Sedimentation 



23 



particles were carried by the streams to the sea or other bodies 

 of water. The settHng of the sand resulted in the forma- 

 tion of deep layers of such material, and under great pressure 

 these layers became cemented into sandstone (Fig. 2). An 

 examination of chalk deposits shows the rock to consist of a 

 sediment of microscopic shells like those formed today by ani- 

 mals similar to the ameba. A handful of slime taken from 



Fig. 2. Sedimentation 



Vast areas have been dug up by water and wind, and by the hand of man, ex- 

 posing layers of the earth's crust that had been slowly deposited in past ages, grain 

 upon grain. From photograph by the Gobi Desert Expedition of the American 

 Museum of Natural History. 



the bottom of the sea in some places would show millions of 

 such microscopic shells. Limestone, which has the same 

 chemical composition as chalk and marble, contains bits of 

 shells of larger animals as well as skeletons of different types, 

 and the geologist assumes that these structures are the remains 

 of animals that lived many years ago. Coal has also been 

 considered a kind of sedimentary rock bearing the remains of 

 plants which grew in swamps ages ago. In other rocks we 



