742 The American Naturalist. [August, 
slopes rapidly down on the northern side into very deep water, exceed- 
ing 1,000 fathoms as far to the eastward as Unimak Island: but from 
the vicinity of Unimak pass (longitude 165° west) the depths to the 
north of the islands are consistantly less than 100 fathoms. Beginning 
near the Unimak pass, the edge of the hundred-fathom bank runs 
northwestward, passing to the west of the Pribilovs and Saint Matthew 
Island and meeting the Asiatic coast in the vicinity of Cape Navarin, 
in about north latitude 60°. Thus all parts of Behring Sea to the north 
and east of this line, together with Behring Straits and much of the 
Arctic Ocean beyond, must be considered physiographically as belong- 
ing 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 Attu Island, is chiefly 
built up of volcanic material. Its eastern part, in the Alaskan 
peninsula 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 volcanism, 
runs off the edge of the plateau and rises distinctly from the ocean- 
bed. 
“ The available evidence goes to show that the submarine plateau of 
the eastern part of Behring 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. 
“ Evidence has recently been obtained of an important factor in regard 
to late changes of climate in this region, in the observations of Mr. I. 
C. Russel, which show that the great mountain range of the Saint Elias 
Alps must have been entirely formed in Pliocene or post-Pliocene times, 
The crumpling and upheaval of the beds which now form this range 
must have relieved a notable and accumulating tangential pressure of 
the earth’s crust, the result of which it is yet difficult to trace; but 
that it must have brought about extensive changes of level throughout 
the region over which this pressure was exerted seems certain, and I 
