538 R. Mountford Deeley—The Ice Age 
away. On the other hand, if the total precipitation per square yard 
remained unaltered, and the snow-line were lowered owing to a colder 
atmosphere, then the snow collected in the larger névé (alimentation 
area) would be greater in quantity and the ablation area would have 
to be increased by the lengthening and broadening of the tongue 
(ablation) portion. 
The Figure is a map of a small glacier showing the snow-line. 
A is the alimentation area, and B is the ablation area. These two 
areas of any glacier must be properly proportioned the one to the 
other.’ Taking a large number of glaciers the average area of the 
alimentation region is about three times that of the ablation region. 
A comparatively small change in the position of the snow-line, in 
the case of such a glacier as is shown in the Figure, would cause 
a considerable retreat or advance of the glacier tongue. 
Map of a small glacier showing (A) the alimentation area and (B) the 
ablation area. 
Although it is clear that either of the theories here considered 
explains how glacier variations may occur, they are so fundamentally 
different in principle that it is of the utmost importance that they 
should both receive very careful consideration when dealing with 
glacier fluctuations, large or small. They may be referred to as the 
precipttation and temperature theories respectively. 
Any increase in the precipitation per square yard requires an 
increased rate of evaporation over the water-covered areas, and to 
obtain this a hotter sun is required. Such an increase of temperature 
would tend to raise the snow-line, whilst the increased precipitation 
of snow would tend to lower it. On this point? Tyndall remarks: 
1 Die Gletscher, 1904, p. 84. 
2 Heat considered as a mode of motion, chap. vi. 
