422 ANIMAL STUDIES 



bottom of the lake or ocean and forms there a layer which 

 gradually hardens under pressure to become rock. This is 

 called sedimentary rock, or stratified rock, because it is 

 composed of sediment, and sediment always arranges itself 

 in layers or strata. In sedimentary or stratified rocks 

 fossils are found. The commonest rocks of this sort are 

 limestone, sandstone, and shales. Limestone is formed 

 chiefly of carbonate of lime ; sandstone is cemented sand ; 

 and shales, or slaty rocks, are formed chiefly of clay. 



337. Sedimentary rocks. The formation of sedimentary 

 rocks has been going on since land first rose from the level 

 of the sea ; for water has always been wearing away rock 

 and carrying it as sediment into rivers, and rivers have 

 always been carrying the worn-ofi 2 lime and sand and clay 

 downward to lakes and oceans, at the bottoms of which the 

 particles have been piled up in layers and have formed new 

 rock strata. But geologists have shown that in the course 

 of the earth's history there have been great changes in the 

 position and extent of land and sea. Sea-bottoms have 

 been folded or upheaved to form dry land, while regions 

 once land have sunk and been covered by lakes and seas. 

 Again, through great foldings in the cooling crust of the 

 earth, which resulted in depression at one point and eleva- 

 tion at another, land has become ocean and ocean land. 

 And in the almost unimaginable period of time which has 

 passed since the earth first shrank from its condition of 

 nebulous vapor to be a ball of land covered with water such 

 changes have occurred over and over again. They have, 

 however, all taken place slowly and gradually. The princi- 

 pal seat of great change is in the regions of mountain 

 chains, which, in most cases, are simply the remains of old 

 folds or wrinkles in the crust of the earth. 



338. Deposition of fossils. Now we may see how fossils 

 come to exist in the sedimentary rocks. When an aquatic 

 animal dies it sinks to the bottom of the lake or ocean, un- 

 less, of course, its flesh is eaten by some other animal. 



