Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: more compact clays and marls, resisting the penetration of this liquid, will retain their soda, lime, and magnesia, and by subsequent alteration will give rise to basic feldspars containing lime and soda, and if lime and magnesia predominate, to hornblende or pyroxene. The presence or absence of iron in sediments demands especial consideration, since its elimination requires the interposition of organic matters, which, by reducing the peroxide to the condition of protoxide, render it soluble in water, either as bicarbonate or combined with some organic acid. This action of waters holding organic matter upon sediments containing iron-oxide has been described by Bischof and many other writers, particularly by Dr. J. W. Dawson in a paper on the coloring matters of some sedimentary rocks, and is applicable to all cases where iron has been removed from certain strata and accumulated in others. This is seen in the fire-clays and iron-stones of the coal-measures, and in the white clays associated with great beds of green-sand (essentially a silicate of iron) in the cretaceous series of New Jersey. Similar alternations of white feldspathic beds with others of iron-ore occur in the Green Mountain rocks of Canada, and on a still more remarkable scale in those of the Laurentian series. We may probably look upon the formation of beds of iron-ore as in all cases due to the intervention of organic matters ; so that its presence, not less than that of graphite, affords evidence of the existence of organic life at the time of the deposition of these old crystalline rocks. The agency of sulphuric and muriatic acids, from volcanic and other sources, is not, however, to be excluded in the solution of oxide of iron and other metallic oxides. The oxidation of pyrites, moreover, gives rise to so... --This text refers to the Paperback edition.
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