Agassiz discovered that the central portion moved faster than the marginal, and he was the first to correct the erroneous views into which he had been led by others on this point, from the fact of the great cracks generally lying in curved lines with the convexity directed up the course of the glacier. (Systeme glaciaire, by Agassiz, Guyot, and Desor, p. 462.) The upward convexity of the fissures is accounted for by the fact that, if the central portion moves fastest, the lines of greatest tension are downward and toward the middle, and the ice gives way at right angles to these lines. Forbes, by careful instrumental observations in 1842, detected the rate of movement in periods of 24 hours, and was able even to notice that which took place in an hour and a half. He proved the faster rate of the central portions, and also that the portions of the glacier near the surface moved faster than those near the bottom. The motion he found was greatest on the slopes of greatest descent; in warm weather more rapid than in cold; yet always continuous, and not exhibited in the manner of jerks.

Such facts are opposed to the theory of I)e Saussure, that the glaciers move by'slipping along upon their bed, the motion being made more easy by the buoyant property of the water flowing beneath them, and the propelling force being that of gravitation. Moreover, the ice, without being broken up, was observed not to be interrupted in its movement by the contracted passages through which it was sometimes forced to pass, nor by solid hills of rock, which lay like islands in its path. The theory maintained by Charpentier, and supported by Agassiz in his Etudes sur les glaciers, was that the glacier slid upon its bed, not necessarily in large bodies pushed on by gravitation, but that different portions were impelled by different degrees of force, arising from the expansion of the water congealing in all the fissures and capillary tubes of the ice into which it found its way. The facts developed by Forbes-that the motion was greatest in the warm summer weather, when the temperature did not descend below the freezing point, and that it did not cease when the ice was no longer liquefied in the cold of winter-demanded some new explanation With the other phenomena they were regarded by him as sufficient to establish the fact that ice in large bodies is not a brittle solid, but that it possesses, particularly when saturated with water, so much plasticity, that with time it can yield to a stupendous and steadily exerted force, and move somewhat like a body of viscous pitch or lava, which, while it appears brittle when suddenly struck, can yet mould itself in the mass to the surface upon which it rests.

By this theory, which was generally received even by those who first opposed it, all the diffi-culties attending the explanation of the movement disappear. It was confirmed by a simple experiment made by Mr. Christie, secretary to the royal society. He filled with water a 10-inch hollow shell of iron, the shell itself being besides 1 1/2 in. thick, and exposed this to severe cold. As the water expanded in freezing, a cylinder of ice was pushed up through the fuse hole, and it continued to increase in length as the water continued to freeze. As the outer portions of the water must have been first converted into ice, it is plain that it was this so-called solid, material which was forced through the narrow aperture and made to assume the form of a cylinder of its diameter. But the peculiar nature of this quality of mobility belonging to ice has been more perfectly explained, together with some of the other phenomena of glaciers, by the researches of Tyndall and Huxley, an account of which is published in the "Philosophical Magazine," vol. xv. (4th series), 1858. The property of particles of ice when exposed to higher temperatures than the freezing point to adhere, and. under pressure to unite in one mass, was observed by Faraday, and was afterward made the subject of various experiments by Tyndall and Huxley. They found that compact transparent ice might be crushed to fragments, and these be made by a hydraulic press to assume in a few seconds the shape of any mould, recovering in their new form perfect solidity and transparency.

A straight bar of ice was bent into a semicircular form by using a succession of four moulds of gradually increasing curvature. As the prism conformed itself to these, cracks were produced, and crackling sounds were emitted, reminding one of those which are so often noticed among the phenomena of glaciers. By reference to this before unobserved property of ice the movement and unbroken continuity of glaciers and their branches are now explained.-The glaciers from their very source present a series of changes of structure, which have been critically observed and traced, and in some instances illustrated and explained by experiments on a small scale with other materials. The snowy region known by the French term neve is formed of dry and granular snow, which extends for miles, sometimes broken by immense chasms, and at others presenting no irregularities of surface such as are common to the glacier below, no streams, crevices, moraines, or cones. The snow lies in strata, which reach to great depth, each representing the accumulation of a single year, the lowest the most dense and approaching the blue color of ice. These bodies move onward to form the glacier proper; and as they pass into this, their material assumes more and more the character of compact ice.

But a remarkable and peculiar feature is the veined or laminated structure, real or only apparent, which it assumes. This is noticed in the walls of the fissures, and is also displayed upon the surface of the glacier itself, when this has been wasted by rain. Thin lamina) of transparent blue ice alternate with others of white porous ice, and standing together in a vertical position the edges of the former project a little above those of the latter, which more readily melt, and thus a ribbed appearance is produced. The direction of the laminae is across the fissures, and as observed by Tyndall and Huxley these are produced at right angles to the direction of greatest tension. They find an analogy between the lamination of the ice and the slaty cleavage of the clays and slates, both which they refer to pressure causing the development of divisional planes in lines approximately at right angles to the direction of pressure. Hence the obliquity of the lamination to the sides of the glacier as the lines extend from the margin toward the middle and down its course; and the deviation directly across the glacier, or at right angles with this and parallel with its axis, as the form of its bed or other causes produce a pressure in the one case exerted longitudinally and in the other laterally with the line of the glacier.

By submitting plastic materials, as wax, to pressure, and observing the laminated structure these assumed, these investigators were led to this explanation of the phenomenon as developed in glacier ice; but others, as Prof. Forbes, describe the white ice as produced merely by lines of cavities or of air bubbles in the blue ice itself, the result, according to the observation of Prof. James Thompson, of partial liquefaction induced by pressure; and Prof. William Thomson attempts to prove that the first effect of pressure not equal in all directions on a mass of snow ought to be, according to the theory, to convert it into a stratified mass of layers of alternately clear and vesicular ice, perpendicular to the direction of maximum pressure." But the complete explanation of this structure will require experimental researches upon ice which have not vet been made.-Another interesting feature in the appearance of glaciers, to which attention was first directed by Forbes, is the distribution of what he called the dirt bands, discolored streaks seen upon the surface, which he supposed were connected with the veined structure, appearing where this is more energetically developed than elsewhere, and caused by the collection of sand and dirt in the decomposed portions of the softer laminae.

These are arranged in curves, the convexity of which is turned down the glacier, and are frequently so obscure that they are distinguished only by looking down upon them from some elevation. Tyndall and Huxley describe them as spread out upon the smooth ice below ice cascades, and caused a similar sym-metrical arrangement of dark-colored sand distributed upon the surface of a current of fine mud, which they made to flow from a reservoir down an inclined trough, through a narrow channel, which spread out below over a widened area.-Various other phenomena connected with the structure and motion of glaciers are discussed in the treatises on this subject already referred to. Besides the works mentioned, see "Norway and its Glaciers" (1853), and "On the Theory of Glaciers" (1859), by James D. Forbes; Die Gletscher der Jetztzeit, by A. Mousson (1854); Neue Unter-suchungen uber die physikalische Geographie und die Geologie der Alpen, by the brothers Schlagintweit (1854);The Glaciers of the Alps," by John Tyndall (1860);The Old Glaciers of North Wales and Switzerland," by A. C. Ramsay (1860); and "The Land of Desolation," by Dr. Isaac I. Hayes (1871).