152 ISLAND LIFE 



have, I think, attached far too little importance to these 

 changes. We know that they have always been in pro- 

 gress to a sufficient extent to produce important climatal 

 effects ; and we shall probably be nearest the truth if we 

 consider, that great extremes of cold have only occurred 

 when astronomical and geographical causes were acting in the 

 same direction and thus produced a cumulative result, while, 

 through the agency of warm oceanic currents, the latter 

 alone have been the chief cause of mild climates in high 

 latitudes, as we shall attempt to prove in our next chapter.^ 

 On the Theory of Inter- glacial Feriods and their FrohaUc 

 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 conse- 

 quences and modifications both of climate and physical 

 geography which follow or accompany such changes. It is 

 essentially a theory of alternation; and it is certainly 



^ 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 stiff*ened 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 efl'ect 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 sufiicient 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 glacia- 

 tion," (Manual of Geology ^ 2nd Edition, pp. 541-755, 756.) 



