250 BAILEY WILLIS 
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. 
4. SILURIAN NORTH AMERICA 
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. 
