humble Mount Sion was, as is said by M. Arnold Guyot, to whom we owe this beautiful discovery, the point of conver gence for these powerful glaciers which have so essentially modified the surface of the plain comprised between the Alps and the Jura. We shall not follow them in all their windings, for all of them present us with particulars analogous to those of the glacier of the Arve. Let us merely trace the great features of the limits of the ancient extension of these glaciers.

The glacier of the Rhone had its origin in all the lateral valleys which intersect the two parallel chains of the Valais, where the most elevated mountains in Switzerland are found, Mont Rosa, Mont Cervin, the Jungfrau, Velan, &c. This glacier filled the Valais, and spread itself in the plain comprised between the Alps and the Jura, from Fort l'Ecluse as far as the environs of Aarau. It was the principal glacier of Switzerland; it is it which has conveyed the innumerable blocks which cover the Jura to a height of 1040 metres above the sea. The other glaciers were only feeble affluents of the glacier of the Rhone, incapable of making it deviate from its direction. Thus, when the glacier of the Arve met it on the ridge of the Salèves or on the sides of the Voirons, we perceive, by the disposition of the moraines, that the glacier of the Rhone continued its progress, while that of the Arve was suddenly arrested. In like manner, a rapid river repels the feeble rivulet which brings the tribute of its waters.

Other secondary glaciers occupied the principal valleys of Switzerland. Such were the glacier of the Aar, whose last moraines crown the hills in the neighbourhood of Berne; that of the Reuss, which has covered the shores of the Lake of the Four Cantons with blocks severed from the peaks of St Gothard. That of De la Linth stops at the extremity of the Lake of Zurich, and the town is built upon its terminal moraine. Finally, that of the Rhine, less studied than the others, occupied all the basin of the Lake of Constance, and extended to the adjacent parts of Germany.

It appears, therefore, that during the period of cold which preceded the appearance of man upon the earth, Switzerland was a vast sea of ice, one of whose roots were buried in VOL. XLIII. NO. LXXXV.-JULY 1847.

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the high valleys of the Alps, while the terminal escarpment rested on the Jura. In like manner, on the southern declivity of the chain, the glaciers descended into the plains of Piedmont and Lombardy. Those of the southern side of Mont Blanc united to form the glacier of the Val d'Aoste. Its terminal moraine rose like a gigantic mound in the neighbourhood of the town of Yvrée; it is the Serra of Piedmont, The greater part of the lakes of Upper Italy owe their existence to the frontal moraines of these great glaciers; by obstructing the course of the rivers, they have forced them to extend themselves in the form of liquid sheets. Among the most evident moraines, I may mention the three concentric arcs which bound the extremity of the Lago Maggiore, near Sesto-Calende; those of the Lake Garda are not less distinctly characterised, in the environs of Desenzano and Peschiera.

6. Of the Climate of the Glacial Epoch.

When the imagination conceives all the countries which surround the Alps buried under the ice to the distance of many leagues, it trembles, so to speak, at the idea of the dreadful cold which must result from this prodigious development of the Alpine glaciers. It would seem that the climate of Siberia presents nothing sufficiently rigorous to explain the permanent existence of this mantle of ice extended over countries which now enjoy a temperate climate. It is easy to shew that these notions are exaggerated,

In fact, what we have said respecting the transformation of snow into ice by repeated meltings and congelations, must lead us to understand that there could not be glaciers with a climate of extreme severity, such as that in the north of Siberia. Spitzbergen, which realises in the highest degree the conception of a country enveloped in glaciers, since they descend in all directions even to the sea, has a mean temperature of 8 centigrade degrees below zero; that of summer, is 2°4 above it. Iceland, where the glaciers rest on the shores of the sea, but never pass beyond them, like those of Spitzbergen, presents, at different points, a mean temperature comprised between zero and +4°. We could form an idea, by means of a very simple calculation, of the climate which would bring the glaciers of Mont Blanc to the margin of the

Lake of Geneva, The mean temperature of that town is 9.56. On the surrounding mountains, the limit of perpetual snow, as we have seen, is found to be 2700 metres above the sea. The great glaciers of the valley of Chamonix descend to 1550 metres below that line. This settled, suppose that the mean temperature of Geneva was lowered 4 degrees only, and became 5°56; the decrease of the temperature with the height being one degree for 188 metres, the limit of perpetual snow would descend to 750 metres, and would be more than 1955 metres above the sea. It will be readily allowed that the glaciers of Chamonix would descend below this new limit in a proportion at least equal to that which exists between the actual limit and their lower extremity. Now, at present the foot of these glaciers is at 1150 metres above the ocean; with a climate 4 degrees colder, it would be 750 metres lower, that is to say, it would come to the level of the Swiss plain. Thus, then, the lowering of the line of eternal snow would suffice to make the glacier of the Arve descend to the neighbourhood of Geneva. But we must not forget that a glacier descends so much lower in proportion to the extent of the amphitheatre from which it proceeds; now, the glaciers, having, as a source of supply all the valleys and all the gorges elevated above 1950 metres, would descend, from that cause alone, much lower than before. Accordingly, the united action of these two causes, the lowering of the line of eternal snow, and the enlargement of the amphitheatres, causes, each of which, taken by itself, would be sufficient to explain the ancient extension of glaciers, enable us readily to comprehend how that of the Arve could formerly advance to the environs of Geneva. Let it not be forgotten, that this extension has been the work of a long series of ages, the number of which is, so to speak, declared to us by those millions of blocks which the glacier has slowly and successively carried from the foot of Mont Blanc to the margin of the Leman lake,

The climate which has favoured this prodigious development of glaciers has nothing of which we cannot form a very exact idea; it is the climate of Upsal, Stockholm, Christiania, and the northern part of America in the state of New York. Geologists who do not hesitate to raise the mean tempera

ture of cold or temperate zones from 10 to 20 degrees in order to explain the presence, in the bowels of the earth, of tropical ferns or the animals of warm countries, would with very bad grace, in my opinion, shew alarm at this alteration of the mean annual temperature, although the proposed change takes place in an opposite sense, and the thermometer descends instead of ascends. If we admit that the climate of one portion of the globe may have undergone change, it is as legitimate to suppose that it has become colder, as to maintain that it has become warmer; and to diminish by 4 degrees the mean temperature of a country, in order to explain one of the greatest revolutions of the globe, is assuredly one of the least hazardous hypotheses which a geologist may be permitted to form.

To discuss the causes which have produced this sinking of temperature,-to indicate the geological or meteorological changes which have brought on this long period of cold, appears to me an attempt altogether premature. It would be necessary, before every thing else, to draw up a map of the extension of glaciers; now, this can scarcely be said to have been attempted even for the Alps, the Vosges, and the mountains of Scotland. Ancient moraines exist in the Pyrenees, Altai, Caucasus, and Atlas; but no one has yet examined the topography of the glaciers which have carried these before them. Sweden, Norway, Denmark, Finland, and North America, were covered with great fields of ice. What of a positive nature can, therefore, be said respecting a phenomenon with whose extent we are yet unacquainted? Let us not imitate our predecessors, whose brilliant imagination rested the boldest generalisations on the frail basis of a few insulated and incomplete facts. All these hasty works are destined to perish. Science has revealed to us a new epoch in the history of our planet; a vast field opens before natural philosophers, astronomers, and naturalists. Let us not fear to turn a searching look into the depths of this remote distance, the traces of which have been preserved by the surface of the earth, but let us reject hypotheses which outstrip facts, and which a fact the most trifling in appearance overturns without mercy. By the side of the diluvian period, we

see the glacial period take its place; let us hail the appearance of this latter, for it has been revealed to us by the attentive study of well observed facts, and not by vain speculations of the fancy. Let us not renew the idle quarrels of the Neptunians and Vulcanists; impartial posterity has decided between them. They were equally wrong as impassioned partisans of an exclusive idea; each of them had reason on their side by the facts and observations they brought forward in support of their absolute theories. All existing geologists are at once Vulcanists and Neptunians; science has taken the part of water and fire. It will be the same with glaciers and currents. Both of them have played their part in times past, and it is the same at the present time. The phenomena have remained the same; but, instead of those gigantic manifestations, which is the character of the geological epochs anterior to our own, they are included within the limits of action imposed upon them by the equilibrium of the period of repose which the appearance of man has introduced to the earth.-From Revue de Deux Mondes, t. xvii. 1st March 1817.

On the Colour of the Ice of Glaciers, and that of the Waters which flow from them. By M. CH. MARTINS.

At the meeting of the 15th of March last, the Academy received a communication from M. Durocher, entitled Etudes sur les Glaciers du Nord et du centre de l'Europe. This note contains some assertions which it appears to me of importance to rectify. The author has stated (p. 144)," that the interposition of water between the pores and fissures of granulated ice contributes largely to develop in them that beautiful blue colour which is so much admired." It is long, indeed, since M. Agassiz has proved that blue ice contains more water than white ice; but the blue colour appears to me to be owing rather to the absence of air than the presence of water, for the white ice is infiltrated with water like the blue, but the white ice is wholly filled with spherical bubbles of air. In the blue ice, these are replaced by water of infiltration. M. Celestin Nicollet has proved this by direct measurement, shewing that 500 grammes of blue ice contain only 0.5 cubic centimetres of air, while the same weight of white ice contains 7.5 centimetre cubes; accordingly, when we look at a fragment of ice half blue and white, we see that the white part is filled with bubbles of air, while the blue part is almost wholly