Below the névé, the glacier commonly finds its way, amid various depressions of different forms, to the lower ground, far beneath the line which marks the usually constant presence of snow throughout the year. The accompanying view (fig. 84, p. 212) of Mont Blanc, taken from the Bréven, a mountain rising high above the valley of Chamonix, which separates the Bréven from the Mont Blanc, will give a better idea of the passage of the glaciers downwards into the lower valley, than a verbal description, and more especially as the altitude and position of the Bréven itself prevents that foreshortening and less instructive view obtained from beneath.

As the great icy mass descends from the region of the névé to the lower ground, the crevasses vary much in length and breadth, sometimes extending across the whole glacier,* and this, as might be expected, according to the character of the surface on which it may repose. As it descends into warmer regions, the glacier is necessarily exposed to the influence of higher temperature, and if it did not obtain the needful supply from above, it would there diminish in bulk and disappear. As this supply varies, the extension of a glacier will correspond with the kind of seasons experienced, so that it may descend further into the lower valleys at one time than another;† and thus its actual amount of protrusion into a valley may depend, for the time, upon effects produced through many seasons, and be liable to frequent change.

accompanying his remarks by ideal sections of glaciers and real sections of viscous bodies experimented upon for illustration. He observes, "that this ribboned structure follows a very peculiar course in the interior of the ice, of which the general type is the appearance of a succession of oval waves on the surface, passing into hyperbolas, with the greater axis directed along the glacier. That this structure is also developed throughout the thickness of a glacier, as well as from the centre to the side, and that the structural surfaces are twisted round in such a manner, that the frontal dip, as we have called it, of the veins, as exhibited on a vertical plane cutting the axis of a glacier, occurs at a small angle at its lower extremity, and increases rapidly as we advance towards the origin of the glacier" (p. 372).

* In the account of his passage over the Col de Géant, Professor Forbes mentions an immense chasm or crevasse, extending wholly across a glacier in the descent on the Chamonix side, and at least 500 feet in width. "It terminated opposite to the precipices of the Aiguille Noire in one vast enfoncement of ice, bounded on the hither side by precipices not less terrible." Travels through the Alps &c., p. 238.

Among the numerous examples of the varied extension and volume of glaciers known in modern times, there would appear none more illustrative than that of the Brenva, on the Italian side of Mont Blanc. In 1818 it attained a height different from that found by Professor Forbes, in 1842, of at least 300 feet, as proved by that of a rock, upon which a well-known chapel (Chapelle de Berrier) was placed, which, with the rock on which it stood, was heaved and fissured by the rise of the ice. This great increase of volume, and its decrease in the 24 years, is well attested. The Professor remarks, that the mean temperature for the five years preceding 1818, when the glacier was thus of such increased volume, presented no marked change, the mean temperature at Geneva being for that time 70-61, Reaumur (490-12 Fahr.); the mean for the last 40 years, in the same town, being 7075 (490.44 Fahr.)-Travels, &c., p. 205.

The turbid waters, rushing out from beneath the glaciers of the Alps will be familiar to all who have visited those mountains, as also the caverns of ice through which these waters commonly find their way when they are most abundant. With respect to such waters, Professor Forbes has pointed out that they may not only be due to the ice melted by contact with the rocks on which it moves, to the fall of rain upon the ice drained by the glacier valley, in the season when rain falls, and to the waste of the glacier itself by the sun and rain, but also to the natural springs rising from beneath the ice, as in any other locality.*

With respect to the cause producing the motion of glaciers, different views have been taken :† Professor James Forbes considers it as "convincingly proved, that the motion of a glacier varies not only from one season to another, but that it has definite (though continuous) changes of motion, simultaneous throughout the whole, or a great part of its extent, and therefore due to some general external change," and that "this change has been shown to be principally or solely the effect of the temperature of the air, and the conditions of wetness or dryness of the ice."§ With regard to the movement itself, the Professor has pointed out "that the ice does not move as a solid body,—that it does not slide down with uniformity in different parts of its section,-that the sides, which might be imagined to be most completely detached from their rocky walls during summer, move slowest, and are, as it were,

* With respect to the waste of the glacier by the sun and rain, Professor Forbes remarks, that it is "a most important item, and which constitutes the main volume of most glacier streams, except in the depth of winter. It is on this account that the Rhine and other great rivers, derived from Alpine sources, have their greatest floods in July, and not in spring or autumn, as would be the case if they were alimented by rain-water only. On the same account, the mountain torrents may be seen to swell visibly, and roar more loudly, as the hotter part of the day advances, to diminish towards evening, and in the morning to be smallest." "Winter is a long night amongst the glaciers. The sun's rays have scarcely power to melt a little of the snowy coating which defends the proper surface of the ice; the superficial waste is next to nothing, and the glacier torrent is reduced to its narrowest dimensions."Travels through the Alps of Savoy, &c., 2nd edit., pp. 20, 21.

The chief of these views will be found in the works of MM. de Saussure, De Charpentier, Agassiz, Élie de Beaumont, Mr. William Hopkins, and of Professor James Forbes. In the latter they will be seen discussed in much detail, and the Professor's own views advocated, especially in his Travels through the Alps of Savoy, &c., in chap. xxi. (entitled "An attempt to explain the leading Phenomena of Glaciers"), and in his papers entitled "Illustrations of the Viscous Theory," published in the Philosophical Transactions for 1846. A very detailed account of the works and views respecting glaciers will also be found in the Histoire des Progrès de la Géologie, de 1834-1845, by the Vicomte d'Archiac, Paris, 1847.

Travels through the Alps of Savoy, &c., and alluding to chap. vii. § Ib., chap. xxi., p. 363.

dragged down by the central parts." * His theory is, that "a glacier is an imperfect fluid, or a viscous body, which is urged down slopes of a certain inclination by the mutual pressure of its parts." He does not, however, "doubt that glaciers slide on their beds, as well as that the particles of ice rub over one another, and change their mutual positions." ‡

The movement of glaciers is important as regards the transport of mineral substances, inasmuch as by it they bear onwards upon their surfaces any fragments of rock that may fall upon them from the heights amid which they pass; thrust before them any loose accumulations of blocks, gravel, sand, and earth which may oppose their course, and even break off portions of rocks where the resistance of the latter is less than the force of the glacier, divisional planes, such as joints and cleavage, with the natural bedding of rocks, often rendering the mass of such rocks less resistant than it would otherwise be. The observer, accustomed often to see the steep cliffs of many a mountain region covered towards their bases

* Travels through the Alps of Savoy, &c. chap. xxi. p. 363. As to the different rates of motion of a glacier, it is observed that a glacier. "like a stream, has its still pools and its rapids. Where it is embayed by rocks it accumulates-its declivity diminishes, and its velocity, at the same time. When it passes down a steep, or issues by a narrow outlet, its velocity increases. The central velocities of lower, middle, and higher regions of the Mer de Glace are-1.398, 574, and 925; and if we divide the length of the glacier into three parts, we shall find these numbers for its declivity, 15°, 41°, and 8°.-Forbes' Travels, &c., p. 371.

Ib., p. 365. It would be somewhat out of place in this work to enter more fully into the theory of glacier movements. Respecting the theory of the viscous condition of a glacier, Professor Forbes alludes to its spreading as a viscous body would do when a glacier passes out of a narrow gorge into a wide valley, stating that this fact had been first brought prominently forward by M. Rendu, now Bishop of Annecy (Travels, p. 367.) M. Rendu (Théorie des Glaciers de la Savoie, Chambery, 1840) divides glaciers into glaciers réservoirs and glaciers d'écoulement, the former in the high regions, and the latter descending into the lower valleys. He estimates the height of the separation between the two in Savoy at 2,923 metres (9,590 English fect). He points to the accumulation of snow in the higher regions, the rain, when it falls there, freezing, and to the feeding of the lower glaciers by the descent of this snow and ice.

"But," he adds, "I maintain that the former motion is caused by the latter, and that the motion impressed by gravity upon the superficial and central parts of a glacier (especially near its lower end), enables them to pull the lateral and inferior parts along with them. One proof, if I mistake not, of such an action is, that a deep current of water flows under a smaller declivity than a shallow one of the same fluid. And this consideration derives no slight confirmation, in its application to glaciers, from a circumstance mentioned by M. Élie de Beaumont, which is so true that one wonders that it has not been more insisted on-namely, that a glacier, where it descends into a valley, is like a body, pulled asunder or stretched, and not a body forced on by superior pressure alone" (p. 370). In a note to this passage, the Professor remarks, "that a state of universal distension, or a state of universal compression, is equally incompatible with the existing phenomena of most glaciers, and that compression in some parts and distension in others are plainly indicated by their natural features."

by the débris detached by atmospheric influences from above, and especially in climates or regions where frosts and thaws often alternate, would readily expect these fragments to move onwards with the glacier on the edges of which they may fall.* Instead, therefore, of accumulating in a talus of débris, as can be well studied in many mountainous regions, the mass of fragments moves slowly onwards, and the protection from atmospheric influences afforded by this talus to the solid rocks beneath it (in some mountain countries collectively very considerable), is removed precisely in localities where the vicissitudes of climate are often so great that it can be the least spared.

The blocks and smaller fragments (necessarily very variable in form and volume, according to the character of the overhanging rocks, and the amount of their decomposition anterior to their fall), thus strewed upon the glaciers, are well known as moraines.† These moraines also necessarily differ in general volume, according to the amount of matter which may be detached from the heights above the glacier, and the rate of movement of the glacier itself on the sides adjoining the sources of the detached fragments. Two lines of moraine will mark the edges of a glacier, should the heights on either side of it afford the needful supply, as also a mass of rock rising through a glacier, should it also afford fragments, as has been pointed out by Professor Forbes. When two or more streams of glaciers unite, each bearing its two, or even as we have seen, three lines of rock fragments, the union will so dispose of the lines as to form a less number for the remaining course of the glacier, as in fig. 85, where the glaciers coming down the valleys A and B, and uniting the four moraines, two on the sides of each glacier, become three (a), (b), (c), by the union of the lateral moraines 2 and 3 into a central moraine (6). Various other unions, easily imagined, are produced, as minor contribute to main glaciers. A great central moraine may be established by the junction of two long lines of glacier sides, unbroken for a considerable distance, and upon which

The débris on mountain sides often completely masks their character as left anterior to such coverings. There are few mountainous regions which do not show this when carefully examined. Mining operations often prove it on the sides of hills. Ravines, where ravines may not be uncommon, are usually favourable for observations of this kind; as, for example, many instances are found in Derbyshire, where the faces of steep cliffs are often modified in this manner, the long-continued action of atmospheric influences having smoothed off many a precipitous hill side, where the same effects may be seen in daily progress. This action has greatly modified the face of most countries, especially when combined with landslips.

+ This name has become common with us from the works of De Saussure and others writing in French. Guffer is the German term for them.