216 
THE FORMATION OF GLACIERS. 
subjected by him to microscopic examination as 
accurate as if made upon any living animal. 
We have as yet no clew to the source of this 
great and sudden change of climate. Various 
suggestions have been made, — among others, 
that formerly the inclination of the earth’s axis 
was greater, or that a submersion of the conti- 
nents under water might have produced a de- 
cided increase of cold ; but none of these expla- 
nations are satisfactory, and science has yet to 
find any cause which accounts for all the phe- 
nomena connected with it. It seems, however, 
unquestionable, that since the opening of the Ter- 
tiary ago a cosmic summer and winter have suc- 
ceeded each other, during which a Tropical heat 
and an Arctic cold have alternately prevailed 
over a great portion of the present Temperate 
Zone. In the so-called drift (a superficial de- 
posit subsequent to the Tertiaries, of the origin 
of which I shall speak presently) there are found 
far to the south of their present abode the re- 
mains of animals whose home now is in the Arc- 
tics or the coldest parts of the Temperate Zones. 
Among them are the Musk-Ox, the Reindeer, the 
Walrus, the Seal, and many kinds of Shells char- 
acteristic of the Arctic regions. The northern- 
most part of Norway and Sweden is at this day 
the southern limit of the Reindeer in Europe ; 
but their fossil remains are found in large quan- 
