THE ORIGIN AND DESCENT OF ROCKS. 



423 



several classes of material become cemented or otherwise hardened, 

 they give rise to shales (cemented muds), sandstones (cemented sands), 

 and conglomerates (or pudding-stones, cemented gravel, Fig. 346). If 

 the coarse material remains angular, they form breccia instead of 

 conglomerates (Figs. 347 and 348). For the most part, the deposits of 

 mud, sand, and gravel are made under the sea or in lakes and estuaries, 

 but they are also formed on the land in lodgment basins, in low-gradient 



Fig. 351. Fig. 352. 



Fig. 351. — Globigerina ooze. Magnified 20 times. (Murray and Renard.) 

 Fig. 352. — Pteropod ooze. Magnified 4 times. (Murray and Renard.) 



valleys, and on base-plains. The deposits of sediment on land have 

 received less recognition than they deserve. When formed under the 

 sea or in other. hfe-sustaining waters, shells and other organic material 

 are liable to be entrapped and to form a part of the rock. These organic 

 remains, or fossils, greatly aid in interpreting the deposits in which 

 they occur. Fossils are less liable to be preserved in sedimentary 

 deposits formed on land. There is, therefore, some ground to suspect 

 that great series of sandstones and shales which do not contain 

 marine or fresh-water fossils were formed in lodgment basins on land. 



