GENERAL REMARKS. 145 



The evident marks of prolonged subaerial denudation which exist on 

 all the islands of this chain which I have seen, appear further to show 

 that it has been long exj^osed to such action since the main period of 

 its formation; that as a rule it has stood unsubmerged since the Miocene, 

 and that though it may at some period have been more elevated, it has 

 either not been more deeply submerged than it is at present, as, if so, that 

 such submergence has endured for a comparatively very brief period. 



Saint Matthew and its adjacent islands, with the Commander islands, 

 appear to have much the same history with the Aleutians, and may 

 very well have been coeval with them in origin. The later eruptions, to 

 which the Pribilof islands and Nunivak island are due, have doubtless 

 also left their traces in the Aleutian chain, while the volcanoes of Kam- 

 chatka may have originated at this later period and have continued 

 their activity with little relaxation to the present time. 



The planes of marine denudation, noted particularly at Saint Law- 

 rence island and at cape Japounski, on or near the western border of 

 Bering sea, seem to require prolonged stability at a level some hundreds 

 of feet lower than the present in that part of the region, and the fact 

 that this plane appears to be capped by volcanic rocks at Saint Law- 

 rence island (particularl}^ if no evidence of existing volcanoes is found 

 there), makes the date of this submergence somewhat remote. It may 

 be conjectured that it corresponds with the general submergence of the 

 later Miocene. That the amount of such submergence should vary in 

 different localities is quite in accord with what might be expected, per- 

 liaps, in any region, certainly in one in which volcanic forces of a local 

 kind have to be allowed for. 



The difference, of climate which would result in the northwestern part 

 of North America from the closing of Bering strait and the addition of 

 the shallow eastern part of Bering sea to the continental land may not 

 have been very great, inasmuch as the strait is even now a shallow one 

 and no very great volume of abnormall}^ cold or warm water flows 

 through it in either direction. The effect would be to slightly lower the 

 temperature and decrease the precipitation on the adjacent lands. Evi- 

 dence has, however, recently been obtained of a much more important 

 factor in regard to late changes of climate in this region, in the observa- 

 tions of Mr I. C. Russell, 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 tangen- 

 tial 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 



♦National Geographic Magazine, Washington, p. 174. Bull. U. S. Geol. Surv., no. 84, p. 259. 



