GLACIER 829 advance, the glacier recedes to its former bounds, the surface it covered is found to be changed into a dismal waste of loose stones. The gathering and distribution of these mate- rials by action of glaciers have been subjects of special interest, from the resemblance in most of the phenomena exhibited to those con- nected with the distribution of the geologi- cal formation known as the drift. The loose rocks are worn into the rounded forms of bowl- ders, and are similarly striated and grooved upon their surface, and sometimes polished. The rocks upon and against which the glaciers have pressed are found, wherever exposed to view, to be ground smooth and deeply marked with lines corresponding in direction with the course of the glacier at the spot. It is upon these resemblances, and others connected with minor details of the two classes of pheno- mena, that the glacial theory of Venetz and Charpentier, so fully elaborated by Agassiz, is based, accounting for the distribution of geo- logical formations like the drift. The trans- porting power of glaciers was recognized by Prof. Playfair of Edinburgh as far back as the year 1816, and the occurrence of the enormous bowlders on the Jura was attributed by him to glaciers, whose track he supposed lay at one time across the valley of Switzerland and the lake of Geneva, which now separate the Jura from the opposite summits of Mont Blanc. It is on these summits, at the distance of from 70 to 80 m., that are found the ledges of granite and other rocks, which are recognized as iden- tical with the great bowlders scattered over the surface of the Jura limestone. (See DI- LUVIUM.) The quantity of stony material, and the enormous size of the masses of rock carried along by glaciers, are little appreciated, even by many who have seen the loads apparently resting quietly on their surface. Sometimes the ice is almost concealed by the accumulated piles of stone. These do not sink into the ice, except as they occasionally fall into the chasms, and even then they are sometimes brought again to the surface by the action of the forces which keep most of them there. As the rock protects the ice beneath it from the action of the sun, which has its melting effect around, the rock is thus gradually lifted upon a pedes- tal of ice, at the same time that the whole is slowly moving down to a lower level. When the pedestal at last gives way, the rock slips down and the process is repeated. When once in the ice, the superficial melting may bring it again to the surface. The size of the frag- ments is often immense. Prof. Forbes saw one in the valley which must have been brought down by the glacier, which was nearly 100 ft. long, and from* 40 to 50 high ; and at the .foot of the glacier of Swartzburg in the valley of Saas was another estimated to contain 244,000 cubic feet, requiring an average diameter of nearly 62 ft. The rate of progress of glaciers, dependent upon various conditions, is no more uniform than that of rivers. It can in no case be correctly estimated except by observations extending over many years. On the glacier of Aar M. Hugi erected a hut in 1827 at the foot of a fixed and well known rock. In 1836 the hut was 2,200 ft. from the rock, and in 1840 this distance had doubled. In the first period its progress had been 250 ft. per annum, and in the second 550. Forbes in 1842 found the remains of a ladder, which, it is believed, was the one left by De Saussure in 1788 at a point 16,500 ft. further up the glacier ; if so, its yearly progress had been 375 ft. This move- ment extends through valleys in which the surface of the glacier appears to lie almost on a dead level. It is made manifest day by day by a row of stakes set up in a straight line across the glacier, and ranging with fixed points on the land at the sides. These are after a time observed to stand upon a semicircular line, the stakes near the middle moving faster than those near the margin. The importance of cor- rectly estimating the rate of movement at short intervals and in different parts of a glacier, in order to determine the nature of the motion, appears to have been first appreciated by Agassiz in 1841, and by Forbes, who was en- gaged about the same time in his explorations. 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 down- ward and toward the middle, and the ice gives way at right angles to these lines. Forbes, by careful instrumental observations in 1842, de- tected 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 sur- face 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 De 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 inter- rupted in its movement by the contracted pas- sages 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
