, 39; and tidal action, 131; earliest study of, 10; fixed, important observations by the ancients of planets and, 43; not isolated suns, 38, 39; variation in the light of, 46; limit of, seen by the naked eye, 11; revolution of one star about another, 46, 47; shooting, 47; speed of certain, 51; study of, 31-80; sudden flashing forth of, due to catastrophe, 46; voyage through the, 44, 45; star, wandering, 74. 

 Stellar realm, 31-80. 

 Storms, circular, 111; desert, 121, 122; expansion of air contained in a hollow wall during the passage of, 114; great principle of, 105, 106; in the Sahara, 121; lightning, more frequent in summer, 167; paths of, 115; secondary, of cyclones, 112; spinning, 115; thunder, 165-167; whirling, 106, 124; whirling peculiarity of, 108, 109. 

 Strabo, writings of, 18. 

 Sun, atmosphere of the, 73; constitution of the, 72; distance of the earth from the, 29; effect from changes in the, and earth, 59; envelope of the, 73, 74, 97; experiments illustrating consolidation of disseminated materials of planets and, 40; finally, dark and cold, 42; formation of the eight planets of the, 53; heat leaving the, 41; heat of the, 76; imaginary journey from the, into space, 44; mass of the, 76, 77; path of the earth around the, 55; physical condition of the, 71; Prof. Newcomb's belief regarding the heat of the, 52; spots, 75; abundant at certain intervals, 72; difficulty in revealing cause of, 75; structure of the, a problem before the use of the telescope, 72; table of relative masses of, and planets, 77; three stages in the history of the, 71; tides, 126; why does it not act in the same manner as the moon? 78. 

 Surfaces, level, 90. 

 Surf belt, swayings of the, 137. 

 Swamps, diagram showing remains of, 335; Dismal Swamp, 95, 333; drainage of, 334, 335; fresh-water, 334, 335; phenomena of, 167, 168. 

 Table-lands, 91. 

 Table of relative masses of sun and planets, 77. 

 Telescopes, 11, 12, 45; first results of, 72; power of, 11; revelations of, 45. 

 Temperature, effects of, produced by vibration, 42; in the doldrum belt, 118; of North America, 118; of the Atlantic Ocean, 118. 

 Tempests, rate of, 99, 100. 

 Thunder, 166; more pronounced in the mountains, 166. 

 Thunderstorms, 165, 166; distribution of, 166, 167. 

 Tidal action, recent studies of, 131, 132; marshes of North America, 340. 

 Tides, carving channels, 129; effecting the earth's rotation, 130; effect of, on marine life, 130; height of, 128, 129; moon and sun, 126, 127; normal run of the, 127; production of, 131; of the trade winds, 150; solar, 127; travelling of, 127, 128. 

 Tillage introducing air into the pores of the soil, 331. 

 Tornadoes, 112, 113, 317; development of, 113; effect of, on buildings, 113; fiercest in North America, 113; length of, 115; resemblance of, to hurricanes, 115; upsucking action of, 114, 115. 

 Torrents, 177-179, 204. 

 Trade winds. See under WINDS. 

 Training in language, diminishing visual memory, 401; soldiers to measure distances, 28; to measure intervals of time, 28; for a naturalist, 25-29. 

 Tunnels, natural, 257. 

 Uranus, 70. 

 Valley of Val del Bove formed from disturbances of Mount Ætna, 294. 

 Valleys, diversity in the form of river, 188-191; river, 193. 

 Vapour, 156, 157, 159, 163; gravitative attraction of, 34, 35; nebular theory of, 52, 53; original, of the solar system, 52, 53. 

 Vegetation, and dew, 160; in a measure, independent of rain, 160; protecting the soil, 316, 317. 

 Vein, diagram of a, 260. 

 Venus, 64, 78; recent observations of, by Mr. Percival Lowell, 64. 

 Vesuvian system, study of the, 285. 

 Vesuvius. See MOUNT VESUVIUS. 

 Visualizing memories, 402, 403. 

 Volcanic action, 268-276. 

 Volcanic eruption of A. D. 79, 288; important facts concerning, 276-279; islands, 272; lava a primary feature in, 266; observations of, made from a balloon, 301; peaks along the floor of the sea, 272, 273; possibility of throwing matter beyond control of gravitative energy, 300. 

 Volcanoes, 125, 203, 263; abounding on the sea floor, 302; accidents from eruptions of, 288; along the Pacific coast, 271; ash showers of, maintaining fertility of the soil, 289; distribution of, 271; eruption of, 286-294, 368; explosions from, coming from a supposed liquid interior of the earth, 275; exporting earth material, 310; little water, 375; Italian, considered collectively, 296, 297; Neapolitan eruptions of and the history of civilization, 288; subsidence of the earth after eruption of, 287, 291; origin of, 263-274; phenomena of, 263-267; submarine, 301; travelling of ejections from, 287, 288. 

 Waters, crevice, 250; of the earth, 250, 251; cutting action of, 117, 192; drift, from the poles, 151; journey of, from the Arctic Circle to the tropics, 151, 152; dynamic value of, 171; expansion of, in rocks, 270; geological work of, 168-206; in air, 99; of the clouds usually frozen, 207; pure, no power for cutting rocks, 204; rock, 250, 263; sea, minerals in, 185; store of the earth, 125; system of, 125, 156; tropical, 151; velocity of the, under the equator, 150; wearing away rocks, 178, 179; underground, carrying mineral matter to the sea, 193; chemical changes of, leading to changes in rock material, 262, 263; effect of carbonic-acid gas on, 251; operations of the, 126; wearing away rocks, 178, 179; work of, 250. 

 Waterfalls, 189-193; cause of, 191; the Yosemite, 192; Niagara, 191, 192; numerous in the torrent district of rivers, 192; produced by dikes, 192; valuable to manufactures, 192, 193. 

 Waterspouts, 115, 116; atmospheric cause of, 116; firing at, 116; life of a, 116; picturesqueness of, 116; the water of fresh, 117. 

 Waves, 128, 129, 132, 145; action of friction on, 135, 136; break of the, 136; endurance of sand against the, 145; force of, 133, 136, 139; marine, caused by earthquakes, 387; of earthquakes, 389; peculiar features in the action of, 137; size of, 137, 138; stroke of the, 144; surf, 135; tidal height of, 132; undulations of, 132; wind, 132; wind influence of, on the sea, 134, 135; wind-made, 128. 

 Ways and means of studying Nature, 9. 

 Weeds of the sea, 155. 

 Well, artesian, 258, 259. 

 Whirling of fluids and gas, 36, 37. 

 Whirlwinds in Sahara, 121. 

 Will-o'-the-wisp, 167. 

 Winds, 101, 110, 122, 317; effect of sand, 122; hurricane, 110; illustration of how they are produced, 101; in Martha's Vineyard, 120; of the forests, work of the, 317; of tornadoes, effect of, 113; on the island of Jamaica, 119, 120; regimen of the, 119; variable falling away in the nighttime, 100; trade, 102-105; 145, 146, 150; action of, on ocean currents, 145: affected by motion of the earth, 103; belt, motion of the ocean in, 146; flow and counter-flow of the, 150; tide of the, 150; uniform condition of the, 102; waves, work of, 132, 134, 135. 

 Witchcraft, belief of, in the early ages, 21. 

 Zoölogy, rapid advance in, 14, 15.