which alone the more regular crevasses generally exist to any great extent; and it therefore follows that, in those localities, the surfaces of structure must be perpendicular, at every point, to the directions of greatest pressure. We can also prove this law to hold in cases where regular crevasses are either non-existent or comparatively very rare, as at the bottoms of ice-falls and along the axial portions of a glacier. In the first case, there must necessarily be an enormous longitudinal pressure from the accumulation of ice à tergo; and in the latter case, the axis of the glacier (as finely illustrated on the Aar glacier) is often indicated by a great central moraine, formed by the junction of two great tributaries. In such instances, the ice of the two tributary streams is forced into the same bed, and must usually produce an enormous transverse pressure in the united glacier. In the first case here cited, the structural curves are directly transverse, and in the second they are entirely longitudinal, and are consequently in both cases perpendicular to the directions of greatest pressure. All other observed cases lead to the same inference. Some time ago Dr. Tyndall made certain experiments, which, together with others made by Mr. Sorby, led him to suppose that the cleavage structure in rocks was due to the great pressure to which they had been subjected, the planes of cleavage being perpendicular to the directions of maximum pressure. This suggested to him the idea that the veined structure might also be due, in like manner, to pressure. The analogy between the two cases is manifest; but as the theory of rock-cleavage is uncertain, that of the veined structure, so far as it rests on this analogy, must, à fortiori, be so likewise. Dr. Tyndall has also made experiments on the liquefaction of ice by pressure, which afford an additional presumptive proof in favour of the theory above mentioned. We must refer the reader to the Glaciers of the Alps' (p. 408) for an account of these ingenious experiments. Though we may not yet regard this phenomenon of the veined structure as unequivocally accounted for by the analogy and experiments here spoken of, it seems not improbable that they may lead in the path towards the right solution. Principal Forbes also, as is well known, put forward, many years ago, his theory of the veined structure. He conceived that, as different parts of the glacier move faster or slower than the adjoining parts, two contiguous particles moving along adjoining parallel lines must generally be moving with different velocities; and thus, if in contact at any proposed instant, the one having the greater velocity would slide past the other, and in time get separated from it. Thus, suppose the velocity of every particle in a vertical plane parallel to each side of a regular regular canal-shaped glacier to move with the same velocity, and suppose the axial parts of the mass to move the fastest; then will every particle in one of these planes tend to slide past the neighbouring particle in one of the adjoining planes; and thus there will be a tendency to make the whole of one of these planes slide on the surface of the adjoining one, and thus also to break the cohesion between them. It was in this presumed bruising and rupturing along these parallel planes that the author of this theory considered the veined structure to originate. His first idea was that a greater facility was thus afforded for the infiltration of water between those bruised laminæ, and that this infiltrating water became frozen by the winter cold, and formed the more compact and transparent ice of the lamina. A real physical cause was thus assigned for the veins, but it was entirely inconsistent with the internal temperature of the glacier, into which, as above explained, the winter cold does not penetrate many feet. The idea was afterwards abandoned, but I am not aware that the author substituted for it any other physical cause. The veins appear to have been attributed only to the bruising of the mass, as above described, and therefore to a mechanical rather than to any determinate physical cause. Principal Forbes did not determine the positions of the planes or surfaces of the veins, as above, by the simple consideration of the relative motions of contiguous particles, which, in a canalshaped glacier, would give the marginal lines of structure necessarily parallel to the sides,-a direction from which they are often observed to deviate very considerably. His explanation was, that a drag towards the centre of the glacier, in consequence of its more rapid motion there, caused an oblique motion of the marginal particles. This explanation was founded on a demonstrable mechanical error; and the Ripple Theory, by which he attempted to explain his conclusions, has now been proved to be entirely fallacious. Again, it has been stated that under the great central moraine of the Aar glacier the veined structure is very finely developed where there can be no difference of motion in adjoining particles. It is also impossible, in our opinion, to give any real explanation of the positions of the surfaces of structure near the foot of an ice-fall, consistent with this theory. If the surfaces of structure be considered as due to the actual difference of motion of contiguous particles, the problem becomes only a geometrical one, and we conceive it to have been shown demonstrably that the positions of the veins or surfaces of *See References in second footnote, p. 106. structure structure could not coincide in that case with their observed positions.* It is impossible, we think, to accept this theory if Principal Forbes's 'differential motion' of two contiguous par ticles means the actual difference between their instantaneous motions; and yet, if it do not mean this actual difference, it is inconceivable to us what intelligible meaning can be assigned to it. Priority in the observation of this phenomenon of the veined structure, immediately after Principal Forbes had remarked it in 1841, was made a subject of controversy. M. Agassiz stated himself to have previously observed it; but in his 'Système Glaciaire' (p. 208) he claims for M. Guyot the credit of having first distinctly noted this structure in 1838 on the Glacier du Gries. In support of this claim he gives a quotation from a communication made by that observer to the Swiss naturalists at Bâle in the year just mentioned, and which is now placed in the archives of the Society of the Natural Sciences at Neuchâtel. The quotation is too long for insertion here, but we may cite the following passage from it as in itself conclusive. M. Guyot says that, being on the Glacier du Gries, 'Je vis sous mes pas la surface du glacier entièrement couverte de sillons réguliers de 1 ou 2 pouces de largeur, creusés dans une masse à demi-neigeuse, séparés par des lames saillantes, d'une glace plus dure et plus transparente. Il était évident que la masse du glacier étoit ici composée de deux sortes de glace, l'une, celle des sillons, encore neigeuse et plus fusible, l'autre, celle des lames, plus parfaite, cristalline, vitreuse, et plus résistante, et que c'était à l'inégale résistance qu'elles présentaient à l'action de l'atmosphère qu'était dû le creux des sillons et la saillée des lames plus dures.' It was at once admitted, we believe, by Principal Forbes himself and all other glacialists, that the evidence in favour of M. Guyot's priority of discovery was established. Principal Forbes's claims, as regards these phenomena, do not rest on the precedency due to his observations, but on his recognition of the importance of this peculiar and curious structure as a general character of glacial ice. The difficulty of explaining the adequacy of the forces acting on a glacier to enable it to overcome the numerous and apparently insurmountable obstacles to its motion, has always been one which has been more or less experienced by most glacialists. A prevailing idea has been that the lower portions of a glacier are crushed simply by the weight of the superincumbent mass-that the cohesion of those portions is thus destroyed and the mass Memoir in the Transactions of the Royal Society,' 1862, p. 725. pushed And pushed outwards, where it meets with the fewest obstacles. yet the Peak of Teneriffe, for example, does not crush its basal strata into atoms, and thrust out its own foundations. If it were possible that the weight of that mountain could be suddenly superimposed on terrestrial rocks which had been solidified under a comparatively small pressure, it seems probable that those rocks would be thus crushed into powder, if sufficiently brittle; but Nature does not work in this manner. She educates the back, as it were, to prepare it for the load it has to bear, by the slow and gradual superposition of the superimposed weight. And similarly if a stratum of ice, frozen under the mere pressure of the atmosphere, could be placed under the weight of a glacier at a temperature below 32 (Fahr.), it would be instantly crushed into powder, and its cohesive power so far destroyed as to make it capable of being thrust outwards on a horizontal plane by a comparatively small vertical force. But if the temperature should be exactly 32 (Fahr.), as in the lower parts of a glacier, the structure and cohesion of the crushed ice would be immediately restored by regelation, and it would be, at least, an apparent contradiction to suppose that the ice would be again crushed by the pressure under which it had just before been regeled and consolidated. We doubt whether any mass of ice producing a pressure within the limit of regelation (if there be such a limit) could squeeze out its lower portions on a horizontal plane, so as to produce any continuous motion like that of a glacier. It is the resolved part of the force of gravity parallel to the bed of the glacial valley (always inclined to the horizon) which we conceive to be the force really effective in urging onwards every part of the glacier. Principal Forbes appears to have been impressed with the difficulty of assigning an adequate cause for the crushing effects which he supposed to be produced in the interior of a glacier, and by which the cohesion was destroyed and its motion facilitated, as if it were viscous. He says that a considerable quantity of water is constantly percolating through the minute fissures of the mass, or held by them in capillary suspension, and that this water exercises a tremendous hydrostatic pressure' to push onwards the whole mass in the direction of least resistance.* Now, we feel ourselves bound to assert that this conclusion is founded on an entire misconception of the mechanical action of this internal water. Admitting the existence of the capillary fissures, filled with water throughout the glacier, what would be the con Occasional Papers,' p. 165. See also a Memoir in the Transactions of the Royal Society for 1846,' Part III. sequence sequence of the supposed enormous hydrostatic pressure in these minute internal tubes and fissures? The answer is obvious: the water would rush forth from every crevice on the exterior surface of the mass. No such hydrostatic pressure, therefore, can exist. In fact, if the water be held at rest in capillary crevices, it will transmit no hydrostatic pressure whatever, but will simply, by its own weight, increase the weight of the mass. Again, if the water flow through these minute fissures with a steady motion (and such its motion must be very approximately), it will produce no hydrostatic pressure at all. The truth of both these assertions may be strictly proved, and is, in fact, sufficiently obvious to any one familiar with such investigations. We have here an example of the incautious appeals which have been made to mechanical principles in the solution of certain glacial problems. It is to the small adhesion of the lower surface of the glacier to its bed, that the enormous power of the internal forces to crush and dislocate the general mass is due. The smallness of this adhesion in a glacier presents a case similar to that of a long beam in a horizontal position, supported principally by forces acting at its two extremities. The more exclusively this force is thrown on these extremities, the more likely will be the beam to break by its own weight. And thus will the glacier be the more likely to be dislocated when the principal forces opposing its motion act along its flanks, while the axial portions are comparatively little impeded by the small friction on the lower surface of the mass. The subjects we have been discussing involve a degree of complexity which may render it desirable, for the clearer comprehension of them, that we should give a brief summary of the contributions which different glacialists have made since the time of De Saussure, to our knowledge of glacial facts and glacial theories. We have already spoken of Rendu's Memoir, and of the claim which it establishes for him of having been the first to recognise clearly and distinctly the pliability of a glacier, and that it moved, speaking generally, as if ice were a viscous substance, and in a manner resembling that in which the water of a river moves. Guyot's claim to having been the first to observe and to describe clearly the veined structure, we conceive to be unequivocally established. Agassiz has probably done more than any other man to diffuse a general interest in glacial subjects throughout the scientific world. He was enabled to accomplish * Memoir 'On the Theory of Glaciers,' Transactions of the Royal Society.' Read May 22, 1862. |