portance of a great revolution, and its theatre may be said. to include a considerable portion of the two hemispheres. If human genius should one day explain the cause of this glacial cataclysm, it will throw the brightest light on the last phase of the geological history of the globe,-on the mysterious epoch which preceded the appearance of Man on the surface of the earth,—and on that universal Deluge, of which traces are found in the traditions of nations in Europe, Asia, and the two Americas. The intimate relation which connects these two phenomena cannot be denied, for they are attested at once by reason and observation. Yet we do not propose to pursue the study of glacial phenomena in all the countries where they have been attended to; we confine ourselves to the study of them among the Alps, where the facts, well known and appreciated, may be verified every year by numerous travellers. Have the glaciers of Switzerland and Savoy always been confined within their present limits, or did they formerly extend into the great plains which lie around the chain of the Alps? Such is the problem, when reduced to a simple form. My object is to explain the facts on which the advocates of the ancient extension of glaciers rest the argument. In order to establish their views, they have to combat, among philosophers, convictions of old standing, resting on the most unexceptionable authorities in geology; among men of the world, the testimony of biblical tradition, and that of all the senses, which revolt against the idea of considering these plains, now so fertile and animated, as having been buried during long periods under an immense sheet of snow and ice. Both parties have a right to demand numerous and positive proofs. These proofs exist; but, before examining them, it is necessary to possess some knowledge of existing glaciers; for the method followed by geologists, to whom we are indebted for the results which we are about to lay before the reader, has always been that introduced into science by M. Constant Prevost, and which may be briefly expressed in this Brongniart, Sefstroem, Keilhau, Boethling, Siljestroem, Daubrée, Murchison, Verneuil, and Durocher, in Scandinavia; Hitchcock and Darwin, in America. way: "To study the mode of action of natural elements which we witness before our eyes, and compare the effects they produce with those of which traces are still preserved on the surface of the globe." By proceeding on this principle, we shall find that we can discover everywhere, in the vast plains which surround the Alps, traces of those gigantic glaciers of which such as now remain may be said to be the mere miniatures. Still, although reduced to small dimensions, existing glaciers present us, on a small scale, with all the phenomena that formerly occurred in fields of ice on a large scale. The effects are the same, and we can infer their identity from that of the agents which have produced them. 1. Of Existing Glaciers. From the summit of the ridge of the Jura, which overlooks the Leman basin, we obtain a bird's-eye view of the whole chain of the Alps, from the Valais to Dauphiny. The colossal mass of Mont Blanc, resting on its broad base, rises majestically and alone above this long serrated ridge. The highest peaks are distinguished from the less elevated summits, by the sparkling whiteness of the snow which covers them. In summer, the lower limit of these perpetual snows forms a straight horizontal line, perfectly marked, which contrasts with the sombre verdure of the forests along the base of the mountains. This line is that of eternal snows. Above it, winter reigns alone; below it, the seasons follow their regular course. Above it, life scarcely exists, and is represented only by a few polar plants, and some ephemeral insects; below, it is manifested in a thousand varied forms, from the highest regions where the pine and the chamois venture to appear, down to the plains inhabited by man, where the harvests become yellow, and the vine ripens its fruit. In Switzerland, the lower limit of perpetual snow is about 2700 metres above the level of the sea; but as we approach the Alps, penetrating into the narrow valleys which intersect the principal masses, such as those of Mont Blanc, Mont Rosa, St Gothard, and the Jungfrau, we perceive that this limit is not a straight line, as it appeared when seen at a distance. The fields of perpetual snow emit branches, so to speak, which descend into the valleys in the form of masse of ice, resembling frozen torrents. These masses are glaciers Their lower extremity is often more than 1500 metres below the limit of perpetual snow, and sometimes approach to the vicinity of large villages, such as those of Chamonix, Cour! mayeur, and Grindelwald, the mean height of which is about 1120 metres above the sea. At the same time, a great number of glaciers exist which do not descend so low, and rest on elevated slopes, where we find only a few scattered chalets, inhabited only a few months in the year. These What are the relations that exist between these glaciers and the fields of snow with which they are connected? is the first question we have to examine. Science has already answered it. In winter, spring, and autumn, considerable masses of snow fall on the summits of the Alps.* snows, driven about by the winds and whirlwinds, accumulate, particularly in the great depressions in the vicinity of the high peaks. These depressions are known by the name of amphitheatres or cirques, for they commonly terminate in a semicircular enclosure, surrounded with elevated summits. Of this description, in the environs of Chamonix, are the cirque which terminates at the Col du Geant; the great plateau, which is only 800 metres below the peak of Mont Blanc; near Grindelwald, the amphitheatre which leads to the Strahleck; at the Grimsel, those of the Lauteraar and Finsteraar. The snows that accumulate in these regions do not remain immoveable; they have a progressive motion, which makes them tend towards the valley. Like the lakes which feed a river, and whose waters begin to flow slowly, as soon as the influence of the declivity is felt, these fields of snow can slide over ground but very slightly inclined. In proportion as this snow descends into the more temperate regions, it undergoes, particularly in the warm season, important modifications, which completely change its nature and aspect: it is * The height of the snow fallen at Grimsel, at 1880 metres above the sea level, was 16 metres 6 decemetres, from the month of November 1845 to April 1846. The sheet of water arising from the melting of this snow would be 1 metre 4 decemetres in thickness. changed into ice. This transformation takes place in the ollowing manner. Under the heat of the sun's rays, the surface of the snow begins to melt; the water thus produced infiltrates into the lower beds, which are changed, under the influence of the night frosts, into a granular mass, composed of small flakes still in a state of disaggregation, but more adherent than the flakes that produced them. This state of the snow has been named by the Swiss Natural Philosopher névé. Throughout the whole summer, this névé becomes infiltrated with additional quantities of water, always proceeding from the superficial melting, or from that of the neighbouring snows, whose waters have united in the depression which forms the cradle of the glacier. In these regions, the thermometer falling every night below zero, even in the middle of summer, the névé is frozen by many successive congelations. In consequence of these successive meltings and congelations, it presents the appearance of a white compact ice, but filled with an infinity of small spherical or spheroidal bubbles of air; this is the vescicular ice (glace bulleuse) of the authors who have written on this subject. The infiltration and congelation of the mass becoming more and more perfect, as the glacier descends towards the inhabited regions, water at length replaces all the air-bubbles; the transformation is then complete; the ice appears homogeneous, and presents those beautiful azure tints which have excited the admiration of travellers. Such, in a few words, is the history of the formation of a glacier; in reality, it is composed, as is seen, of all the layers of snow accumulated during a long series of years, and which is converted by degrees into ice, more or less compact. If the summer heats did not set a limit to the increasing glaciers, they would go on enlarging indefinitely in length and momentum; but every summer causes a considerable decrease in the thickness of the glacial surface.* This is the phenomenon which M. Agassiz has designated by the name of ablation. At the same time, the lower extremity melts rapidly, and the glacier would diminish every year, if an in * Nearly three metres. cessant progression did not counterbalance this effect. A kind of equilibrium is thus established between the summer melting on the one hand, and the annual progression on the other. If the season be warm and dry, the fusion predominates, and the glacier recedes; if the summer be cold and rainy, the progression more than compensates the effects of the fusion, and the glacier advances. We now understand what are the influences which assign to glaciers a mean limit, around which they may oscillate without ever surpassing it. It is less easy to give a reason for certain glaciers descending into inhabited valleys, while others remain suspended on the sides of the highest mountains. These differences are owing to the size and height of the amphitheatres or cirques which feed these glaciers. The more extensive and elevated these cirques are, the more considerable will be the quantity of snow which accumulates on them, and the more also will the outshoots from the fields of snow descend into the low valleys, and regain, so to speak, the ground which had been lost by the annual melting. It is in this way that the glacier of the Bossons, whose source is in the great plateau of Mont Blanc, a vast amphitheatre, situate nearly 4000 metres above the sea, descends to 1040 metres, and advances to the midst of inhabited places, orchards, and cultivated fields. The glaciers of Aletsch, Viesch, Grindelwald, and Zermatt, are in the same state. Every year, the astonished traveller may see golden harvests by the side of the glacier of Brenva, which descends the southern face of Mont Blanc. The influence of the size and elevation of the amphitheatres even counterbalances, according to M. Desor's remark, that of exposure; and explains the surprising fact, that the longest and most important glaciers of the Bernese Alps are found on the southern declivity of the chain. We have seen that these glaciers have a progressive movement which draws them towards the plain. What are the laws which regulate this movement? The investigation of these laws has continually occupied the attention of all the natural philosophers who have devoted themselves to this subject, yet they have not hitherto deduced the cause of this advance from the whole of the singular phenomena which |
