420 University of California Publications. [Geology 



The Eocene and Miocene were apparently periods of com- 

 parative stability and were marked by the reduction of the region 

 of Seward Peninsula to a peneplaiu. The submarine plateau 

 of Bering Sea, which is considered by Dawson 20 as belonging 

 physiographically to the continental plateau region, may have 

 been evolved during those periods, and Asia and America con- 

 nected by a land mass. 



At the beginning of the Pliocene, Seward Peninsula possessed 

 approximately its present shore line. In terms of the physio- . 

 graphic record the facts indicate that the peninsula possessed 

 approximately its present outline at the time of the marine 

 planation of the York bench. The York bench is undoubtedly 

 older than the loose sands and gravels of the Nome beach de- 

 posits, and if we accept the age of the latter as determined 

 paleontologically, is, therefore, of pre-Upper Miocene age. It 

 is difficult to reconcile this great age with the splendid state of 

 preservation of the marine terrace. 



During the remainder of Cenozoic time the dominant move- 

 ment affecting Seward Peninsula has been that of uplift. The 

 crustal instability of the region, the known large differential 

 warping that has accompanied elevatory movements, and the 

 shallow depth of Bering Sea render it, however, highly probable 

 that at various times brief periods of land communication have 

 existed between the continents. 



The general conclusion is therefore borne upon us that if 

 the problems of the intercontinental migration of faunas demand 

 periods of terrestrial communication between the two mainlands 

 during Cenozoic time, the physical evidence, so far as now 

 known, favors the probability of intervals of continuity of the 

 adjoining land masses of Asia and North America. 



20 Op. ext., p. 146. 



Issued May 21, 1910. 



