GRAND DIVISIONS OF THE EARTH 7 



isolated mountains whose heights and slopes would seem extraordi- 

 nary, if the ocean were removed. 



The surface of the land is diversified similarly by broad undula- 

 tions and volcanic peaks, as well as by narrower wrinklings and 

 foldings of the crust, and all of these irregularities have been carved 

 into varied and picturesque forms by erosion. In this respect, the 

 land differs radically from the bed of the sea. 



The outer part of the lithosphere is often called the crust of the 

 earth. The old notion that it was the solid portion overlying a 

 liquid part beneath is now generally abandoned. The crust is 

 merely the outer, cooler portion of the lithosphere. Its thickness 

 is undefined, but a shell several miles thick, and perhaps a few score 

 miles, is generally meant when this term is used. 



Materials of the lithosphere. Mantle rock. The great body of 

 the lithosphere probably is composed of solid rock, but the solid 

 rock is very generally covered by a layer of loose material such as 

 soil, clay, sand, gravel, and broken rock, known collectively as 

 mantle rock. The mantle rock of many places consists of the 

 decayed products of underlying rocks. The upper part of mantle 

 rock constantly is being blown away by wind and washed away by 

 water, while the lower part is being renewed constantly by the 



Fig. i. Soil and subsoil arising from the decay of limestone resting on the 

 uneven surface of the rock beneath. Southeastern Missouri. (Buckley.) 



