chap, viii.] THE CAUSES OF GLACIAL EPOCHS. 
147 
climates in high latitudes, as we shall prove in our next 
chapter . 1 
On the theory of inter-glacial Periods and their 'probable character. 
— The theory by which the glacial epoch is here explained is 
one which apparently necessitates repeated changes from glacial 
to warm periods, with all the consequences and modifications 
both of climate and physical geography which follow or ac- 
company such changes. It is essentially a theory of alternation ; 
and it is certainly remarkable in how many cases geologists have 
independently deduced some alternations of climate as probable. 
Such are the interglacial deposits indicating a mild climate, both 
in Europe and America ; an early phase of very severe glacia- 
tion when the “ till ” was deposited, with later less extensive 
1 The influence of geographical changes on climate is now held by 
many geologists who oppose what they consider the extravagant hypotheses 
of Dr. Croll. Thus, Prof. Dana imputes the glacial epoch chiefly, if not 
wholly, to elevation of the land caused by the lateral pressure due to 
shrinking of the earth’s crust that has caused all other elevations and 
depressions. He says : “Now, that elevation of the land over the higher 
latitudes which brought on the glacial era is a natural result of the same 
agency, and a natural, and almost necessary, counterpart of the coral-island 
subsidence which must have been then in progress. The accumulating, 
folding, solidification, and crystallisation of rocks attending all the rock- 
making and mountain-making through the Palaeozoic, Mesozoic, and 
Cenozoic eras, had greatly stiffened the crust in these parts ; and hence in 
after times, the continental movements resulting from the lateral pressure 
necessarily appeared over the more northern portions of the continent, 
where the accumulations and other changes had been relatively small. To 
the subsidence which followed the elevation the weight of the ice-cap may 
have contributed in some small degree. But the great balancing move- 
ments of the crust of the continental and oceanic areas then going 
forward must have had a greatly preponderating effect in the oscillating 
agency of all time — lateral pressure within the crust.” (American Journal 
of Science and Arts , 3rd Series, Vol. IX. p. 318.) 
“ In the 2nd edition of his Manual of Geology, Professor Dana suggests 
elevation of Arctic lands sufficient to exclude the Gulf Stream, as a source 
of cold during glacial epochs. This, he thinks, would have made an 
epoch of cold at any era of the globe. A deep submergence of Behring’s 
Strait, letting in the Pacific warm current to the polar area, would have 
produced a mild Arctic climate like that of the Miocene period. When 
the warm current was shut out from the polar area it would yet reach 
near to it, and bring with it that abundant moisture necessary for 
glaciation.” ( Manual of Geology, 2nd Edition, pp. 541-755, 756.) 
L 2 
