CHAP. VIII.] 



THE CAUSES OF GLACIAL EPOCHS. 



147 



climates in high latitudes, as we shall prove in our next 

 chapter.^ 



On the theory of inter-glacial Periods and their prolable 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 



^ 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.) 



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