Shaler.} 148 [November 4, 



to suppose that ahhougli the region was covered with glacial accumu- 

 lations fi-om some cause or other, the ordinary evidences of the ice 

 sheet failed to be produced by the glacial mass which covered the 

 region. The first of these suppositions does not seem probable; the 

 rainfall of the valley of the Yukon and of Siberia is great enough to 

 produce considerable rivers, and must, even during the glacial period, 

 have been quite as great as that of Scandinavia, which bears almost 

 precisely the same relation to those sources of moisture, ocean streams 

 and currents of moistened air. It seems far more likely that the 

 latter supposition is true, and that local circumstances have prevented 

 the formation of as distinct evidences of glaciation as at other points. 

 It is important to consider that all the phenomena of erosion and 

 transportation ef ice by glacial masses depend upon the ability of the 

 base of the sheet to slide over the surface on which it rests, and upon 

 the existence of considerable areas of uncovered rock to supply the 

 materials to be ti'ansported. If the circumstances do not favor these 

 conditions, then the ice sheet may exist for any time and in any mag- 

 nitude, produce all its effects upon organic life, and yet leave tew 

 traces of its former existence. The whole of the Alaskan peninsula 

 is bordered on the south by an elevati;d mass of mountains which 

 would present tn insuperable barrier to the escape of a glacial mass 

 in that direction; to the east there could be no outlet. To the noi-th 

 there probably exists a similar mountain barrier, but even if there 

 were not, we ctnnot suppose an outlet in that direction. The only 

 course in whicb a movement of the base of the glacier would be pos- 

 sible is to the vestward into the basin of the sea of Kamtschatka. It 

 seems, however exceedingly probable that, at this time, the whole 

 of this apparertly shallow sea was filled with glacial accumulations to 

 such an extent that no escape in that direction would have been pos- 

 sible. Were this the case, no movement of such a nature as would 

 leave traces ir scored and scratched rock surfaces would be possible. 

 Accumulation might go on until the general surface of the Ice sheet 

 overtopped thi banner to the southward, when there might be suffi- 

 cient escape to prevent farther increase in the thickness of the mass. 

 But this movement could not affect the base of the sheet; that portion 

 would necessarily remain unchanged, except by the melting of the 

 ice, and the ^-earing of the rock by the streams thus produced. 



In the Sixxni way in Siberia, the want of outlet to the southward 

 would prevert the movement of the glacial envelope, which Is mani- 

 festly the firsr condition of the formation of striated surfaces. Thus 



