From these causes, the glaciers will progressively de scend to the plains below ; and the rapidity of their motion will depend upon the inclination of the bed on which they rest, and on the magnitude and velocity of the reinforce ments which they receive from fresh avalanches. The stream which flows from the extremity of the glacier, forms in general a vault of ice above, which gradually widens as the ice melts, and when it can no longer sustain the super incumbent mass, it is crushed by its own weight, and gives place to the masses behind it. This progressive motion of the glaciers, is rendered visible by the variation in the po sition of large stones on the surface, or of trees frozen in the ice. In the glacier of Chamouni, the progressive mo tion has been observed to be 14 feet in the year ; and on those of Grindelwald, 130 feet in six years, or 25 feet in the year.
It has been a question keenly agitated among naturalists, whether the glaciers are in a state of increase or diminu tion, and each party has succeeded in proving the truth of his opinion. We may necessarily infer, therefore, that they sometimes increase and sometimes diminish. The lower extremity which projects into the plain, sometimes con tinues to diminish for a series of years, as the quantity which is annually dissolved is not replaced by the superjacent masses. At other times, when they are copiously suppli ed by h esti avalanches, they advance more rapidly than they dissolve, and therefore encroach upon the cultivated pf.tins. Their augmentation commonly takes place in spring, and when they have made great inroads upon the lower ground, they are generally foundto diminish for some years afterwards.
We have already had occasion to mention both the masses of granite rock, which lie upon the surface of the glacier, and the heaps of enormous stones, called the Mo raine or Marren, which are accumulated at its lower ex tremity. These stones sometimes are totally different front those in the valley where they now lie, and must have been detached from rocks often six or eight leagues distant. These stones are frequently accumulated in separate mounds like hillocks or graves, and arranged in parallel lines of a considerable height and width. These are gene
rally called Gouffrelyncs, and appear on a great scale in the glacier of Rosboden on the Simplon. Sometimes a large regular pyramid of ice is seen, with a huge stone up on its summit. The heaps of stones which we have men tioned, contain in general specimens of the rocks in the higher regions of the mountains.
It has been already stated, that the glaciers are compos ed of different kinds of ice. Some of it is granulated and imperfectly frozen, other parts have a transparent green co lour, as in the chasms and crevices, while that which is near the heaps of gravel, is of a bluish black colour. The only exception to the generality of this remark occurs in the glacier of Rosboden, the whole of which consists of ice, hard, firm, and compact, and of a blackish blue colour.
The vaults of ice are always formed at the exit of the little stream which runs below the glacier. In winter, all these openings are closed up by ice, but the heat of spring speedily dissolves it, and vaults, sometimes 100 feet long, and 50 or 60 wide, are formed. The figure and magnitude of these suffer constant changes. That of the Aveyron, once so much admired, is no longer in existence.
In the extensive alpine chain from Montblanc to the bor ders of the Tyrol, there are no fewer than 400 glaciers, the greater part of which are six or seven leagues long, by one half or three quarters of a league wide, and from 100 to 600 feet thick. A very few of these are so small as a league in length. M. Ebel has calculated, as nearly as can be done, their general extent, and has found that those be tween Switzerland and Montblanc, and on the frontiers of the Tyrol, would form a single glacier of 130 square leagues.
For farther information on this subject, we must refer the reader to the article ALPs, where he will find an ac count of the glacier of Furca, the glacier of the Aar, and the glaciers of Grindelwald and Lauterbrunnen. See also Grouner's Histoire Xaturelle des Glaciers de Suisse. trans lated by Keralio ; Saussure Voyages dans les ?11Jes ; Ebel's Manuel du Voyageur en Suisse ; Lambert's Voyage Pit toresque en Suisse.