IO8 ALASKA GLACIERS 



years ago, and a great retreat has been in progress ever 

 since. On the south and southwest slopes the same period 

 of one hundred years has witnessed a general advance. 

 The century's variation for one side of the range, in a 

 general way and so far as known, is the reverse of the va- 

 riation for the other side. To apply the principle of lag to 

 the phenomena it is necessary to suppose that the glaciers 

 of one slope are very far behind those of the other in some 

 phase of variation. If the southwestern group are con- 

 sidered the slower to respond to variations of neve supply, 

 the inference is that they are now at or near a culmination 

 corresponding to the culmination of the northeastern 

 group a century or more ago. If the northeastern group 

 are considered the slower to respond, the inference is that 

 though now at very low ebb, they have not yet felt the 

 impulse which has carried the southwestern group to a 

 maximum; and many decades, if not a full century, will 

 be required to bring them to the same phase. It is not 

 clear to me which horn of the dilemma should be taken, but 

 in either case the time interval between corresponding 

 phases is greater than can reasonably be ascribed to lag. 

 Associated with the theory of lag as applied to the 

 variations of European glaciers is a generalization that 

 glacial variation is rhythmic, with a period of about thirty- 

 five years, each recurrent cycle of variation being brought 

 about by a corresponding cycle of climatic change. In 

 this respect also the Alaskan phenomena are discordant. 

 It is not credible that the great advance and retreat which 

 occurred in Glacier Bay, involving the extension of glaciers 

 along the main trough for thirty-five miles, or more, and 

 their subsequent melting, could be accomplished in so 

 short a period as thirty-five years; for, though direct ob- 

 servation has covered but a small part of the great oscil- 

 lation, it has shown that in the half of thirty-five years the 

 retreat of the ice front was less than five miles. 



