THE COMPOSITION OF THE EARTH 39 



six-sided prisms. Cemented and compacted clay forms shale. 

 Conglomerate, breccia, sandstone, and shale, since they are 

 made up of fragments of older rocks, are often called fragmen- 

 ted rocks. The remains of organisms that take calcium car- 

 bonate from solution in the waters to form their shells become 

 limestone when cemented or crystallized. Some limestones 

 have been formed by chemical precipitation of calcium car- 

 bonate. Chalk is a very soft limestone of fine texture. Dolo- 

 mite (magnesian limestone) is developed when some considerable 

 proportion of the calcium of a limestone is replaced by mag- 

 nesium. This replacement may take place long after the 

 formation of the limestone, or while the material of the lime- 

 stone is accumulating. 



Flint is a very compact, dark gray, siliceous rock. Chert 

 is an impure flint, usually of light color. These rocks do not 

 in most cases form extended independent beds, but occur 

 chiefly in limestones in the form of irregular masses and thin 

 layers. Both contain fossils of the siliceous parts of various 

 sea animals, particularly sponges and protozoans (p. 294). 

 The silica was taken from sea water by such animals, and at 

 their death formed deposits, often scattered through other 

 sediments. Subsequently, some of it was dissolved by ground 

 water, and redeposited in certain places where conditions 

 favored. (See Concretions, p. 121.) While this seems quite 

 certainly to be. the origin of some flints and cherts, that of 

 others is uncertain. 



The larger part of the land surface is covered with sedimen- 

 tary rocks. Most of these rocks are in layers and contain 

 marine fossils. For these and other reasons, it is concluded 

 that such rocks are consolidated sediments that were deposited 

 beneath the sea in the same manner that offshore sediments 

 are now forming. This conclusion carries with it the inference 

 that at some time in the past the ocean waters have covered 

 large areas which are now land. Since the beds of sediment 

 now forming are nearly horizontal, we conclude further that 

 all sedimentary beds originally had that position, and that 



