GLACIER is a name given to immense masses of ice, which are formed above the snow-line, on lofty mountains, and descend into the valleys to a greater or less distance, often encroaching on the cultivated regions. The materials of the glaciers are derived from the snow which falls during summer as well as winter on the summits of high mountains. Every fresh fall of snow adds a little to the height of the mountain, and, were there no agents at work to get rid of it, the mountains would be gradually rising to an indefinite elevation. Avalanches and glaciers, however, carry the snow into warmer regions, where it is reduced to water; in the one, the snow slips from the steep mountain slopes, and rushes rapidly down; in the other, it gradually descends, and is converted into ice in its progress. The snow which forms the glacier at its origin has a very different appearance and consistence from the ice of which it consists at its lower termination. The minute state of division of the ice, in its snow condition, and the quantity of air interspersed through it, gives it its characteristic white color. Two.
'causes operate in causing this change into ice; first, pressure expels the air, by bringing the particles of the lower layers of snow more closely together; and second, the sum Iner's heat, melting the surface, the water thus obtained percolates through the mass beneath, and as it passes amongst the particles whose temperature is below 32° F., it increases their size by external additions till the particles meet, and the whole becomes .a solid mass. The snowy region of the glacier is called by the French name nae. In large glaciers, the neve is of great extent, a large quantity of material being required to make up the waste. The neve is, however, often confined to narrow valleys, and, as a consequence, produces glaciers which soon perish. The increase of a glacier by snow falling on its surface takes place only above the snow-line—below that line, all the seen -ululated winter's snows are speedily melted by the summer heat. The ice of the glacier sel dom exhibits any traces of the horizontal stratification which is found in the neve, but is generally intersected with vertical veins of clear blue ice.
The most remarkable feature of glaciers is their motion. It has been long known to the natives of the Alps that they move, but it is only within the last few years that it has received due attention from scientific men; the account of their observations, and the theories based upon them, form one of the most interesting chapters in the history •of glaciers. See the writings of Agassiz, Forbes, and Tyndall. The continual waste .of glaciers below the snow-line, both along its surface and at its extremity, is ever repaired, so that the glacier does not recede from the valley, nor decrease in depth. That the materials of the reparation are not derived from the fall of the win ter's snow and the influence of the winter's frost, is evident, inasmuch as these addi tions speedily disappear with the return of the summer's heat, and in the end form but small proportion of the year's total loss. The true repairing agent is the motion of the glacier, which brings down the glaeified snow from the upper regions to be melted below. To account for this motion, Charpantier supposed the water which saturated the glacier in all its parts, and filled the innumerable capillary fissures, was, during night and during the winter, frozen, and that the well known and almost irresistible expansion which would take place in the conversion of the water into ice, furnished the force necessary to move the glacier forwards. This theory, known as the dilatation theory, was for some time adopted by Agassiz, but ultimately abandoned. Agassiz showed that the interior of the glacier had a temperature of 32° F., and subsequent observations have shown that the glacier moves more rapidly in summer than in winter. In 1799, De Saussure published a second theory, known as the gravitation or sliding theory, in which he supposed that the glacier moved by sliding down the inclined plane on which it rested, and that it was kept from adhering to its bed, and sometimes even elevated by the water melted in the contact of the glacier with the naturally warmer earth. While correctly attributing the motion to gravity, De Saussure erred in consid ering glaciers as continuous and more or less rigid solids—indeed, the motion he attrib utes to them would, if commenced, be accelerated by gravity, and dash the glacier from its bed as an avalanche. Principal Forbes was the author of the next important theory. Considerable attention had in the meantime been paid to the subject by Rendu, Agassiz, and others. Rendu had shown that the glacier possessed a semi-fluid or river-like motion, in explaining the difference between observations made by him at the center, which " moves more rapidly," and others made at the sides, " where the ice is retained by the friction against its rocky walls." The results based on Rendu's observations were established by the repeated and exact measurements of Forbes, who, in the progress of his examinations, made the further discoveries, that the surface moves more rapidly than the ice near the bottom, and the middle than the sides; that the rate of motion is greater where the glacier-bed has the greatest inclination; and that the motion is continued in winter, while it is accelerated in summer by the increase of the temperature of the air. The only theory which, as it appeared to Forbes, could
account for these phenomena is thus expressed by him: " 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." This is known as the viscous theory. He considered a glacier as not a crystalline solid, like ice tranquilly frozen in a mold, but that it pos sessed a peculiar fissured and laminated structure, through which water entered into its intrinsic composition, giving it a viscid consistence, similar to that possessed by treacle, honey, or tar, but differing in degree. Prof. Tyndall has published another theory, which he designates the pressure theory. This differs little from that of Forbes, except that it denies that glacier ice is in the least viscid. By a number of independent observations, be established the facts first noticed by Rendu and Forbes, and added the important one, that the place of greatest motion is not in the center of the glacier, but in a curve more deeply sinuous than the valley itself, crossing the axis of the glacier at each point of contrary flexure—in fact that its motion is similar to that of a river whose point of maximum motion is not central, hut deviates towards that side of the valley towards which the river turns its convex boundary. This seems a further corrobora tion of the viscous theory, but Tyndall explained it and the other facts by a theory which, while maintaining the quasi-fluid motion of the glacier, denied that this motion was owing to its being in a viscous condition. The germ of his theory, as he tells us, was derived from some observations and experiments of Faraday's in 1850, who showed " that when two, pieces of ice, with moistened surfaces, were placed in contact, they became cemented together by the freezing of the film of water between them, while, when the ice was below 32° F., and therefore dry, no effect of this kind could be pro duced. The freezing was also found to take place under water." By a further series of experiments, Tyndall found that ice at 32° F. could be compressed into any form, and that no matter how great the bruising of its particles and the change of its shape, it would, from this property of regelation, re-establish its continuous solid condition, if the particles of ice operated on were kept in close contact. These facts he applied to the motion of glaciers, asserting that the pressure of the parts of a glacier on each other, in a downward direction, produced by gravitation, was more powerful than the attraction which held the particles of the ice together—that, consequently, the ice was ruptured, to permit the motion of the glacier, the particles being, however, speedily reunited by regelation. The supposed viscous condition of ice he believed to be refuted by the fact that, whenever the glacier is subjected to tension, as in passing over a cas cade, it does not yield by stretching, but always by breaking, so as to form crevasses. This theory, equally with that of Forbes, explains the known phenomena of glaciers, while the advantage is claimed for it of not drawing upon our imagination as to a required condition of the ice, but, by experiment, exhibiting ice from known causes producing effects on the small scale similar to those produced it nature on the large. Forbes, however, maintained (Occasional Papers, etc., 1859) that all that is peculiar to Tyndall's theory was included in his own; and that the facts discovered and expounded by Faraday in 1850 had already been used by him as part of his theory in 1846. He said that his viscous theory included the notion " of an infinity of minute rents; that it also embraces the substitution of the finite sliding of the internally bruised surfaces over -one another;" and that it includes the " reconsolidation of the bruised glacial substance into a coherent whole by pressure acting upon ice, softened by imminent thaw." Prof. Tyndall re-introduces and re-asserts the gravitation theory of De Saussure as in part the cause of the glacier's motion; but the phenomena which be considers pro duced by a sliding motion of the whole mass over its bed—viz., the polishing and groov ing of the rock below—can be produced by a substance whose motion is the result of a yielding of its parts, if that substance has sufficient consistence to retain firmly imbedded in its lower surface portions -of rock to act as polishers, and; it cannot be doubted that the ice of glaciers has such a consistency.