and Glacier Fluctuations. 5387 
retreats of glaciers in many parts of the world, whilst during 
Pleistocene time very much more pronounced oscillations are known 
to have taken place. ‘he question asto whether the prehistoric great. 
oscillations of climate, and the less pronounced historic oscillations, 
have been due to similar climatic changes is one of considerable interest. 
That glacialists are by no means all of one opinion on this point may 
be gathered from the recent literature of the subject. 
The very existence of glaciers depends upon both temperature and 
amount of precipitation ; for the presence of glacier ice results from 
the fact that over certain areas more snow falls in the winter than 
the summer heat can melt, and as the ice thus collected is a very 
viscous liquid it slowly flows to lower levels and reaches altitudes 
where the temperature is sufficiently high to melt it. In glaciers we 
thus always have alimentation and ablation areas. 
Within the limits of height reached by mountains, as we rise in 
the atmosphere, the temperature falls, and as a glacier is ascended 
the temperature falls about 1° C. for every 140 metres of vertical 
rise.! On this account a greater proportion of the precipitation comes 
down as snow at high levels than is the case at lower levels. However, 
the precipitation per square yard varies considerably with the height 
and aspect of the mountain mass. ‘Thus it has been shown that on 
the declivities of the Aiguille du Gouter the precipitation falls off 
rapidly as we ascend or descend from the level of 2,550 metres. 
Indeed, the precipitation not only varies greatly from place to place 
on a mountain range, but when the precipitation is in the form of 
snow it is blown to great distances, or slides off the slopes and collects 
at low levels in great masses. In the Northern Hemisphere, slopes 
and valleys facing south are much more exposed to the sun’s rays 
than are those having a northerly aspect, and the winds reaching 
them are generally warmer. ‘There is, consequently, no regular 
continuous snow-line, i.e. a regular level above which some of the 
winter snow survives the heat of summer and below which all the 
snow of winter is melted away during the summer. 
Although the snow-line is a very irregular feature as regards 
height, in the case of the majority of glaciers it will be found that 
‘snow-lines can be drawn across them and their tributaries, above 
which some of the snow of winter does not entirely melt each year, 
and below which not only is all the snow melted away but some of 
the ice is melted as well. Such a snow-line cuts the glacier into two 
portions, of which the upper portion is the alimentation area, whilst 
the lower portion is the ablation area. Ifthe meteorological conditions 
remained invariable from year to year, then the glacier would always 
appear to be in the same condition during the corresponding months 
of each year. 
If the precipitation of snow should increase per square yard, then 
the snow-line would be lowered, the névé would get thicker and 
larger in area, and as a result the ablation area of the glacier (the 
tongue portion) would have to grow longer, and if possible wider, so 
as to admit of the greater quantity of ice coming down being melted 
1 Hans Hess, Die Gletscher, 1904, p. 210. 
