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This is probably an exaggeration, Hugi rarely met with any thicker than 150 feet; he estimates the average depih at between 60 and 100 feet, and the greatest thickness of the Mer de glace near Chamouny at 180 feet, Saussure had calculated it at 600 feet.
Notwithstanding their great extent and solidity, the glaciers are by no means stationary, even in the winier. Although the movement is slight, they do not remain quite still. They are undergoing a perpetual process of renovation and destruction. The arms or skirts descending into the lower valleys are gradually dissolved by the increased temperature which prevails at so low a level. The sunimer sun, aided by particular winds, acts upon the surface, so that, in the middle of the day, it abounds in pools, and is traversed by rills of water. The constant evaporation from every part exposed to the air produces great diminution in the upper beds ; but, above all, the temperature of the earth, which is at all seasons greater than that of ice, is constantly melting away its lower surface. The vacancy thus caused from below is partially or entirely filled up from above by the winter's snow falling upon the mountain-tops, and on the whole upper region, which is drifted into the higher valleys, and pressed down by its own weight. After it has concreted into ice, the slope of the mountain-sides, and the descent of the valleys in which the glaciers lie, serve as inclined planes, down which the ice slides by the force of gravity, assisted by the melting on its under surface, which prevents any adhesion to the rock below it. Indeed the German word Gletscher comes from glitschen, to. glide. Hugi, in one of his journeys, found his way under a glacier, by following the bed of a dried-up torrent which passed below it. He wandered about beneath the ice for the distance of a mile. The ice was everywhere eaten away into dome-shaped bollows, varying from 2 to 12 feet in height, so that the whole mass of the glacier rested at intervals on pillars or feet of ice, irregular in size and shape, which had been left standing. As soon as any of these props gave way a portion of the glacier would of course fall in and move on. A dim twilight prevailed in these caverns of ice, not sufficient to allow one to read, except close to the fissures which admitted the day-light from above. The intense blue of the mass of the ice contrasted remarkably with the pure white of the icy stalactytes, or pendants descending from the roof. The water streamed down upon him from all sides, so that after wandering about for 2 hours, at times bending and creeping to get along under the low vaults, he returned to the open air, quite Jrenched and half frozen.
The nature of the upper surface of the ice depends upon that of the ground on which it rests; where it is even or nearly so, the ice is smooth and level; but whenever the supporting surface becomes slanting or uneven, the glacier begins 10. split and gape in all directions. As it approaches a steeper declivity or precipice the layers of ice are displaced, up-heaved, and squeezed one above another; they rise in toppling crags, obelisks, and lowers of the most fantastic shapes, varying in height from 20 to 80 feet. Being unequally melted by the wind and sun, they are continually tottering to their fall, either by their own weight or the pressure of other masses, and tumbling headlong, are shivered to atoms with a roar like thunder.
The glaciers assume this fractured character only when the foundation on which they rest is. very uneven, generally near their lower extremity, when they begin to bend down towards the valley.
The crevices, or fissures, which traverse the upper portion of the glacier, before it becomes entirely fractured and disruptured, run in a transverse direction, never extending quite across the ice-field, but narrowing out at the extremities, so thai when they gape too wide. to leap across they may generally be turned by following them to their termination. These rents and fissures are the chief source of danger to those who cross the glaciers, being often concealed by a treacherous coating of snow, and many a bold chamois hunter has found a grave in their recesses. Ebel mentions an instance of a shepherd who , in driving his flock over the ice to a high pasturage. had the misfortune to tumble into one of these clefts. He fell in the vicinity of a torrent which flowed under the glacier, and, by following its bed under the vault of ice, succeeded in reaching the foot of the glacier with a broken arm. More melancholy was the fate of M. Mouron, a clergyman, of Grindelwald : he was engaged in making some scientific researches upon the glacier, and was in the act of leaning over to examine a singular wellshaped aperture in the ice, when the staff, on whích he rested , gave way; he was precipitated to the bottom, and his lifeless and mangled body was recovered from the depths of the glacier a few days after.
These crevices, though chiefly formed mechanically by the movement of the glacier to fill up vacancies, and the unequal pressure of different parts, are greatly assisted by the action of the sun and wind. The S.E. wind, in Uri and among the Bernese Alps, is very instrumental in causing the
glacier to split, and the loud reports thus occasioned, called by the herdsmen the growlings (brullen) of the glacier, are regarded as a sign of bad weather. The traveller who ventures to cross the Mer de Glace of Chamouny or Bern may, at times, both hear and see the fissures widening around him. The crevices exhibit in perfection the beautiful azure blue colour of the glacier; the cause of which has not been satisfactorily accounted for. It is the same tint of ultramarine which the Rhone exhibits at Geneva, after leaving all its impurities behind it in the lake; and the writer has even observed the same beautiful tint in footmarks and holes made in fresh-fallen snow, not more than a foot deep, among the high Alps on the borders of Tyrol.
The traveller who has only read of glaciers is often disappointed at the first sight of them, by the appearance of their surface, which, except when covered with fresh-fallen snow, or at very great heights, has none of the purity which might be expected from fields of ice. On the contrary it exhibits a surface of dirty white, soiled with mud and often covered with stones and gravel. Such beds of dirt and rubbish are common to most glaciers, and are called, in German, Guffer. They are supposed to be formed in the following manner: —the edge of the glacier receives the masses of stone and sand falling from the mountains above, produced by the disintegrations of moisture and frost. During the summer heat the glacier shrinks away from the rocks that bound it, and carries away the rubbish lying upon it. The intervening space between the foot of the mountains and the ice is filled up by the snow of winter, which is gradually changed into ice, and receives a fresh heap of gravel from above. This again is carried forward by the shrinking of the glacier. Thus these
lines of loose stones are constantly advancing, one behind another, like waves; and where the glacier from one valley joins that out of another, the heaps are often confounded and intermixed.
A singular circumstance occurs when a boulder, or large mass of rock, has fallen upon the glacier; the shade and protection from the sun's rays
afforded by the stone prevents the ice on which it rests from melting, and, while the surface around is gradually diminished, it remains supported on a pedestal or table, often attaining a height of several feet. When a leaf, insect, or such light body falls upon the ice, it gradually sinks, and at length disappears.
Another circumstance peculiar to the surface of the snow-field or upper glacier (firn) is the occurrence of Red Snow. This phenomenon, which at one time was treated with incredulity, is of common occurrence among the high Alps, and is produced by a species of fungus, called Palmella Nivalis, or Protococcus, a true vegetable, which plants itself on the surface of the snow, takes root, germinates, produces seed, and dies. In the state of germination it imparts a pale carmine tint to the snow; this increases, as the plant comes to maturity, to a deep crimson blush, which gradually fades, and, as the plant decays, becomes a black dust or mould. By collecting some of the coloured snow in a bottle, and pouring it on a sheet of paper, the form of the plant may be discovered with a microscope, as soon as the water has evaporated. Increase and Diminution, Advance and Retreat of
the Glaciers, It has been already observed that the vacancy caused by the melting of the lower portion of the