32-t 



to pure water can flourish close to the continental shore, and 

 deposits of a purely organic nature, such us the extensive 

 deposit of pteropod shells constituting the Styliolina lime- 

 stone iu Western New York, may form in comparatively 

 shallow water. 



The epicontinental seas are especially adapted for fhe 

 development of local or provincial faunas. Such provin- 

 cializing of faunas is most marked if, by some oscillatorv 

 movement of the land or some other physical change, the 

 basin of the epicontinental sea should become separated from 

 the extra-continental portion of the littoral district suffi- 

 ciently to prevent intercommunication between the organ- 

 isms of the two provinces. A barrier is thus formed, which 

 need not necessarily be a land barrier, and a great diversity 

 of faunas may result. Such diversity of fauna existed in 

 early Tertiary time between the Mississippi embayment and 

 the Atlantic coast; and in Palaeozoic time, between the Bay 

 of New York and the Central Interior sea. Recent provincial 

 faunas are frequently met with. It requires only a compara- 

 tively slight elevation of the sea fioor, or a moderate deepen- 

 ing of the abyssal oceanic basins, to draw off the water from 

 the shallower regions, and lay large portions of the littoral 

 district dry. Such a change would, of course, result in an 

 extinction of the whole of the littoral flora and fauna thus 

 exposed, and force the survivors to accommodate themselves 

 to a narrower held. Revival of stream activities, conse- 

 quent upon elevation of the land, would result in carrying a 

 large amount of debris into the sea. and thus produce con- 

 ditions unfavorable to the existence of many organisms. 

 Such an elevation of the land and extinction of faunas oc- 

 curred at the close of Ordovicic time* in the area of the 

 central and eastern interior Palaeozoic sea. A few survivors 

 only, of the Ordovician fauna, occur in some of the lower 

 detrital beds of the next succeeding formation, in certain 

 localities. In the Siluric era a new fauna developed, as 

 conditions again became favorable. 



* Welter, '9s, p. 693. 



