8 



PRELIMINARY OUTLINE 



decay of the rock below. The mantle rock of some other areas, as 

 the northern part of North America and the northwestern part of 

 Europe, consists chiefly of an irregular sheet of commingled clay, 

 sand, gravel, and bowlders (drift) deposited by great glaciers, com- 

 parable to that which now covers Greenland. In still other places, 

 especially along the flood plains of streams, the mantle rock consists 

 of deposits made by rivers. Along the shores of lakes and seas, 

 there are beach gravels and sands. The thickness of the mantle 

 rock varies from almost nothing to hundreds of feet (Fig. i). 



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, on the slopes of cliffs which face 

 seas or lakes, and 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 

 to several hundred feet in depth, and occasional wells and mine- 

 shafts go much deeper. In these, and even in many of the shallower 

 excavations, solid rock is encountered, and in most regions excava- 

 tions as much as a few hundred feet deep reach it. We infer, there- 

 fore, that solid rock is nowhere far below the surface. 



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

 the land, the solid part beneath would be found to be made up of 

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



Fig. 2. Stratified rock. Trenton Limestone, Fort Snelling, Minn. (Calvin.) 



