GLACIERS OF THE SIERRA COSTA MOUNTAINS 57 
European ice sheets, as in the bearing which they may have on 
the fascinating and yet unsettled question of the ‘‘Cause of the 
Glacial Period.” After trying unsuccessfully to solve the prob- 
lem through a study of the varied series of drift sheets in the 
Mississippi basin, I have concluded that we will do well to take 
into account such evidence as may be gathered in alpine regions 
of glaciation—outlines of the main sheets, we may say—for 
here the problem of determining climatic changes is less obscure. 
The suspicion is growing in my mind that the ‘Glacial Period” 
in geology, as a glacial or relatively cold epoch of time, was of 
world wide extent in its effects, and the absolute determination 
of the cause of the past accumulation of glacial ice in one sec- 
tion will be the key to the solution of the problem of all terres- 
trial glaciations. 
Oscar H. HERSHEY. 
November 18, 1899. 
