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Show 308 DESCRIPTION OF TilE PLATES A~D MAP. period, the dry land in the northern hemisphere has been co~tinually on the increase, not only because it is now greatly 111 excess beyond the average proportion which land generally bears to water on the globe, but because the comparison of the secondary and tertiary strata implies a passage throughout the space now occupied by Europe, from the condi~ion of an ocean interspersed with islands to that of a large contme~1t. But if it were possible to represent all the vicissitudes in the distribution of land and sea that have occurred during the tertiary period, and to exhibit not only the actual existence of land where there was once sea, but also the extent of surface now submerged, which may once have been land, tl~e m~p would still fail to express all the important revolutiOns m physical geography, which have taken place within the epoch under consideration. The oscillations of level have not merely been such as to lift up the land from below the waters to a small height above them, but in some cases a rise of several thousand feet has been 'effected. Thus the Alps have acquired an additional altitude of from ~000 to 4000 feet, and even in some places still more; and the A pennines owe a great part of their height (from 1000 to ~000 · feet and. upwards) to subterranean convulsions which have happened within the tertiary epoch. On the other hand, some mountain chains may have been lowered, during the same series of ages, in an equal degree, and shoals may have been converted into deep abysses. It would be superfluous to point out in detail the beari~g of the facts exhibited in this map, on the theories proposed m a former part of this volume, respe'cting the migrations of animals and plants, and the extinction of species; and it would be equally unnecessary to enlarge on the va~~ti~ns in local climate, which must have accompanied such vicissitudes in physical geography. But the general temperature, also, of the habitable surface of the o-lobe as well as the local climates, may have been cono ' siderably modified by such extr~ordinary revolutions. The ·· DESCRIPTION OF THE PLATES AND MAP. 309 alteration in climate, implied by a comparison of the organic remains of the older tertiary strata, and the species of living animals and plants, does not appear to be so great as would be produced if the temperature of our tropics were now transferred to the temperate zone, and the temperature of the latter to the arctic. We do not, therefore, anticipate that the reader, who has duly studied the arguments explained by us in the 6th, 7th, and 8th chapters of the first volume, will object to the adequacy of the cause proposed, on the score of the small quantity of geographical change during the time in question. But if there be good reason to conclude that the change would be fully adequate, in point of the magnitude of its effects, this cause, we conceive, ought to supersede every other of a purely speculative nature, until some argument can be adduced to prove that the change has not acted in the right direction *. Some persons, but slightly acquainted with the present state of geology, have objected, that the lands in high northern latitudes have not been recently elevated. If they had reflected that every year we are making some new discoveries respecting the periods when tracts in the immediate neighbourhood of the great European capitals emerged from the deep, and had they sufficiently considered that the antiquity of a group of rocks has no necessary connexion with the date of its elevation, they would proLably have seen the futility of such arguments. As far as we can conjecture, from the very scanty information which we possess of the geology of the arctic region, there is no want of proofs of comparatively recent alterations of level. In conclusion, we may remark that the portion of Europe distinguished in this map by colours and ruled lines, comprises the greater part of the globe now known to geologistsalmost all at least that is known in such a manner as to entitle any one to speculate on the mutations in physical geography which have taken place during the tertiary period. * See Mr. Herschell's remarks_ on a change of climate.-Disc. on the Study of Nat. Phil., pp. 146 and 148. |