1846.] HOW BLOCKS ARRIVE ON THE SURFACE OF glaciers. 203 stones are actually extruded, although in a peculiar manner, from pure ice, or at least exposed by the ablation of the surface. In my Travels, p. 241, referring to the excessively gradual development of the moraine of La Noire on the Mer de Glace of Chamouni, I stated my belief, that this moraine was buried in a fold between two glaciers, one of which had overflowed the other, and that, as the upper glacier decayed away, the rocky fragments were strewed on the surface. A fresh examination of the same localities leaves me in the same want of direct proof of this fact, but the difficulty of explaining it otherwise, makes me suppose my former view correct. The extruded stones on the Glacier du Nant Blanc, near Chamouni (alluded to in the first part of this letter), present a very remarkable appearance, imperfectly shown in Plate VIII. fig. 6. The right bank of this glacier (that which appears on the left of the figure as seen from a distance) is at first bounded by rocky summits, but in the lower part of its course by a mound-like moraine of the usual form. The surface-blocks can only be derived from the precipices near the origin. Yet they do not even appear on the surface opposite to the rocks, but only opposite to the moraine; and they increase in number and quantity towards the lower end of the glacier, where they almost blacken the surface of the right side, the left side remaining almost clean. It is difficult to believe that this accumulation is not due to the gradual denudation of the blocks by the melting of the ice in which they have been, in some way or other, imbedded; but it is scarcely less difficult to admit that, having fallen from the rocks above the Névé, they should have remained unperceived in the ice during all the intermediate space. To take another example. The glacier of the Rhone is distinguished by the extraordinary purity of its surface, and the consequent absence of lateral moraines. But this general freedom from stones on the surface is subject to one exception, which is remarkable :-Stones begin to appear at the surface on the terminal slope at a considerable height. How came they there? Not a stone the size of the fist can be seen on the sur face farther up; and, in examining a number of the crevasses, I could not see any engorged in the ice. The explanation seems to be, that these stones are actually introduced into the ice by friction at the bottom of the glacier, and forced upwards by the action of the frontal resistance which produces the frontal dip of the veined structure, and they are finally dispersed on the surface by the melting of the ice. What is here supposed to occur is illustrated by an ideal section of the glacier of the Rhone, Plate VIII. fig. 7, where the curves of ejection are identical with those of forced separation, causing the frontal dip of the veined structure; and this view is confirmed by what I have often observed, particularly on the Glacier of Bossons, that the veined structure in contact with the lateral moraines becomes soiled, and that dirt and stones may be traced along the course of the structural bands from the moraine to a considerable depth in the ice. The action there is in the horizontal plane, what we here suppose to take place in the vertical, and which the now established retardation of the lower strata permits us to assume as exactly a similar action. I have no doubt that a similar explanation applies to the glacier of the Nant Blanc, and to other glaciers.* Before closing this already too long letter, I wish to record an observation already made by me in 1844, but which I hesitated to publish because the sketch representing it was made from memory, and not upon the spot; but I have now verified it both in the same and another locality. It is represented in Plate VIII. fig. 5, where b d is part of the wall of the glacier which is about to turn at a considerable angle with its former direction (it is at the well-known part of the Mer de Glace named L'Angle). I knew, from long experience, that the ice here pre * It will be seen that this explanation will give an elevatory force to the ice containing blocks similar to that which De Charpentier (Essai sur les Glaciers, 25) ascribed to the expansion of the frozen water. It will also be seen that it renders a perfect account of the "veins of the debris of rocks" in glaciers, particularly near their lower extremities, which that ingenious author has attempted to account for (unsatisfactorily, I think) by the transporting action of streams of water. -(Ibid, 27.) 1846.] THREE ORDERS OF DISCONTINUITY IN GLACIERS. 205 sents, year after year, compact ridges, such as a b, c d, parallel to one another, and separated by a mass of crevasses which it is, generally speaking, impossible to cross; and any one who attempts to traverse the more crevassed portions of the glacier without attending to this peculiarity, will infallibly lose his way. But, as the drawing explains, the direction of these ridges by no means corresponds with that of the individual crevasses, whose grouping subdivides the glacier as I have now described. The crevasses may be nearly transverse to the glacier, whilst the systems of crevasses form an angle of perhaps 30° with the transverse line. The veined structure again cuts the crevasses at right angles, so that these may be regarded as three orders of discontinuity, or tearing surfaces, which occur in systems, that is, regularly repeated at nearly uniform intervals over great portions of the glacier surface. I have this year succeeded, for the first time, in laying down on a map an approximation to the various and complex systems of crevasses which traverse the Mer de Glace, and I have found a repetition of this phenomenon. of a series of discontinuous but parallel fissures ranged along a line or axis oblique to their direction, to recur at several points where the strain is very violent. Let it be remarked, too, that where the violence of the pressure opens a system of such fissures to relieve it, the bands, or system of surfaces of molecular discontinuity, disappear, or are less well developed. remain, etc. 12th December 1846. I XVII. FOURTEENTH LETTER ON GLACIERS. On the Variation of the Motion at different Seasons. Observations on the Glacier of the Aar, considered and compared with the Author's results. My Dear Sir-I am led to request you to reprint in the Edinburgh Philosophical Journal, part of the 9th section of my paper on the Viscous Theory of Glacier Motion from the Philo sophical Transactions for 1846,* for the following reason:There has lately appeared in the Bibliothèque Universelle de Genève, a paper by M. Collomb, in which it is maintained, on the authority of the experiments of M. Dollfuss on the Glacier of the Aar, that the velocity of progress of that glacier is the same at all seasons and in all weathers; and that previous observers who arrived at different conclusions on that glacier were in error. It may be so. The Glacier of Aar is, as I have elsewhere shewn,† in very peculiar conditions of mechanical constraint; but it is important that it should be shewn by the testimony of by far the completest series of observations which has yet been published, and which extends over the entire year, that this independence of the velocity upon external circumstances is not at least the general rule. The passage in M. Collomb's paper, to which I have alluded, may be thus translated :--" It results from the examination of the registers of M. Dollfuss, which contain several thousand observations, that the Glacier of the Lower Aar moves with perfect regularity;-no sudden starts nor pauses; the movement of any particular point taken on the surface of the glacier, whether in the centre or at the edges, is very slow, uniform, and independent of atmospheric influences. (He goes on to say, that, as it is the fastest in the centre of the glacier, and almost nothing at the edge.) Fine weather or rain, dryness of the air or humidity, cold or heat, day or night, or different seasons, have no influence upon the velocity of any particular point of the surface of the glacier. "The observers of previous years believed, on the other hand, that the general march of great glaciers was in intimate connection with the accompanying state of the atmosphere. They believed, for instance, that the movement was retarded in dry and cold weather, and accelerated by moisture and rain. * [Already printed at page 125, etc.] + Ninth Letter on Glaciers, Ed. Phil. Jour., vol. xxxviii., p. 332. [See above, p. 68.] I may add, that M. Martins gives a similar account of the results of Mr. Dollfuss' experiments in the Comptes Rendus, 26th October 1846. 1847.] OBSERVATIONS ON THE AAR GLACIER. 207 The observations of M. Dollfuss, which will, without doubt, be published one day in detail, have proved the contrary, and have shewn that a glacier, like a semifluid body, urged by a mechanical force, moves on without being influenced by the state of the surrounding medium."* Now, Sir, the last passage, the italics of which are in the original, conveys to us, in the first place, the pleasing information that M. Collomb, and probably M. Dollfuss also, have accepted the viscous theory of glacier motion. The word "semifluid" applied to a glacier, is now, notwithstanding its seeming harshness, an adopted word. But I must enter an earnest protest against the supposed discovery of the uniformity of the motion and its entire independence of atmospheric circumstances, being assumed to add any probability to the viscous theory, as the phraseology of the preceding extract seems to infer. the contrary, if there be any glacier which does not present the law of variable velocity, which I established for the first time, on the Mer de Glace of Chamouni in 1842, and which has since been found on the Glacier of Bossons, on the Glacier of Grindelwald, and was supposed to have been found on the Lower Glacier of the Aar, such a glacier affords a proof the less in favour of viscous or semifluid theory. On It is on this account, Sir, that I desire that the readers of the scientific journals may be made fully aware of the amount of the evidence by which, in some glaciers at least, the direct connection between the movements of the glacier and the conditions of temperature and moisture have been established; and it is for this object that I crave a few pages of your Journal, for an extract from a more elaborate and less accessible paper. Before quitting the subject, I wish to add, that I concur with M. Collomb in desiring the full publication of M. Dollfuss' results on the Glacier of the Aar, which, latterly at least, * Bibliothèque Universelle publiée 15 Decembre 1846. p. 212, note. The passage in italics stands thus in the original,—“ Un glacier comme un corps semifluide poussé par une force mécanique, marche en avant sans se laisser influencer par l'état du milieu ambiant.” |