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Show 616 ADDENDA. frigid zone, are the r~sults of a climate which appears favourable. to the passage of tropical forms beyond their proper limits, and to a v1goro~s native vegetation. The climate is one of an equable nature; and th1s must, to a considerable degree, be the effect of the great area of ocean compared with the land of the southern hemisphere. In the northern hemisphere we have proofs, that the productions both of the land and water, during the period antecedent to the present, had a more tropical character than they now have, and there is, also, a high degree of probability that the proportional area of water was much greater. If then we judge from the analogy of the southern hemisphere, the first and simple inference from these facts, is, that the temperature of Europe was formerly more equable, though perhaps with a lower mean, than it now is. It may be asked, as a test of this inference, did the snow-line formerly descend lower than it now does? Was the soil formerly frozen a little beneath the surface in a low latitude? The congealed carcasses of the great Pachydermata of Siberia answer the second question ; and in my journal, I have indirectly considered the first one as answered, by the fact of the many erratic boulders of Europe having travelled from mountains, situated in regions where great bodies of ice do not at present descend to the level of the sea. For on the theory that these boulders were transported by icebergs from glaciers, which formerly descended into the sea in latitudes where perpetual snow is not now found, or if so, only at great heights, the problem receives so simple a solution, that I did not hesitate, having the other data, to assume, that the snow-line in Europe formerly did descend much lower than it does at present. But, had I studied my subject more attentively, I might have taken a higher ground : in a note, indeed (p. 294), I have stated that according to Professor Esmark, it is certain, that the glaciers of Norway formerly descended to a lower level; and I now found that some time since, Messrs. Venetz and Charpentier, and more lately M. Agassiz, have incontestably shown, from the presence of glacier-dikes or moraines, and from the polished and scratched surface of the rocks, that in the Alps enormous bodies of ice formerly descended to the borders even of the lake of Geneva, and therefore much lower than the line of present lowest descent.* With these several facts it might have been boldly asserted, that the clirn_ate of Europe formerly was like * No doubt if much more snow fell formerly than at present, the glaciers would formerly have descended somewhat lower; but as Europe now has a moderately humid climate, it is improbable in the highest degree (if indeed possible) that a difference of that kind could have caused the former extremely low descent of the ancient glaciers of the Alps : therefore we are compelled to attribute the difference to a change of temperature of some kind. ADDENDA. 617 that of the southern hemisphere as it now is; and consequently, as we know, that the sea within recent tertiary periods stood at a higher level over a large portion of our continent, it might have been affirmed, had there been no record of the existence of erratic blocks on this side of the globe, that it would be an anomaly, difficult of explanation, should there not be found around the eminences of central and northern Europe great unrolled fragments, scattered at long distances from their parent sources, and often separated from them by profound valleys. M. Agassiz has lately (Address to the Helvetic Society, July 1837, translated in Jameson's New Philosophical Journal, vol. xxiii., p. 364, and in several communications in the F1·enclt periodical L'Institut) written on the subject of the glaciers and boulders of the Alps. He clearly proves, as it appears to me, that the presence of the boulders on the J nra cannot be explained by any debacle, or by the power of ancient glaciers driving before them moraines, or by the subsequent elevation of the surface on which the boulders now lie. M. Agassiz also denies that they were transported by floating ice, but he does not fully state his objections to this theory; nor does he oppose it, by the argument of the apparent anomaly of a low descent of glaciers, with the generally-received opinion of the more tropical character of the productions of the antecedent periods,-which was philosophical, until the effects of a temperate and equable climate were considered.* On the contrary, he assumes that, during the gradual cooling of the earth, there have been periods of excessive refrigeration. It is needless to state that such an hypothesis is not supported by a single fact-without, indeed, the assumed sudden renewal of life on the surface of the world at successive periods be considered such. During this imagined period of excessive refrigeration, the Alps and the greater part of Europe, and even of Asia, are supposed to have been covered by one immense sheet of ice, and during the assumed sudden elevation of the Alps, fragments of rocks are supposed to have been shot ,over the frozen surface, and, when the ice melted, to have dropped on the surface where they now lie. M. Agassiz considers that this view explains the position of the boulders on pinnacles, and their absence in the valleys. I confess I should have thought, after the flexure and elevation of the ice, these would have been the least probable situations: but neither this, or some * M. Charpentier (in his account of M. Venetz's investigations on tlle Glaciers of the Valais-Edinburg!t New Philosopllical Joumal, vol. xxi., p. 215) was fully aware of this difficulty. His explanation rests on a supposed enormous oscillation of level in the Alps,-an assumption which is unsupported by other facts, and is not applicable to the general case of Europe. |