Snow is constantly accumulating in the recasts or depths of the mountains, dur ing nine months of the year, by the usual fall of moisture, and the descent of vast masses borne down by their weight from the precipices and-crags above. Part of this is necessarily reduced to water by slight thaws and casual rains, and being frozen irahis state, the glacier is com posed of a po'rous opaque ice.
The upper glaciers, Mr. Coxe subdi vides into those which cover the sum mits, and those which extend along the sides of the Alps ; the former originate from the snow frequently falling, and con gealing into a firm body, though not be coming actual ice, which the brilliancy of the projections has induced some phi losophers to suppose it. M. de Saussure, having,,txplored Mont Blanc, ascertained that the top was encrusted with ice, which might be penetrated by a stick, covering a mass of snow on the declivi ties, so chilled and dry as to be incapable of coherence.
The sides of the Alps support a conge lation of half dissolved snow, which is different from the pure snow of the sum mits and the ice of the lower glaciers. Two causes operate to produce this ef fect ; the first is the descent of water from the higher regions, where a dissolu tion of the snow sometimes occurs ; and the second arises from the more favoura ble situatian of these parts for reflecting the rays of the sun, and the consequent melting of the snow. From hence down wards the ice adhering to the cavities be ores gradually more solid by the fr zing of the snow-water, then nearly divested of that air, which in the less dis solved portions renders the ice, formed from it, porous, opaque,and full of bubbles.
Considerable difference of opinion has prevailed amongst philosophers, whether the masses of ice and snow in these regions of endless winter increase, de crease, or remain nearly stationary ; Mr. Coxe seems inclined to think they vary in their size ; that gentleman observes, that the glacier of Montanvert is generally bordered with trees ; near the base of this vast body of frozen matter the ice is ex cavated into an arch,perhaps one hundred feet in height,whence the Arveron rushes with impetuosity and in a large sheet of water. As he approached the ice he passed through a forest of firs, those near the arch were very ancient and about eighty feet high, the trees between them and the glacier were evidently younger, from the inferiority of their size and other intrinsic marks ; others, still less, had been enveloped by the ice, and many were thrown down ; arguing from this gradation in the appearance of the firs, he concludes that the glacier has origi nally extended to the full grown ancient trees, and, dissolving, !young ones have grown on its' former site, which have been overturned by a fresh increase of ice.
This inference seems almost conclusive, but it is still further supported by the fall of large pieces of granite, called moraine by the inhabitants, which, borne along by the ice,sinks through it as it dissolves, and, falling into the plain, form a border along its extremity ; those have been urged forward by the pressure of new ice, and extend even to the place occupied by the large firs.
Exclusive of these circumstances, Mr. Coxe discovered, that the glacier of Grin delwald had diminished, at least, 400 paces between the dates of his two visits in 1776 and 1785 ; and in the valley ,of Chamouny, the Muraille de Glace, which he had described as forming the border of the glacier of Bosson, in 1776, no long er existed in 1785, and young trees had grown on the site of the edge of the glacier of Montanvert.
In,opposition to the evidence thus ad duced, it is argued that the operations ob servable in the vallies, arising from the concentration of solar heat, form no data for judging of those on more elevated situations, where, it is asserted, a greater quantity of snow falls and becomes ice than can possibly be dissolved annually ; and experience proves, beyond doubt, that mountains have been covered, pas sages obstructed, pastures and habita tions destroyed by the ice, within the me mory of man. In replying to these argil, ments, the result obtained is extremely satisfactory. The rain and sleet falling during summer not only thaws the ice and snow, but forms various channels in it, the water descending must wear and carry along part of the frozen sides and depths, and prepare the way for separat.