144 G. M. DAWSON — COASTS OF BERING SEA AND VICINITY. 



with Bering straits and much of the Arctic ocean beyond, must be con- 

 sidered physiographically as belonging to the continental plateau region 

 and as distinct from that of the ocean basin proper, and there is every 

 reason to suppose that it has in later geologic times more than once and 

 perhaps during prolonged periods existed as a wide terrestrial plain 

 connecting North America with Asia. 



In all probability this portion of the continental plateau is a feature 

 much more ancient than the mountain range of which the outstanding 

 parts now form the Aleutian islands. This range, though to some ex- 

 tent due to uplift, as for instance in the case of Attn island, is chiefly 

 built up of volcanic materials. Its eastern part, in the Alaskan penin- 

 sula and as far as the Unimak pass, must be regarded as having been 

 built upon the edge of the old continental plateau. Its western part, 

 though certainly the continuation of the same line of vulcanism, runs 

 ofl* the edge of the plateau and rises directly from the ocean-bed. 



The available evidence goes to show that the submarine plateau of the 

 eastern part of Bering sea, together with much of the flat land of western 

 Alaska, was covered by a shallow sea during at least the later part of 

 the Miocene period, while the most recent period at which this plateau 

 stood out as land is probably that at which, according to facts previously 

 noted, the mammoth reached the Pribilof islands and Unalaska island 

 across it. 



As to the date of the formation of the Aleutian chain, Dall inclines to 

 the belief that it marks a line of weakness or faulting which has been in 

 course of development since early Mesozoic times.* This may be the 

 case, but I have found nothing on record nor have I myself met with 

 any facts which appear to require so early a date of origin. The associ- 

 ation of the volcanic materials of the islands in some cases with Miocene 

 marine fossils and with plant-remains, noted b}^ Dall, shows that early 

 in that period, or possibly before it, the islands existed in some form, for 

 the organic remains are those of shores and shallow water, not of the 

 deep sea. The existence of very ancient volcanic products forming well 

 bedded rocks on Attn island, and perhaps elsewhere in its vicinity, does 

 not appear to have any necessary connection with the date of the vul- 

 canism to which the islands as a whole are referable, for such rocks are 

 very common in formations of many periods on both sides of the Pacific, 

 and may be due to volcanic action along lines entirely distinct from that 

 now occupied by the Aleutians and long since extinct. We may there- 

 fore, I believe, assume that the building of the Aleutian islands began 

 in the later Eocene or earlier Miocene, that it was continued with vigor 

 throughout the Miocene, and in an intermittent and declining way has 

 survived up to the present tinle. 



* Op. cit., p. 242. 



