PALEOGEOGRAPHIC MAPS 91 



Chamberlin's suggestion of an inversion of oceanic circulation which 

 is based upon the possibihty that sahne equatorial waters may have 

 been denser than, and have sunk beneath, relatively fresh and lighter 

 polar waters. The conditions of circulation today are determined 

 by temperature and not by salinity, but the differences of temperature 

 which developed during the glacial period and which still persist are 

 exceedingly great, and it may well be doubted whether temperature 

 had a like effect in Ordovician time, when, as is established by the 

 distribution of faunas, climatic conditions were relatively very equable. 

 The period covered by this composite map is essentially that of 

 the Niagara. North America was still an archipelago. The conti- 

 nental plateau was widely submerged on the north and was still 

 covered by an interior sea. On the east, however, lands appear to 

 have been elevated in consequence of the Taconic orogenic movements 

 which proceeded from the Atlantic basin, and shallow seas or lands 

 appear to have extended across the region which is now that of the 

 Gulf states. It has been suggested that the sea was generally absent 

 from the western portion of the continent, but recent investigations 

 in Alaska and Utah indicate the presence of a Silurian fauna and we 

 are at least justified in an alternative assumption that marine condi- 

 tions existed quite extensively, but presented a habitat unfavorable 

 to the rich fauna that occupied the interior Niagaran sea. The 

 conditions of marine circulation appear to have restricted the equa- 

 torial currents to the Atlantic on the east and to the Gulf on the south, 

 while the polar currents coming along the coast of Siberia and along 

 Greenland penetrated into the interior sea where the slowly circu- 

 lating waters became warm enough to afford a very genial habitat. 

 As in the middle Ordovician, the climatic conditions were equable 

 throughout wide ranges of latitude and marked differences of tem- 

 perature probably did not exist. 



