THE ORDOVICIAN PERIOD 



419 



shown by the character of the sediments, perhaps not exceeding 200 

 to 300 feet in depth. It is impossible to characterize the rocks of a 

 system in a few words, since at all times in the earth's history sedi- 

 mentary deposits of every description were being laid down in some 

 portion of the world. This is also true of the Ordovician, during 

 which gravels and sands were deposited in certain places, but lime- 

 stones and shales form a much larger proportion of the deposits 

 than perhaps in any other Paleozoic period. One of the physi- 

 cal conditions which brought this about was the limited area and 

 probably slight elevation of the land, which consequently yielded 



Fig. 389. — Probable distribution of land and water in the Middle Ordovician. 

 The continent has probably never been so completely submerged since that time. 

 (Modified after Schuchert.) 



little sediment, leaving extensive areas of the seas free from sands 

 and muds. However, although built up largely of the remains 

 of lime-secreting animals, such as brachiopods, corals, and bryo- 



