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Show 154 TSLAND LTFK [ PART T. - · ·- .~: ll durincr summer and off the sun-heat, and then snow 1a s even. o Wh the stored-up cold does not diminish dunn?' the year. . h .en, l ll t. of the surface 1s covered wit Ice, however,on y a sma por wn th. the exposed earth becomes heated by the hot ~nn, IS warms the air and the warm air melts the adjacent rc~. It follows, that to' wards the equaton·a 1 1u' m' ts of a gla.c rated c. ountry alternatw· ns of ch· mate may occur durinoo· a penod of hrcor h ex. - centn·c i·t y, wh 1'l e nearer. the pole , where the whole country rs comp1 e t el y I.C e-c 1a d , no a·m elioration. may. take. p. lace. . Exa.c tly the same thing will occur inversely with n:rld A1ct1C climates, but this is a subject which will be discussed m the .next chapter: This view of the character. of the last glac1al epoch st.nctly correspon d s wr' th the .1t; :acts adduce.d b. y g. eologists. . The mter-glacial deposits never exhibit any m~rcatron of a chma.teO' whose warmth corresponded to the seventy of the pre~edi.nt:~. cold, but rather of a partial amelioration of that cold; wh1le It IS only the very latest of them, which we may su~p~s~ to have occu~r~d when the excentricity was considerably dunmrshed, that exh1b1t any indications of a climate at all warmer than that which now prevails.1 1 In a recent number of the Geological Magazine(April, 1880) Mr. Searles V. ·wood adduces what he considers to be the "conclusive objection' ' to Dr. Croll's excentricity theory, which is, that during the last gl acial epoch Europe and North America wete glaciated very much in propor tion to their respective climates now, which are generally admitted to be due to the distribution of oceanic currents. But Dr. Croll admits his theory "to be baseless unless there was a complete diversion of the warm ocean currents from the hemisphere glaciated," in which case there ought t o be no difference in the extent of glaciation in Europe and North America. Whether or not this is a correct statement of Dr. Croll's theory, the above objection certainly does not apply to the views here advocated; but as I also hold the "excentricity theory" in a modified form, it may be as well to show why it does not apply. In the first place I do not believe that the Gulf Stream was "completely diverted" during the glacial epoch, but that it was diminished in force, and (as described at p. 138) pa1·tly diverted southward. A portion of its influence would, however, still remain to cause a difference between the climates of the two sides of the Atlantic ; and to this must be added two other causes-the far greater penetration of warm sea-water into the European than into the North American continent, and the proximity to America of the enormous ice-producing mass of Greenland. We have thus three distinct causes, all combining to CHAP. YIH.] 'l'RE CAUSES OF GLACIAL EPOCHS. 155 r:ob.able .date of the Glacial Epoch.-The state of extreme glaCiatwn m . th.e northeru hemisphere , of wh r' c h we gave a general d. escnptwn at the commenceme n t of th e prece dm' g chap:er, rs a fact of which there can be no doubt whatever, and It occurred at a period so recent geologically that all the mollus.c a were the same as species still l1'vi· ng. The re r· s c1 e ar geologwal proof, however, that considerable changes of sea and land, ~nd a large ~mount of valley denudation, took place during and smce the glacial epoch, w bile on the other hand the surface markin~s produced by the ice have been extensively preserved; and takmg all these facts into consideration, the period of about 200,000 years since it reached its maximum and about 80 000 years since it p::~ssed away, is generally considered by geol~~ists to be ample. There seems, therefore, to be Httle doubt that in increased excentricity we have found one of the chief excitincr causes of the glacial epoch, and that we are therefore able t~ fix its date with a considerable probability of being correct. The enormous duration of the glacial epoch itself (including its interglacial, mild, or warm phases), as compared with the lapse of time since it finally passed away, is a consideration of the greatest importance, and has not yet been taken fully into account in the interpretation given by geologists or" the physical and biological changes that were coincident with, and probably dependent on, it. Changes of the Sea-level dependent in Glaciation.-It has been pointed out by Dr. Croll, that many of the changes of level of produce a more severe winter climate on the west than on the east of the Atlantic during the glacial epoch, and though the first of these-the Gulf Stream-was not nearly so powerful as it is now, neither is the difference indicated by the ice-extension in the two countries so great as the present difference of winter-temperature, which is the essential point to be considered. The ice-sheet of the United States is usually supposed to have extended about ten, or, at most, twelve, degrees further south than it did in Western Europe, whereas we must go twenty degrees further south in the former country to obtain the same mea~ winter temperature we find in the latter, as may be seen by examining any map of winter isothermals. This difference very fairly corresponds to the difference of conditions existing during the glacial epoch and the present time, so far as we are able to estimate them, and it certainly affords no grounds of objection to the theory by which Lhe glaciation is here explained. |