and Glacier Fluctwations. 541 
rises, but as we go further north or south the effect of latitude begins 
to tell, and the snow-line falls rapidly as the Polar areas are reached. 
However, even in the Polar areas, the snowfall is often so small in 
many districts that it is all evaporated or melted by the summer sun. 
A change in the direction of the winds, and, therefore, of the 
areas of greatest precipitation, might cause some of the present 
glaciers to disappear and fresh ones to appear in other places, but it ‘ 
could scarcely cause intense glaciation over large areas which are 
now free from ice. The greater prevalence of northerly winds in 
the Northern Hemisphere would lead to drier conditions there, and if 
such occurred British Columbia, Alaska, and Greenland would be less 
ice-covered than at present. But the Ice Age does not appear to 
have been either due to an alteration of any note in the direction of 
the winds, or to any notable increase in the precipitation. 
Tutkowski has maintained that dry conditions existed at the edges 
of ice-sheets owing to cold winds blowing off them. That this has 
been the case is very likely, but here we are only dealing with local 
effects, the precipitation being decreased outside the margins of the 
ice-sheets and increased to a like extent on the ice- sheets themselves. 
With regard to the greater glacier advances which brought the 
ice down on to the lowlands in many continental areas, W. B. Wright? 
takes the view that it is to change of temperature, and not increased 
precipitation, that we owe the Ice Age. Some other glacialists do 
not seem to be of this opinion, for quite recently it has been main- 
tained that the minor oscillations are due to increased precipitation. 
As far as can be judged from the evidence of glacial moraines, 
ete., it would appear that even the greater advances and retreats of 
‘the glaciers in Pleistocene time did not take place in a regular 
manner. ‘lhe ice margins appear to have always advanced or 
retreated in an irregular way. The moraines formed during the 
retreat of the ice-sheets show this most clearly. These moraines lie 
one behind the other, each moraine showing where the ice rested for 
a time, or where a temporary advance stopped. It might be urged 
that the greater oscillations are due to one kind of climatic variation, 
whereas the smaller oscillations are due to another kind. Indeed, it 
would appear that in some quarters the greater variations are 
considered to be due to changes of temperature and the smaller ones 
to precipitation changes. But it is impossible to divide the glacier 
fluctuations into two groups, the large and the small. Glacier 
variations are of all magnitudes, and it appears to be more reasonable 
to regard all glacier fluctuations, except perhaps some of the annual 
or very inconsiderable ones, as being due to fall of temperature. 
That neighbouring glaciers very often do not advance and retreat 
together is often difficult to explain. Throughout any particular 
region, however, where there are a large number of glaciers, the 
majority of them move in close agreement the one with the other. 
! The Quaternary Ice Age, p. 146. 
