an introduction to geology

Book cover
Purchase of this book includes free trial access to where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER IV. OF DIVISIONAL PLANES IN ROOKS. In addition to the planes of bedding, which were produced during the deposition of rocks, other planes are found affecting them, which require some notice. Joints. If you enter a quarry of sandstone or granite you will probably notice a number of fissures in the rock, which in many cases are nearly or quite upright. In the sandstone quarry it may be observed that the most important of these lines run at right angles to the bedding planes, or nearly so, and they are obviously not bedding planes. They are in fact cracks in the rock, and are spoken of as joints. In the case of the sediments it is often found that one important set of joints traverses the rocks in the direction of their dip and another in that of their strike. These two systems are called dip joints and strike joints. Joints may be caused by more than one process. Some are due to shrinkage owing to loss of water or in the case of igneous rocks to consolidation, for the solid rock occupies less space than the molten rock. Others again may be formed by earth movement. The rocks when bent may eventually snap across, giving rise to joints. In sedimentary rocks, the important joints which follow the direction of dip and strike are quadrangular joints. The most regular of the joints producedduring the cooling of igneous rocks often assume the form of six-sided columns, as seen in the basalt of Staffy, and the Giant's Causeway. These are columnar joints. In Fig. 1 the upright lines cutting the beds of sandstone represent joints. Faults. When beds are displaced along a joint, so that one end of a stratum abuts against a different stratum on the opposite side of the crack, the beds are said to be faulted. A fault is represented as it would appear in an upright cutting i...
add to favoritesadd

Users who have this book

Users who want this book

What readers are saying

What do you think? Write your own comment on this book!

write a comment

What do you think? Write your own comment on this book

Info about the book






3/5 (4)

Your rating:




Do you want to exchange books? It’s EASY!

Get registered and find other users who want to give their favourite books to good hands!