CAUSE OF VARIATIONS I07 
For the explanation of irregularity there are two promi¬ 
nent hypotheses. The first, regarding oscillation of long 
period as climatic, ascribes minor oscillations to the 
rhythmic gorging and disgorging of the neve reservoir. 
The rhythmic period, being connected with topographic 
conditions, is different in different glaciers. 
The second, which has been more fully developed, ap¬ 
peals to the principle of lag. The chief cause of variation 
of the wasting end of the glacier is believed to reside in 
variation of snow accumulation on the neve fields, but con¬ 
siderable time is required for the transmission of the effect 
from end to end of the glacier. For average Alpine 
glaciers this time is believed to amount to several decades, 
but it varies with the length, slope, and other peculiarities 
of the individual ice streams, and the general result of its 
variation is that ice streams of the same mountain slope, 
or streams flowing from the same neve, initiate a period 
of advance or retreat in different years or even different 
decades. 
It is impossible to compare the first hypothesis with the 
behavior of Alaska glaciers, on account of the meagerness 
of observational data. As yet we know nothing of perio¬ 
dicity of variation. The comparison of the second is ob¬ 
structed by the lack of meteorologic records for the glacier 
region, and by the fact that very few of the determinations 
of variations are associated with definite dates; no com¬ 
parison with climatic variation is possible, and the com¬ 
parison of individual histories of variation, one with 
another, is approximate only. Nevertheless, some judg¬ 
ment may be formed of the general competence of the 
second theory; and it seems to me not fully adequate for 
the explanation of Alaskan disparities. 
Consider, for example, the contrasted histories on oppo¬ 
site sides of Fairweather Range. On the northeast slope 
a great advance culminated not less than one hundred 
