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Show 1413 ISLAND LIFffi. (PART L - -- bable changes which would It would be easy to suggest ot~er pr~ b t we will only refer to d l d effect on chmate' u . pro uce a mar ce f p rna which has certamly b 'd f th Isthmus o ana ' the su SI ence 0 e . T j.~ times. If this subsidence d than on.ce m erlJol.ary happene _more . ld h allowed much of the accumulated derable It wou ave · were consi . . . . h G lf Stream to pass mto the t whiCh mitiates t e u warm wa er . . d while astronomical causes were P ·n. . nd If th1s occurre . t aCdIm. co -' toa bn.n o- a b ou t a co ld period in the n. orthern hemisphere, r en o . 0 • • iaht be exceptiOnally severe. The the resultmo0- glaCiation m o 1' d 'f t th effect of this change wou ld h owev er b.e neutra tse I a e same epoch the L esser and Greater Antilles formed a connected land. b bl h' 1 Now as sue h posst' ble and even pro a e geograp wa chano-es' are very num erous ' they must have produced .i mportant effecots . and th ouoo- h we may admit that the astro· nomd iCal ca· u·s es alread y' exp1 a m· e d were the most important In etermm.m g the last glacial epoch, we must also allow that geographical changes must often have had an equally_ important and perhaps even a preponderating influence on chmate. We must also rememb er tha t Chancor es of land and . sea are almost al. wa. ys accompam· e d b Y elevation or depressiOn of the . pre-existing l d . d whereas the former produces its chief effect by an . an · t th d. ·t· o- the course of warm or cold oceamc curren s, e IVei In0 d' · . h' l a tte r 1· s of not less importance in adding to o·r 1m1m· 1sh mg tho se areas Of Condensation and ice-accumulatiO. n w 1Ic , . as we have seen, are the most efficient agents m producmg glaciation. · 1 If then Sir Charles Lyell may have somewhat erred m attac - ina- too exclusive an importance to geographical changes as bringing about mutations of climate, his critics have, I think, attached far too little importance to these changes. .we know that they have always been in progress to a suffiCient extent to produce important climatal effects; and we shall probably be nearest the truth if we consider, that great extre~es 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 ocea~t~ currents, the latter alone have been the chief cause of ml L IIAP. VIII.] TilE CAUSES OF GLACIAL EPOCHS. 147 climates in high latitudes, as we shall prove in our next chapter.1 On the theory of inte1·-glacial Pe1·iods and theitt pTobable chaTacter. -The theory by which tbe 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 accompany 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 glaciation 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 rockmaking and mountain-making through the Palreozoic, Mesozoic, and Cenozoic eras, had greatly stifFened the crust in these partA; 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 movements of the crust of the continental and oceanic areas then going forward must have had a greatly preponderating effect in tl1e oscillating agency of all time-lateral pressure within the crust." (.Arnm·ican Jmmwl of Science and .Arts, 3rd Series, Vol. IX. p. 318.) "In the 2nd edition of his llfanual 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 o£ cold at any era o£ 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 |