10 GEOLOGY 



islands constitute peaks whose heights and slopes would seem 

 extraordinary, if the ocean were removed. 



The surface of the land is diversified in a similar way by broad 

 undulations and volcanic peaks, and also by narrower wrinklings 

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

 carved into varied and picturesque forms by subaerial erosion. In 

 this respect the surface of the land differs radically from the bed 

 of the sea. 



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

 earth. No definite lower limit can be assigned to the crust, and 

 the former notion that it is the solid portion overlying a liquid 

 part beneath, is now generally abandoned. The term crust as 

 now used means the outer, cooler portion of the lithosphere. Its 

 thickness is undefined, but it includes a shell several miles thick, 

 at least, and perhaps a few score miles. 



The interior. Concerning the great interior of the earth, little 

 is known except by inference. From the weight of the earth, 1 it 

 is inferred that its interior is much more dense than its surface. 

 From its behavior under the attraction of other bodies, it is be- 

 lieved to be at least as rigid as steel, and its interior cannot, there- 

 fore, be liquid, in the usual sense of that term. From the phenom- 

 ena of volcanoes, and from observations on temperature in deep 

 borings, it is inferred that its interior is very hot. Further infer- 

 ences concerning its character are less simply stated, and will be 

 referred to later. 



1 Its specific gravity as a whole is about 5.57, and the specific gravity 

 of its outer portion is about 2.7. 



