12 GEOLOGY 



parable to those which now cover Greenland and Antarctica. In 

 still other places, especially along the flood plains of streams, the 

 mantle rock consists of deposits made by streams. Along the 

 shores of lakes and seas, there are beach gravels and sands. The 

 thickness of the mantle rock varies from a few inches to hundreds 

 of feet (Fig. 1). 



The solid rock. Mantle rock is absent in some places, and 

 there the surface of solid rock appears. It is common on the slopes 

 of steep-sided valleys and mountains, and on the slopes of cliffs 

 which face seas or lakes, and is frequently seen in the channels of 

 swift streams, especially where there are falls or rapids. In all 

 lands inhabited by civilized peoples, there are numerous wells and 

 other excavations ranging from a few feet to several hundred feet 

 in depth, and occasional wells and mine-shafts go much deeper. 

 Even in the shallower excavations solid rock is often encountered, 

 and in most regions excavations as much as two or three hundred 

 feet deep reach it. It may, therefore, be accepted as a fact that 

 the upper surface of the solid rock is nowhere far below the surface. 



Varieties of solid rock. If the mantle rock were stripped from 

 the land, the rock beneath would be found to be made up of many 

 kinds of rocks, all of which may be grouped into three great classes. 

 By far the larger part of the land surface would be of rock arranged 

 in layers (stratified rock), and the remainder would be of rock which 

 is without distinct stratification. The rocks without distinct strati- 

 fication are divided into two great groups, igneous rocks, and meta- 

 morphic rocks. 



The essential feature of stratified rock is that it is arranged in 

 layers. The layers may be distinct or indistinct, and they may 

 be thick or thin. Thick layers are often made up of many thinner 

 ones. Fig. 2 shows rock which is distinctly stratified. 



In composition, much stratified rock corresponds somewhat 

 closely with the sediments now being carried from the land and 

 deposited in the sea; that is, these rocks are made up of pebbles, 

 sand grains, or particles of mud, cemented together. The bedded 

 arrangement of stratified rocks and of recent sediments is the same, 

 and the markings on the surfaces of the layers, such as ripple- 

 marks, rill-marks, wave-marks, etc., are identical. Furthermore, 



