78 FIRST YEAR SCIENCE 



they will, under certain conditions, harden into rocks, 

 The shells and remains of sea animals also accumulate, 

 and after a time consolidate into rock. The remains of 

 plants may accumulate under such conditions that they 

 will not rot but will solidify into rock which we call 

 bituminous or soft coal. About four fifths of the land 

 surface of the earth is composed of sedimentary rocks. 

 They vary greatly in color, durability and usefulness 

 to men. 



The sandstones, which are composed of little grains of 

 sand cemented together, are used for buildings and for 

 many other purposes. The limestones, which are mostly 

 made from the remains of sea animals, are the source of 

 our lime and are also used for building and for other pur- 

 poses. The shales are finely stratified mud deposits often 

 having many layers in an inch of thickness. Bituminous 

 coal, which is formed from plants of former ages, is the 



OIL WELLS. 

 Tapping the rock layers containing petroleum. 



most useful and valuable of all mineral products. None 

 of these rocks is crystalline. They are composed of frag- 



