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. 
