270 J. Croll—Geological Climatology. 
of that hemisphere, more particularly the Gulf Stream, begin 
to decrease in volume, while those on the southern or warm 
hemisphere begin part passu to increase.* This withdrawal of 
heat from the northern hemisphere favors the accumulation of 
snow and ice, and as the snow and ice accumulate the ocean 
currents decrease. On the other hand, as the ocean currents. 
diminish, the snow and ice still more accumulate. Thus the 
two effects in so far as the accumulation of snow and ice is 
concerned mutually strengthen each other. 
The same process of mutual action and reaction takes place 
among the agencies in operation on the warm hemisphere, only 
the result produced is diametrically opposite to that produced 
in the cold hemisphere. On this warm hemisphere action and 
reaction tend to raise the mean temperature and diminish the 
quantity of snow and ice existing in temperate and polar 
regions, 
The primary cause of all these physical agencies being set 
in operation is a high state of eccentricity of the earth’s orbit, 
and with a continuance of that state a glacial epoch becomes. 
inevitable, 
telling it backward. ; 
err Woeikof on the cause of Glaciation—In an article by 
A. Woeikof on ‘Glaciers and Glacial Periods in their rela- 
tions to Climate’ (Nature, March 2d, 1882), it is maintained 
that the chief cause which leads to the formation of snow, a0 
consequently to a glacial condition, is a low surface-tempera- 
ture of the sea surrounding or adjoining the land. When. the 
surface temperature of the water much exceeds the freezing- 
* Professor Dana has shown that in North America those areas which at pres~ 
ent have the greatest rain-fall are, as a rule, the areas which were most glacia 
Wood maintains that this fact uc eh T 
if as I have mai : 
the Glacial period, still I think it would follow, other things being equal, tha 
areas whi test rain would duri 
é test snow- ni . ene 
The amount of precipitation might be less than at present, but this would _ 
prevent the areas which had the greatest snow-fall from being most eoverse . 
en 
