glaciers of the posterior Rhine, Alalein, Tschingel, and Grünberg. The ancient glaciers of Switzerland present still more striking examples of this mode of progression. I shall recall to M. Durocher's recollection the succession of hills which he must have seen in descending the valley of the Aar; 1st, the two Baerenbühl; 2d, the groups of moutonnéed rocks which we cross in going from the Aar to the hospice of the Grimsel, and which form the contrefort of Naegelisgraeth; 3d, the Spitalnollen; 4th, the Kirchet, which are covered on all sides with striæ characteristic of the action of glaciers, identical in every respect with those which the Aar is engraving in the present day under our own observation. I was astonished, I must confess, to find the following remark among the objections to the ancient extension of glaciers: "In the country of Areskutan," says M. Durocher, "the erratic agent must necessarily have had an ascending motion; it has set out from a lower country to ascend to another 222 metres higher than its point of departure; it is not, therefore, a glacier which has engraved the stria of Areskutan." This difficulty is indisputable; but it appears to me, that if it be difficult to admit that a glacier can raise raise itself up a slope, it is still more difficult to conceive that a diluvian current could flow up one; and if we were forced to choose between the two suppositions, there is no one who could not decide for the glacier. In truth, it has not been demonstrated that a glacier cannot ascend a declivity by resting itself against an obstacle at the bottom, whilst water cannot move against the laws of gravity. This alleged objection, therefore, against glaciers is only a local problem, of which M. Durocher has been unable to find the solution. There is one, however, which I have endeavoured to give, in favour of currents as well as of glaciers. The coast of Norway, as is well known, is subject to considerable oscillations; and we may affirm, that, in the glacial epoch, the relative levels of the sea, of the coast, and of the interior of the country, were not the same as they now are. Everywhere in the Gulf of Christiania, and elsewhere, the striæ are continued under the sea. It is, therefore, extremely probable, that at the era of glaciers the coast was more elevated than at the present. In fact, it is necessary, in order that glaciers might slide along the bottom of the sea, that its temperature should be constantly below zero; for, if the temperature of the latter were maintained above the freezing point during several months of the year only, the glacier would float upon the surface of the sea, and not striate its bottom. I think that I have established this point in my observations on the glaciers of Spitzbergen.* During the glacial epoch, it is, therefore, probable that the coast was higher than at present, for we must suppose it to have been more elevated before all the striæ could be submerged. But since it has undergone remarkable oscillations, as is proved by the balani of Udevalla, seen by M. Brongniart,† 60 metres above the sea; the serpulæ fixed on polished rocks between Christiania and Aggersbach; and above all, by the bed of clay and shells (Skalenschicht) described by M. Keilhau, and which are found to the height of 188 metres above the sea, and 12 myriametres from the coast.§ The oscillation of the coast of the peninsula is, therefore, a fact mathematically demonstrated. Let us now see whether the portion of the Norwegian coast, of which M. Durocher speaks, presents any marks of these changes of level. Areskutan is situate under the same latitude as Drontheim, but 10 myriametres into the interior of the country. Now, in the Beistadfiord, 11 myriametres to the north of Drontheim and Areskutan, M. de Buch|| found, at a height of 140 metres, marine shells, which he considers the same as those of the deposit near Drammen, the greater part of them now living in the North Seas. These are, Mytilus edulis, Cardium eduli, Mya truncata, Venus Islandica, * Bibliotheque Universelle de Geneve, July 1849; Bulletin de la Société Geologique, Meeting of 4th May 1840; and Edin. New Philosophical Journal, vol. xxx., p. 284, 1841. † Notice of the blocks of rocks found in transported formations, Annales des Sciences Naturelles, t. xiv., 1828. Scheerer, Beytraege zur Kentniss des Seftstroem'schen Frictions Phenomen., Annales de Physique de Poggendorff, 3d series, t. vi., 1844. § Daubree, Note on the Erratic Phenomena of the North of Europe. Bulletin de la Société Geologique, t. xiv., p. 574, 19th June 1843. || Reise durch Norwegen und Lappland, t. i., 251. &c. A little to the south of Drontheim, at the entry of the Joergenfiord, latitude 62° 20′, M. Keilhau* observed terraces rising 200 metres above the ocean. Thus, then, the coast of Drontheim, which corresponds to the mountain Areskutan, has been subject, like the rest, to considerable oscillation, and we cannot infer, from its present height, what was its elevation at the period when the striae were produced. The difficulty started by M. Durocher is not, therefore, without a solution, and all his calculations on the ascending motions of a glacier are without foundation, since it is all but demonstrated that, at the glacial era, the relative levels of the sea, the coast, and the interior of the country, were not the same as now. It remains to be determined, whether the mean direction of the striæ is really that which he describes; for I must observe that M. Siljestroem, who visited this locality before M. Durocher, mentions only striæ,† directed from north to south, and others from east to west; and none of those running from 1'W. to 1'E., on which M. Durocher founds his reasoning. M. Durocher's fourth difficulty is one of a similar nature. He cannot comprehend how a glacier could cross obliquely the deep depression of the Gulf of Bothnia, and then ascend the hills of Finland. I can still less understand how a current could do this; in fact, a current from the NW. towards this gulf, would have evidently deviated on entering this depression, and would have followed its direetion, which is from NE. to SW., in the same manner as a river which enters a valley, immediately takes the direction of that valley. But we may conceive, how a mass of ice could traverse a depression obliquely. The glaciers of Switzerland present us with numerous examples. We must remember, that the maximum specific gravity of the ice of glaciers is, according to M. Dollfus' experiments, to that of pure water as 909 is to 1000, a proportion which, with muddy water becomes 883 * Nyt Magazin fur Naturviderskaberne, 1837, p. 220; and Bravais, On the Lines of the Ancient Level of the Sea in Finmark. Voyages du Commission du Nord en Scandinavia. Geographie Physique, t. i., p. 117. † Carte de la Scandinavie, par M. Bravais. Atlas des Voyages de la Commission du Nord. to 1000. This ice, therefore, partly swims on water; and if a glacier supported its sides on the shore, that would be sufficient to sustain it above a liquid mass. These consequences are verified in nature. The glacier of Aletsch, in Switzerland, rests on the surface of Lake Morill; all the glaciers which occupy the bottom of the Spitzbergen bays, advance above the sea, to a certain distance, supporting themselves on the shores of the bay.* For all these reasons, it does not appear to me absurd to suppose, that, by enlarging the proportions of the phenomenon, the glaciers formerly crossed the Gulf of Bothnia, as they have probably traversed the lakes of Switzerland. In this country, we can demonstrate, from the altitudinal limit of the ancient lateral moraines left by these glaciers, that their base did not rest on the bottom of the lake, but corresponded very nearly to the actual level surface of the water. The portion of his map of the direction of the striæ, which M. Durocher has derived from that of M. Sefström,† presents, on the shores of the Gulf of Bothnia, between Gefle and Oeregrund, arrows running from NE. to SW., as if the grooving force advanced from the sea towards the interior of the country. From this M. Durocher draws a great argument against glaciers, which, he says, cannot rise from the bottom of a basin, like the Gulf of Bothnia, upon the shore. The objection applies equally to currents; but I shall not discuss it; for I have strong reasons for thinking, that the direction indicated by M. Sefström is not the mean direction, but that he has been led into error by local deviations. The recollections of my journey in 1839, in which I travelled from Gefle to Upsal, crossing this region, recall nothing of this nature. Such are the objections M. Durocher makes to the ancient extension of the glaciers of Scandinavia. Not to prolong the discussion, I shall not enter upon the enumeration of the difficulties that may be raised against the hypothesis of diluvial currents; my task would be difficult from other causes, *See my observations on the glaciers of Spitzbergen, Bibliotheque Universelle de Genève, t. xxviii.. p. 160. July 1848. † Annales de Physique de Poggendorff, t. xliii. 1838. for in these two memoirs the author has not come to a conclusion. Formerly, he admitted a single current,* which extended from Spitzbergen to Germany; now, I suppose that he must admit ten different currents, successive or simultaneous,† running towards the four cardinal points, according to the mean direction of his arrows. The author not having explained himself, I prefer to wait till he has himself pointed out the consequences of his observations, and shewn how one or more diluvial currents better account for the facts observed, than the supposition of the ancient extension of glaciers. Reply to the Objections to the Ancient Extension of Glaciers among the Alps. M. Durocher terminates his memoir by drawing a parallel between the erratic phenomena of the North, and those of the Alps, Pyrenees, and Vosges. He admits the analogy of the effects produced; but as he does not take into account the powerful action of the sea during the time of the immersions and emersions of the land of Scandinavia, he carries the analogy, in my opinion, much too far. In the Alps, Pyrenees, and Vosges, the aqueous effects are confined to such as resulted from the melting of glaciers, and the incessant action of atmospheric agents. In Scandinavia, there were, besides these, the powerful effects of the sea, which has covered, at different periods, a considerable portion of the Swedish plain. This part of the memoir contains a multitude of observations and details already known, and objections long since refuted. Thus, for example, M. Durocher draws attention to the fact, that, at the contraction of valleys the striæ are ascending. Let him visit the glacier of the Aar, and he may touch, at the lower part of the promontory which supports M. Agassiz's pavilion, the identical striæ rising 64° from before backwards. He will again meet with the same phenomenon at 8 kilometres distance, at the lower contraction of the small valley of Ræterichsboden, where the striæ are inclined 48°, on a * Comptes Rendus de l'Acadamie des Sciences, t. xiv., p. 109. January 17, 1842. † See his Map. |