254 



THE EARTH'S CRUST 



tain conditions, harden into rocks. The shells and remains 

 of sea animals also accumulate, and after a time consolidate 

 into rock. 



About four fifths of the land surface of the earth is com- 

 posed of sedimentary rocks. They vary greatly in color, 



durability, and use- 

 fulness to man. 



The sandstones, 

 which are composed 

 of little grains of 

 sand cemented to- 

 gether, are used for 

 buildings and for 

 many other pur- 

 poses. The lime- 

 stones, which are 

 mostly made up of 

 the remains of sea 

 animals, are the 

 source of our lime 

 and are also used 



A sedimentary rock formed from old gravel 

 beds. 



CONGLOMERATE 



and are 



for building and for 

 other purposes. The 



shales are finely stratified mud deposits often having many 

 layers in an inch of thickness. These rocks are not crystal- 

 line. They are composed of fragments of other rocks or 

 remains of plants or animals and usually occur in layers 

 or strata. 



Bituminous coal is sedimentary rock, formed from plants 

 of ages ago which have been compressed and solidified by 

 enormous and long-continued pressure. 



The metamorphic rocks have a crystalline structure, 



