PRELIMINARY OUTLINE. 13 



of North America and the northwestern part of Europe/ the soil gradu- 

 ates down into an irregular sheet of mixed clay, sand, gravel, and bowl- 

 ders, known as drijt. From this and other evidence it is inferred 

 that at a time not greatly antedating our own, ice, chiefly in the form 

 of glaciers, spread extensively over the high latitudes of the northern 

 hemisphere. In some parts of the earth the surface is still covered 

 by fields of snow and ice, comparable to those which formed the drift. 

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

 mantle rock consists of deposits made by streams which were unable to 

 carry their loads of sediment to the sea. 



The crust of the lithosphere — Much of the detritus washed down 

 from the land finds its way to bodies of standing water, and beneath 

 lakes and seas the mantle of loose material is made up largely of the 

 gravel, sand, and mud derived from the land. Before deposition these 

 materials are more or less assorted and arranged in layers by waves 

 and currents. Wlien consolidated they constitute rock. The weather- 

 ing of the rocks of the land, the wearing away of the resulting detritus, 

 and its deposition beneath standing water, are among the most impor- 

 tant processes of geologic change. .j, 



On the land, the mantle of loose material is sometimes absent, and 

 in such places the surface of solid rock of the crust appears. Bare 

 surfaces of rock are most commonly seen where the topography is 

 rough, especially on the slopes of steep-sided valleys and mountains, 

 and on the slopes of chffs which face seas or lakes. Sohd rock, without 

 covering of soil or loose material of any sort, is also frequently seen 

 in the channels of streams, especially where there are fahs or rapids. 



We have but to note the effects of a vigorous shower on a steep 

 slope, or of a swift stream on its channel, or of waves on the cliffs which 

 face lakes and seas, to understand at least one of the reasons why loose 

 materials are frequently absent from steep slopes The very general 

 exposure of solid rock where conditions favor surface erosion suggests 

 that rock is everywhere present beneath the soil or subsoil. For- 

 tunately there is an easy way of testing the universality of the crust 

 beneath the mantle. 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 reach 

 depths of several thousand feet. Even in shallow excavations rock 

 is often encountered, and in most regions excavations as much as two 

 or three hundred feet deep usually reach rock^ and no really deep boring 



