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Show '144 ISLAND LIFE. [rAnT I. . . t' f much of the land would f 1 t tual glacm 1on o . state o a mas per~e b h 1 earth should theoretically 'th t d (]' that t e w o e result, notw1 s an me t Two main causes would h h' h tempera ure. be. at a somewh ' at 1 Ig. te'r n A verY large area of elevated land bnng about t 1s g acia 10 • • powerful condenser of the . . 1 't d ould act as a 1n h1gh at1 u es w d ed bv the whole of the 't f vapour pro uc J • enormous quanti Y 0 h t perate regions bemg areas of . 1 d ch of t e em ' equatona. an dm tuh us a (]'rea t er accumulation of snow and . ice evaporatiOn, an ound both poles than would be possible would take place ar . . In the second place there would d ther cond1twns. . un er any 0 h' umulation of ice, because, owmg l. 1 heck to t 1s aco . be 1tt e or not ' c 1 the polar areas warm oceamc t f land arounc ' to the quan 1 Y 0 · h th m while the warm winds would t ld not reac e , curron s 'l coub . so much . t that they would help on IDOlS ure n. ecte ssda n fY chnencgk m. ct tll e pro cess of ice-accumulation. If we Ins ea oth e cont.m een ts t o b e of the same total area and to have sthuep psoasme e extent and a lt1' t u de of mountain ranges as the pres. ent ones, these moun t am. s mu st necessar. ily of, fer an almost condtm hu- ous barn.e r to t h e •vapour- bearing wmds from the sout1h , da n t lde result would pro b a bl y b e that three-fourths .o f the an wt'o u l be in t h e I.C e-c 1a d condl. tion of Greenland, while a com para Ifvfe dy b lt f the more southern lowlands would alone a or narrow e o d ve(J'etation h bitable surfaces or produce any woo y e . a N otw1. t l 1stan dm' g, therefore ' the criticism abov· e 1r efe· rrhetd tod, . h t s· :Charles Lyell was subst::mtlal y ng , an I beheve t a Ir · · G z (11th . th t the two ideal maps given .in the Prtnctples of eo ogy a . 270) if somewhat modified so as to allow a ~r~er ed. Vol. l.fp. ent's l·n the tropics do really exhibit a conditiOn · passage o curr ' ld b · of the earth which, by geographical change.s alone, ~au rmg about a perpet ua1 Summer or an almost univers· al whm tetr . Butt we have seen 1. n our si'xth chapter that there IS t e s· rongthe s t latl.ve evidence almost amounting to demonstratwn, a cumu ' · d ns have for all known geological periods our contments an ocea h occupied the same general posi.t i.O n th ey d o now ' andd t1 atd no h radical chanfY'es in the distribution of sea an an as s. ue · d by way oef hypothesi.s by S.u Ch ar1 e s L ye 11 ' have ev.e r nnaame · 'th t 1ts occ:rred. Such an hy~othesis, . however, ~s not Wl ~u clear use in our present inqmry, for If we obtam thereby CHAP. \'111.] TllE CAUSES Oli' GLACTAL EPOCHS. 145 C'onccption of the influence of such great changes ou climate, we are the better able to appreciate the tendency of lesser changes such as have undoubtedly often occurred. Land as a baT1·ie1· to ocean CUTTents.-We have seen already the great importance of elevated land to Rerve as condensers and ice-accumulators; but there is another and hardly less important effect that may be produced by an extension of land in high latitudes, which is, to act as a barrier to the flow of ocean currents. In the region with which we are more immediately interested it is easy to see how a comparatively slight alteration of land and sea, such as has undoubtedly occurred, would produce an enormous effect on climate. Let us suppose, for instance, that the British Isles again became continental, and that this continental land extended across the Faroe Islands and Iceland to Greenland. The whole of the warm waters of the Atlantic, with the Gulf Stream, would then be shut out from Northern Europe, and the resu'lt would almost certainly be that snow would accumulate on the high mountains of s~andinavia till they became glaciated to as great an extent as Greenland, and the cold thus produced would react on our own country and cover the Grampians with perpetual snow, like mountains of the same height at even a lower latitude in South America. If a similar change were to occur on the opposite side of the Atlantic very different effects would be produced. Suppose, for instance, the east side of Greenland were to sink considerably, while on the west the sea bottom were to rise in Davis' Strait so as to unite Greenland with Baffin's Land, thus stopping altogether the cold Arctic current with its enormous stream of icebergs from the west coast of Greenland. Such a change might cause a great accumulation of ice in the higher polar latitudes, but it would certainly produce a wonderful ameliorating effect on the climate of the east coast of North America, and might raise the temperature of Labrador to that of Scotland. Now these two changes have almost certainly occurred, either together or separately, during the Tertiary period, and they must have had a considerable effect either in aiding or checking the terrestrial and astronomical causes affecting climate which were then in operation. L |