book I

have briefly referred to the force of crystallization. To permit this force to exercise its full influence, it must have free and unimpeded action; a crystal, for instance, to be properly built, ought to be suspended in the middle of the crystallizing solution, so that the little architects can work all round it; or if placed upon the bottom of a vessel, it ought to be frequently turned, so that all its facets may be successively subjected to the building process. In this way crystals can be _nursed_ to an enormous size. But where other forces mingle with that of crystallization, this harmony of

## action is destroyed; the figures, for example, that we see upon a glass

window, on a frosty morning, are due to an action compounded of the pure crystalline force and the cohesion of the liquid to the window-pane. A more regular effect is obtained when the freezing particles are suspended in still air, and here they build themselves into those wonderful figures which Dr. Scoresby has observed in the Polar Regions, Mr. Glaisher at Greenwich, and I myself on the summit of Monte Rosa and elsewhere.

Not only however in air, but in water also, figures of great beauty are sometimes formed. Harrison's excellent machine for the production of artificial ice is, I suppose, now well known; the freezing being effected by carrying brine, which had been cooled by the evaporation of ether, round a series of flat tin vessels containing water. The latter gradually freezes, and, on watching those vessels while the action was proceeding very slowly, I have seen little six-rayed stars of thin ice forming, and rising to the surface of the liquid. I believe the fact was never before observed, but it would be interesting to follow it up, and to develop experimentally this most interesting case of crystallization.

[Sidenote: DISSECTION OF ICE BY SUNBEAM.]

The surface of a freezing lake presents to the eye of the observer nothing which could lead him to suppose that a similar molecular architecture is going on there. Still the particles are undoubtedly related to each other in this way; they are arranged together on this starry type. And not only is this the case at the surface, but the largest blocks of ice which reach us from Norway and the Wenham Lake are wholly built up in this way. We can reveal the internal constitution of these masses by a reverse process to that which formed them; we can send an agent into the interior of a mass of ice which shall take down the atoms which the crystallizing forces had set up. This agent is a solar beam; with which it first occurred to me to make this simple experiment in the autumn of 1857. I placed a large converging lens in the sunbeams passing through a room, and observed the place where the rays were brought to a focus behind the lens; then shading the lens, I placed a clear cube of ice so that the point of convergence of the rays might fall within it. On removing the screen from the lens, a cone of sunlight went through the cube, and along the course of the cone the ice became studded with lustrous spots, evidently formed by the beam, as if minute reflectors had been suddenly established within the mass, from which the light flashed when it met them. On examining the cube afterwards I found that each of these spots was surrounded by a liquid flower of six petals; such flowers were distributed in hundreds through the ice, being usually clear and detached from each other, but sometimes crowded together into liquid bouquets, through which, however, the six-starred element could be plainly traced. At first the edges of the leaves were unbroken curves, but when the flowers expanded under a long-continued

## action, the edges became serrated. When the ice was held at a suitable

angle to the solar beams, these liquid blossoms, with their central spots shining more intensely than burnished silver, presented an exhibition of beauty not easily described. I have given a sketch of their appearance in Fig. 34.

[Sidenote: LIQUID FLOWERS IN ICE.]

[Illustration: Fig. 34. Liquid Flowers in lake ice.]

I have here to direct attention to an extremely curious fact. On sending the sunbeam through the transparent ice, I often noticed that the appearance of the lustrous spots was accompanied by an audible clink, as if the ice were ruptured inwardly. But there is no ground for assuming such rupture, and on the closest examination no flaw is exhibited by the ice. What then can be the cause of the noise? I believe the following considerations will answer the question:--

Water always holds a quantity of air in solution, the diffusion of which through the liquid, as proved by M. Donny, has an immense effect in weakening the cohesion of its particles; recent experiments of my own show that this is also the case in an eminent degree with many volatile liquids. M. Donny has proved that, if water be thoroughly purged of its air, a long glass tube filled with this liquid may be inverted, while the tenacity with which the water clings to the tube, and with which its

## particles cling to each other, is so great that it will remain securely

suspended, though no external hindrance be offered to its descent. Owing to the same cause, water deprived of its air will not boil at 212 deg. Fahr., and may be raised to a temperature of nearly 300 deg. without boiling; but when this occurs the particles break their cohesion suddenly, and ebullition is converted into explosion.

Now, when ice is formed, every trace of the air which the water contained is squeezed out of it; the particles in crystallizing reject all extraneous matter, so that in ice we have a substance quite free from the air, which is never absent in the case of water; it therefore follows that if we could preserve the water derived from the melting of ice from contact with the atmosphere, we should have a liquid eminently calculated to show the effects described by M. Donny. Mr. Faraday has proved by actual experiment that this is the case.

[Sidenote: WATER DEPRIVED OF AIR SNAPS ASUNDER.]

Let us apply these facts to the explanation of the clink heard in my experiments. On sending a sunbeam through ice, liquid cavities are suddenly formed at various points within the mass, and these cavities are completely cut off from atmospheric contact. But the water formed by the melting ice is less in volume than the ice which produces it; the water of a cavity is not able to fill it, hence a vacuous space must be formed in the cell. I have no doubt that, for a time, the strong cohesion between the walls of the cell and the drop within it augments the volume of the latter a little, so as to compel it to fill the cell; but as the quantity of liquid becomes greater the shrinking force augments, until finally the particles snap asunder like a broken spring. At the same moment a lustrous spot appears, which is a vacuum, and simultaneously with the appearance of this vacuum the clink was always heard. Multitudes of such little explosions must be heard upon a glacier when the strong summer sun shines upon it, the aggregate of which must, I think, contribute to produce the "crepitation" noticed by M. Agassiz, and to which I have already referred.

[Sidenote: FIGURES IN ICE; VACUOUS SPOTS.]

In Plate VI. of the Atlas which accompanies the 'Systeme Glaciaire' of M. Agassiz, I notice drawings of figures like those I have described, which he has observed in glacier-ice, and which were doubtless produced by direct solar radiation. I have often myself observed figures of exquisite beauty formed in the ice on the surface of glacier-pools by the morning sun. In some cases the spaces between the leaves of the liquid flowers melt partially away, and leave the central spot surrounded by a crimped border; sometimes these spaces wholly disappear, and the entire space bounded by the lines drawn from point to point of the leaves becomes liquid, thus forming perfect hexagons. The crimped borders exhibit different degrees of serration, from the full leaves themselves to a gentle undulating line, which latter sometimes merges into a perfect circle. In the ice of glaciers, I have seen the internal liquefaction ramify itself like sprigs of myrtle; in the same ice, and

## particularly towards the extremities of the glacier, disks innumerable

are also formed, consisting of flat round liquid spaces, a bright spot being usually associated with each. These spots have been hitherto mistaken for air-bubbles; but both they and the lustrous disks at the centres of the flowers are vacuous. I proved them to be so by plunging the ice containing them into hot water, and watching what occurred when the walls of the cells were dissolved, and a liquid connexion established between them and the atmosphere. In all cases they totally collapsed, and no trace of air rose to the surface of the warm water.

No matter in what direction a solar beam is sent through lake-ice, the liquid flowers are all formed parallel to the surface of freezing. The beam may be sent parallel, perpendicular, or oblique to this surface; the flowers are always formed in the same planes. Every line perpendicular to the surface of a frozen lake is in fact an axis of symmetry, round which the molecules so arrange themselves, that, when taken down by the delicate fingers of the sunbeam, the six-leaved liquid flowers are the result.

In the ice of glaciers we have no definite planes of freezing. It is first snow, which has been disturbed by winds while falling, and whirled and tossed about by the same agency after it has fallen, being often melted, saturated with its own water, and refrozen: it is cast in shattered fragments down cascades, and reconsolidated by pressure at the bottom. In ice so formed and subjected to such mutations, definite planes of freezing are, of course, out of the question.

[Sidenote: CONSTITUTION OF GLACIER-ICE.]

The flat round disks and vacuous spots to which I have referred come here to our aid, and furnish us with an entirely new means of analysing the internal constitution of a glacier. When we examine a mass of glacier-ice which contains these disks, we find them lying in all imaginable planes; not confusedly, however--closer examination shows us that the disks are arranged in groups, the members of each group being parallel to a common plane, but the parallelism ceases when different groups are compared. The effect is exactly what would be observed, supposing ordinary lake-ice to be broken up, shaken together, and the confused fragments regelated to a compact continuous mass. In such a jumble the original planes of freezing would lie in various directions; but no matter how compact or how transparent ice thus constituted might appear, a solar beam would at once reveal its internal constitution by developing the flowers parallel to the planes of freezing of the respective fragments. A sunbeam sent through glacier-ice always reveals the flowers in the planes of the disks, so that the latter alone at once informs us of its crystalline constitution.

[Sidenote: VACUOUS CELLS MISTAKEN FOR AIR-CELLS.]

Hitherto, as I have said, these disks have been mistaken for bubbles containing air, and their flattening has been ascribed to the pressure to which they have been subjected. M. Agassiz thus refers to them:--"The air-bubbles undergo no less curious modifications. In the neighbourhood of the _neve_, where they are most numerous, those which one sees on the surface are all spherical or ovoid, but by degrees they begin to be flattened, and near the end of the glacier there are some that are so flat _that they might be taken for fissures when seen in profile_. The drawing represents a piece of ice detached from the gallery of infiltration. All the bubbles are greatly flattened. But what is most extraordinary is, that, far from being uniform, _the flattening is different in each fragment_; so that the bubbles, according to the face which they offer, appear either very broad or very thin." This description of glacier-ice is correct: it agrees with the statements of all other observers. But there are two assumptions in the description which must henceforth be given up; first, the bubbles seen like fissures in profile are not air-bubbles at all, but vacuous spots, which the very constitution of ice renders a necessary concomitant of its inward melting; secondly, the assumption that the bubbles have been _flattened_ by pressure must be abandoned; for they are found, and may be developed at will, in lake-ice on which no pressure has been exerted.

[Sidenote: CELLS OF AIR AND WATER.]

But these remarks dispose only of a certain class of cells contained in glacier-ice. Besides the liquid disks and vacuous spots, there are innumerable true bubbles entangled in the mass. These have also been observed and described by M. Agassiz; and Mr. Huxley has also given us an accurate account of them. M. Agassiz frequently found air and water associated in the same cell. Mr. Huxley found no exception to the rule: in each case the bubble of air was enclosed in a cell which was also

## partially filled with water. He supposes that the water may be that of

the originally-melted snow which has been carried down from the _neve_ unfrozen. This hypothesis is worthy of a great deal more consideration than I have had time to give to it, and I state it here in the hope that it will be duly examined.

My own experience of these associated air and water cells is derived almost exclusively from lake-ice, in which I have often observed them in considerable numbers. In examining whether the liquid contents had ever been frozen or not, I was guided by the following considerations. If the air be that originally entangled in the solid, it will have the ordinary atmospheric density at least; but if it be due to the melting of the walls of the cell, then the water so formed being only eight-ninths of that of the ice which produced it, _the air of the bubble must be rarefied_. I suppose I have made a hundred different experiments upon these bubbles to determine whether the air was rarefied or not, and in every case found it so. Ice containing the bubbles was immersed in warm water, and always, when the rigid envelope surrounding a bubble was melted away, the air suddenly collapsed to a fraction of its original dimensions. I think I may safely affirm that, in some cases, the collapse reduced the bubbles to the thousandth part of their original volume. From these experiments I should undoubtedly infer, that in lake-ice at least, the liquid of the cells is produced by the melting of the ice surrounding the bubbles of air.

But I have not subjected the bubbles of glacier-ice to the same searching examination. I have tried whether the insertion of a pin would produce the collapse of the bubbles, but it did not appear to do so. I also made a few experiments at Rosenlaui, with warm water, but the result was not satisfactory. That ice melts internally at the surfaces of the bubbles is, I think, rendered certain by my experiments, but whether the water-cells of glacier-ice are entirely due to such melting, subsequent observers will no doubt determine.

[Sidenote: "LIQUID LIBERTY."]

I have found these composite bubbles at all parts of glaciers; in the ice of the moraines, over which a protective covering had been thrown; in the ice of sand-cones, after the removal of the superincumbent debris; also in ice taken from the roofs of caverns formed in the glacier, and which the direct sunlight could hardly by any possibility attain. That ice should liquefy at the surface of a cavity is, I think, in conformity with all we know concerning the physical nature of heat. Regarding it as a motion of the particles, it is easy to see that this motion is less restrained at the surface of a cavity than in the solid itself, where the oscillation of each atom is controlled by the

## particles which surround it; hence _liquid liberty_, if I may use the

term, is first attained at the surface. Indeed I have proved by experiment that ice may be melted internally by heat which has been conducted through its external portions without melting them. These facts are the exact complements of those of "regelation;" for here, two moist surfaces of ice being brought into close contact, their liquid liberty is destroyed and the surfaces freeze together.

THE MOULINS.

(25.)

[Sidenote: MOULIN OF GRINDELWALD GLACIER.]

[Sidenote: DEPTH OF THE SHAFT.]

The first time I had an opportunity of seeing these remarkable glacier-chimneys was in the summer of 1856, upon the lower glacier of Grindelwald. Mr. Huxley was my companion at the time, and on crossing the so-called Eismeer we heard a sound resembling the rumble of distant thunder, which proceeded from a perpendicular shaft formed in the ice, and into which a resounding cataract discharged itself. The tube in fact resembled a vast organ-pipe, whose thunder-notes were awakened by the concussion of the falling water, instead of by the gentle flow of a current of air. Beside the shaft our guide hewed steps, on which we stood in succession, and looked into the tremendous hole. Near the first shaft was a second and smaller one, the significance of which I did not then understand; it was not more than 20 feet deep, but seemed filled with a liquid of exquisite blue, the colour being really due to the magical shimmer from the walls of the moulin, which was quite empty. As far as we could see, the large shaft was vertical, but on dropping a stone into it a shock was soon heard, and after a succession of bumps, which occupied in all seven seconds, we heard the stone no more. The depth of the moulin could not be thus ascertained, but we soon found a second and still larger one which gave us better data. A stone dropped into this descended without interruption for four seconds, when a concussion was heard; and three seconds afterwards the final shock was audible: there was thus but a single interruption in the descent. Supposing all the acquired velocity to have been destroyed by the shock, by adding the space passed over by the stone in four and in three seconds respectively, and making allowance for the time required by the sound to ascend from the bottom, we find the depth of the shaft to be about 345 feet. There is, however, no reason to suppose that this measures the depth of the glacier at the place referred to. These shafts are to be found in almost all great glaciers; they are very numerous in the Unteraar Glacier, numbers of them however being empty. On the Mer de Glace they are always to be found in the region of Trelaporte, one of the shafts there being, _par excellence_, called the Grand Moulin. Many of them also occur on the Glacier de Lechaud.

As truly observed by M. Agassiz, these moulins occur only at those parts of the glacier which are not much rent by fissures, for only at such portions can the little rills produced by superficial melting collect to form streams of any magnitude. The valley of unbroken ice formed in the Mer de Glace near Trelaporte is peculiarly favourable for the collection of such streams; we see the little rills commencing, and enlarging by the contributions of others, the trunk-rill pouring its contents into a little stream which stretches out a hundred similar arms over the surface of the glacier. Several such streams join, and finally a considerable brook, which receives the superficial drainage of a large area, cuts its way through the ice.

[Sidenote: MOULINS EXPLAINED.]

But although this portion of the glacier is free from those long-continued and permanent strains which, having once rent the ice, tend subsequently to widen the rent and produce yawning crevasses, it is not free from local strains sufficient to produce _cracks_ which penetrate the glacier to a great depth. Imagine such a crack intersecting such a glacier-rivulet as we have described. The water rushes down it, and soon scoops a funnel large enough to engulf the entire stream. The moulin is thus formed, and, as the ice moves downward, the sides of the crack are squeezed together and regelated, the seam which marks the line of junction being in most cases distinctly visible. But as the motion continues, other portions of the glacier come into the same state of strain as that which produced the first crack; a second one is formed across the stream, the old shaft is forsaken, and a new one is hollowed out, in which for a season the cataract plays the thunderer. I have in some cases counted the forsaken shafts of six old moulins in advance of an active one. Not far from the Grand Moulin of the Mer de Glace in 1857 there was a second empty shaft, which evidently communicated by a subglacial duct with that into which the torrent was precipitated. Out of the old orifice issued a strong cold blast, the air being manifestly impelled through the duct by the falling water of the adjacent moulin.

These shafts are always found in the same locality; the portion of the Mer de Glace to which I have referred is never without them. Some of the guides affirm that they are motionless; and a statement of Prof. Forbes has led to the belief that this was also his opinion.[A] M. Agassiz, however, observed the motion of some of these shafts upon the glacier of the Aar; and when on the spot in 1857, I was anxious to decide the point by accurate measurements with the theodolite.

My friend Mr. Hirst took charge of the instrument, and on the 28th of July I fixed a single stake beside the Grand Moulin, in a straight line between a station at Trelaporte and a well-defined mark on the rock at the opposite side of the valley. On the 31st, the displacement of the stake amounted to 50 inches, and on the 1st of August it had moved 74-1/2 inches--the moulin, to all appearance, occupying throughout the same position with regard to the stake. To render this certain, moreover we subsequently drove two additional stakes into the ice, thus enclosing the mouth of the shaft in a triangle. On the 8th of August the displacements were measured and gave the following results:--

Total Motion. First (old) stake 198 inches. Second (new) do. 123 " Third 124 "

[Sidenote: MOTION OF THE MOULINS.]

The old stake had been fixed for 11 days, and its daily motion--_which was also that of the moulin_--averaged 18 inches a day. Hence the moulins share the general motion of the glacier, and their apparent permanence is not, as has been alleged, a proof of the semi-fluidity of the glacier, but is due to the breaking of the ice as it passes the place of local strain.

[Sidenote: DEPTH OF "GRAND MOULIN" SOUGHT.]

Wishing to obtain some estimate as to the depth of the ice, Mr. Hirst undertook the sounding of some of the moulins upon the Glacier de Lechaud, making use of a tin vessel filled with lumps of lead and iron as a weight. The cord gave way and he lost his plummet. To measure the depth of the Grand Moulin, we obtained fresh cord from Chamouni, to which we attached a four-pound weight. Into a cavity at the bottom of the weight we stuffed a quantity of butter, to indicate the nature of the bottom against which the weight might strike. The weight was dropped into the shaft, and the cord paid out until its slackening informed us that the weight had come to rest; by shaking the string, however, and walking round the edge of the shaft, the weight was liberated, and sank some distance further. The cord partially slackened a second time, but the strain still remaining was sufficient to render it doubtful whether it was the weight or the action of the falling water which produced it. We accordingly paid out the cord to the end, but, on withdrawing it, found that the greater part of it had been coiled and knotted up by the falling water. We uncoiled, and sounded again. At a depth of 132 feet the weight reached a ledge or protuberance of ice, and by shaking and lifting it, it was caused to descend 31 feet more. A depth of 163 feet was the utmost we could attain to. We sounded the old moulin to a depth of 90 feet; while a third little shaft, beside the large one, measured only 18 feet in depth. We could see the water escape from it through a lateral canal at its bottom, and doubtless the water of the Grand Moulin found a similar exit. There was no trace of dirt upon the butter, which might have indicated that we had reached the bed of the glacier.

FOOTNOTES:

[A] "Every year, and year after year, the watercourses follow the same lines of direction--their streams are precipitated into the heart of the glacier by vertical funnels, called 'moulins,' at the very same points."--Forbes's Fourth Letter upon Glaciers: 'Occ. Pap.,' p. 29.

[Illustration: DIRT-BANDS OF THE MER DE GLACE, AS SEEN FROM A POINT NEAR THE FLEGERE. Fig. 35. _To face p. 367._]

DIRT-BANDS OF THE MER DE GLACE.

(26.)

[Sidenote: DIRT-BANDS FROM THE FLEGERE.]

These bands were first noticed by Prof. Forbes on the 24th of July, 1842, and were described by him in the following words:--"My eye was caught by a very peculiar appearance of the surface of the ice, which I was certain that I now saw for the first time. It consisted of nearly hyperbolic brownish bands on the glacier, the curves pointing downwards, and the two branches mingling indiscriminately with the moraines, presenting an appearance of a succession of waves some hundred feet apart."[A] From no single point of view hitherto attained can all the Dirt-Bands of the Mer de Glace be seen at once. To see those on the terminal portion of the glacier, a station ought to be chosen on the opposite range of the Brevent, a few hundred yards beyond the Croix de la Flegere, where we stand exactly in front of the glacier as it issues into the valley of Chamouni. The appearance of the bands upon the portion here seen is represented in Fig. 35.

It will be seen that the bands are confined to one side of the glacier, and either do not exist, or are obliterated by the debris, upon the other side. The cause of the accumulation of dirt on the right side of the glacier is, that no less than five moraines are crowded together at this side. In the upper portions of the Mer de Glace these moraines are distinct from each other; but in descending, the successive engulfments and disgorgings of the blocks and dirt have broken up the moraines; and at the place now before us the materials which composed them are strewn confusedly on the right side of the glacier. The portion of the ice on which the dirt-bands appear is derived from the Col du Geant. They do not quite extend to the end of the glacier, being obliterated by the dislocation of the ice upon the frozen cascade of Des Bois.

[Sidenote: DIRT-BANDS FROM LES CHARMOZ.]

Let us now proceed across the valley of Chamouni to the Montanvert; where, climbing the adjacent heights to an elevation of six or eight hundred feet above the hotel, we command a view of the Mer de Glace, from Trelaporte almost to the commencement of the Glacier des Bois. It was from this position that Professor Forbes first observed the bands. Fifteen, sixteen, and seventeen years later I observed them from the same position. The number of bands which Professor Forbes counted from this position was eighteen, with which my observations agree. The entire series of bands which I observed, with the exception of one or two, must have been the _successors_ of those observed by Professor Forbes; and my finding the same number after an interval of so many years proves that the bands must be due to some regularly recurrent cause. Fig. 36 represents the bands as seen from the heights adjacent to the Montanvert.

[Illustration: DIRT-BANDS OF THE MER DE GLACE, AS SEEN FROM LES CHARMOZ. Fig. 36. _To face p. 368._]

I would here direct attention to an analogy between a glacier and a river, which may be observed from the heights above the Montanvert, but to which no reference, as far as I know, has hitherto been made. When a river meets the buttress of a bridge, the water rises against it, and, on sweeping round it, forms an elevated ridge, between which and the pier a depression occurs which varies in depth with the force of the current. This effect is shown by the Mer de Glace on an exaggerated scale. Sweeping round Trelaporte, the ice pushes itself beyond the promontory in an elevated ridge, from which it drops by a gradual slope to the adjacent wall of the valley, thus forming a depression typified by that already alluded to. A similar effect is observed at the opposite side of the glacier on turning round the Echelets; and both combine to form a kind of skew surface. A careful inspection of the frontispiece will detect this peculiarity in the shape of the glacier.

[Sidenote: FROM THE CLEFT-STATION.]

From neither of the stations referred to do we obtain any clue to the origin of the dirt-bands. A stiff but pleasant climb will place us in that singular cleft in the cliffy mountain-ridge which is seen to the right of the frontispiece; and from it we easily attain the high platform of rock immediately to the left of it. We stand here high above the promontory of Trelaporte, and occupy the finest station from which the Mer de Glace and its tributaries can be viewed. From this station we trace the dirt-bands over most of the ice that we have already scanned, and have the further advantage of being able to follow them to their very source.

This source is the grand ice-cascade which descends in a succession of precipices from the plateau of the Col du Geant into the valley which the Glacier du Geant fills. We see from our present point of view that the bands _are confined to the portion of the glacier which has descended the cascade_. Fig. 37 represents the bands as seen from the Cleft-station above Trelaporte.

[Illustration: DIRT-BANDS OF THE MER DE GLACE, AS SEEN FROM THE CLEFT STATION, TRELAPORTE. Fig. 37. _To face p. 369._]

We are now however at such a height above the glacier and at such a distance from the base of the cascade, that we can form but an imperfect notion of the true contour of the surface. Let us therefore descend, and walk up the Glacier du Geant towards the cascade. At first our road is level, but we gradually find that at certain intervals we have to ascend slopes which follow each other in succession, each being separated from its neighbour by a space of comparatively level ice. The slopes increase in steepness as we ascend; they are steepest, moreover, on the right-hand side of the glacier, where it is bounded by that from the Periades, and at length we are unable to climb them without the aid of an axe. Soon afterwards the dislocation of the glacier becomes considerable; we are lost in the clefts and depressions of the ice, and are unable to obtain a view sufficiently commanding to subdue these local appearances and convey to us the general aspect. We have at all events satisfied ourselves as to the existence, on the upper portion of the glacier, of a succession of undulations which sweep transversely across it. The term "wrinkles," applied to them by Prof. Forbes, is highly suggestive of the appearance which they present.

[Sidenote: SNOW-BANDS ON THE GLACIER DU GEANT.]

From the Cleft-station bands of snow may also be seen partially crossing the glacier in correspondence with the undulations upon its surface. If the quantity deposited the winter previous be large, and the heat of summer not too great, these bands extend quite across the glacier. They were first observed by Professor Forbes in 1843. In his Fifth Letter is given an illustrative diagram, which, though erroneous as regards the position of the veined structure, is quite correct in limiting the snow-bands to the Glacier du Geant proper.

At the place where the three welded tributaries of the Mer de Glace squeeze themselves through the strait of Trelaporte, the bands undergo a considerable modification in shape. Near their origin they sweep across the Glacier du Geant in gentle curves, with their convexities directed downwards; but at Trelaporte these curves, the chords of which a short time previous measured a thousand yards in length, have to squeeze themselves through a space of four hundred and ninety-five yards wide; and as might be expected, they are here suddenly sharpened. The apex of each being thrust forward, they take the form of sharp hyperbolas, and preserve this character throughout the entire length of the Mer de Glace.

I would now conduct the reader to a point from which a good general view of the ice cascade of the Geant is attainable. From the old moraine near the lake of the Tacul we observe the ice, as it descends the fall, to be broken into a succession of precipices. It would appear as if the glacier had its back periodically broken at the summit of the fall, and formed a series of vast chasms separated from each other by cliffy ridges of corresponding size. These, as they approach the bottom of the fall, become more and more toned down by the action of sun and air, and at some distance below the base of the cascade they are subdued so as to form the transverse undulations already described. These undulations are more and more reduced as the glacier descends; and long before the Tacul is attained, every sensible trace of them has disappeared. The terraces of the ice-fall are referred to by Professor Forbes in his Thirteenth Letter, where he thus describes them:--"The ice-falls succeed one another at regulated intervals, which appear to correspond to the renewal of each summer's activity in those realms of almost perpetual frost, when a swifter motion occasions a more rapid and wholesale projection of the mass over the steep, thus forming curvilinear terraces like vast stairs, which appear afterwards by consolidation to form the remarkable protuberant wrinkles on the surface of the Glacier du Geant."

[Sidenote: FORBES'S EXPLANATION.]

With regard to the cause of the distribution of the dirt in bands, Professor Forbes writes thus in his Third Letter:--"I at length assured myself that it was entirely owing to the structure of the ice, which retains the dirt diffused by avalanches and the weather on those parts which are most porous, whilst the compacter portion is washed clean by the rain, so that those bands are nothing more than visible traces of the direction of the internal icy structure." Professor Forbes's theory, at that time, was that the glacier is composed throughout of a series of alternate segments of hard and porous ice, in the latter of which the dirt found a lodgment. I do not know whether he now retains his first opinion; but in his Fifteenth Letter he speaks of accounting for "the less compact structure of the ice beneath the dirt-band."

It appears to me that in the above explanation cause has been mistaken for effect. The ice on which the dirt-bands rest certainly appears to be of a spongier character than the cleaner intermediate ice; but instead of this being the cause of the dirt-bands, the latter, I imagine, by their more copious absorption of the sun's rays and the consequent greater disintegration of the ice, are the cause of the apparent porosity. I have not been able to detect any relative porosity in the "internal icy structure," nor am I able to find in the writings of Professor Forbes a description of the experiments whereby he satisfied himself that this assumed difference exists.

[Sidenote: TRANSVERSE UNDULATIONS.]

[Sidenote: INFLUENCE OF DIRECTION OF GLACIER.]

Several days of the summer of 1857 were devoted by me to the examination of these bands. I then found the bases and the frontal slopes of the undulations to which I have referred covered with a fine brown mud. These slopes were also, in some cases, covered with snow which the great heat of the weather had not been able entirely to remove. At places where the residue of snow was small its surface was exceedingly dirty--so dirty indeed that it appeared as if peat-mould had been strewn over it; its edges particularly were of a black brown. It was perfectly manifest that this snow formed a receptacle for the fine dirt transported by the innumerable little rills which trickled over the glacier. The snow gradually wasted, but it left its sediment behind, and thus each of the snowy bands observed by Professor Forbes in 1843, contributed to produce an appearance perfectly antithetical to its own. I have said that the frontal slopes of the undulations were thus covered; and it was on these, and not in the depressions, that the snow principally rested. The reason of this is to be found in the _bearing_ of the Glacier du Geant, which, looking downwards, is about fourteen degrees east of the meridian.[B] Hence the frontal slopes of the undulations have a _northern aspect_, and it is this circumstance which, in my opinion, causes the retention of the snow upon them. Irrespective of the snow, the mere tendency of the dirt to accumulate at the bases of the undulations would also produce bands, and indeed does so on many glaciers; but the precision and beauty of the dirt-bands of the Mer de Glace are, I think, to be mainly referred to the interception by the snow of the fine dark mud before referred to on the northern slopes of its undulations.

[Sidenote: BANDS DO NOT CROSS MORAINES.]

Were the statements of some writers upon this subject well founded, or were the dirt-bands as drawn upon the map of Professor Forbes correctly shown, this explanation could not stand a moment. It has been urged that the dirt-bands cannot thus belong to a single tributary of the Mer de Glace; for if they did, they would be confined to that tributary upon the trunk-glacier; whereas the fact is that they extend quite across the trunk, and intersect the moraines which divide the Glacier du Geant from its fellow-tributaries. From my first acquaintance with the Mer de Glace I had reason to believe that this statement was incorrect; but last year I climbed a third time to the Cleft-station for the purpose of once more inspecting the bands from this fine position. I was accompanied by Dr. Frankland and Auguste Balmat, and I drew the attention of both

## particularly to this point. Neither of them could discern, nor could I,

the slightest trace of a dirt-band crossing any one of the moraines. Upon the trunk-stream they were just as much confined to the Glacier du Geant as ever. If the bands even existed east of the moraines, they could not be seen, the dirt on this part of the glacier being sufficient to mask them.

The following interesting fact may perhaps have contributed to the production of the error referred to. Opposite to Trelaporte the eastern arms of the dirt-bands run so obliquely into the moraine of La Noire that the latter appears to be a tangent to them. But this moraine runs along the Mer de Glace, not far from its centre, and consequently the point of contact of each dirt-band with the moraine moves more quickly than the point of contact of the western arm of the same band with the side of the valley. Hence there is a tendency to _straighten_ the bands; and at some distance down the glacier the effect of this is seen in the bands abutting against the moraine of La Noire at a larger angle than before. The branches thus abutting have, I believe, been ideally prolonged across the moraines.

[Illustration: Fig. 38. Plan of Dirt-bands taken from Johnson's 'Physical Atlas.']

On the map published by Prof. Forbes in 1843 the bands are shown crossing the medial moraines of the Mer de Glace; and they are also thus drawn on the map in Johnson's 'Physical Atlas' published in 1849. The text is also in accordance with the map:--"Opposite to the Montanvert, and beyond les Echelets, the curved loops (dirt-bands) extend _across the entire glacier_. They are single, and therefore _cut_ the medial moraine, though at a very slight angle."--'Travels,' p. 166. The italics here belong to Prof. Forbes. In order to help future observers to place this point beyond doubt, I annex, in Fig. 38, a portion of the map of the Mer de Glace taken from the Atlas referred to. If it be compared with Fig. 35 the difference between Prof. Forbes and myself will be clearly seen. The portion of the glacier represented in both diagrams may be viewed from the point near the Flegere already referred to.

[Sidenote: ANNUAL "RINGS."]

The explanation which I have given involves three considerations:--The transverse breaking of the glacier on the cascade, and the gradual accumulation of the dirt in the hollows between the ridges; the subsequent toning down of the ridges to gentle protuberances which sweep across the glacier; and the collection of the dirt upon the slopes and at the bases of these protuberances. Whether the periods of transverse fracture are annual or not--whether the "wrinkles" correspond to a yearly gush--and whether, consequently, the dirt-bands mark the growth of a glacier as the "annual rings" mark the growth of a tree, I do not know. It is a conjecture well worthy of consideration; but it is only a conjecture, which future observation may either ratify or refute.

FOOTNOTES:

[A] 'Travels,' page 162.

[B] In the large map of Professor Forbes the bearing of the valley is nearly sixty degrees west of the meridian; but this is caused by the true north being drawn on the wrong side of the magnetic north; thus making the declination easterly instead of westerly. In the map in Johnson's 'Physical Atlas' this mistake is corrected.

THE VEINED STRUCTURE OF GLACIERS.

(27.)

[Sidenote: GENERAL APPEARANCE.]

The general appearance of the veined structure may be thus briefly described:--The ice of glaciers, especially midway between their mountain-sources and their inferior extremities, is of a whitish hue, caused by the number of small air-bubbles which it contains, and which, no doubt, constitute the residue of the air originally entrapped in the interstices of the snow from which it has been derived. Through the general whitish mass, at some places, innumerable parallel veins of clearer ice are drawn, which usually present a beautiful blue colour, and give the ice a laminated appearance. The cause of the blueness is, that the air-bubbles, distributed so plentifully through the general mass, do not exist in the veins, or only in comparatively small numbers.

In different glaciers, and in different parts of the same glacier, these veins display various degrees of perfection. On the clean unweathered walls of some crevasses, and in the channels worn in the ice by glacier-streams, they are most distinctly seen, and are often exquisitely beautiful. They are not to be regarded as a partial phenomenon, or as affecting the constitution of glaciers to a small extent merely. A large portion of the ice of some glaciers is thus affected. The greater part, for example, of the Mer de Glace consists of this laminated ice; and the whole of the Glacier of the Rhone, from the base of the ice-cascade downwards, is composed of ice of the same description.

[Sidenote: GROOVES ON THE SURFACE OF GLACIERS.]

Those who have ascended Snowdon, or wandered among the hills of Cumberland, or even walked in the environs of Leeds, Blackburn, and other towns in Yorkshire and Lancashire, where the stratified sandstone of the district is used for building purposes, may have observed the weathered edges of the slate rocks or of the building-stone to be grooved and furrowed. Some laminae of such rocks withstand the action of the atmosphere better than others, and the more resistant ones stand out in ridges after the softer parts between them have been eaten away. An effect exactly similar is observed where the laminated ice of glaciers is exposed to the action of the sun and air. Little grooves and ridges are formed upon its surface, the more resistant plates protruding after the softer material between them has been melted away.

One consequence of this furrowing is, that the light dirt scattered by the winds over the surface of the glacier is gradually washed into the little grooves, thus forming fine lines resembling those produced by the passage of a rake over a sanded walk. These lines are a valuable index to some of the phenomena of motion. From a position on the ice of the Glacier du Geant a little higher up than Trelaporte a fine view of these superficial groovings is obtained; but the dirt-lines are not always straight. A slight power of independent motion is enjoyed by the separate parts into which a glacier is divided by its crevasses and dislocations, and hence it is, that, at the place alluded to, the dirt-lines are bent hither and thither, though the ruptures of continuity are too small to affect materially the general direction of the structure. On the glacier of the Talefre I found these groovings useful as indicating the character of the forces to which the ice near the summit of the fall is subjected. The ridges between the chasms are in many cases violently bent and twisted, while the adjacent groovings enable us to see the normal position of the mass.

[Sidenote: GUYOT'S OBSERVATIONS.]

The veined structure has been observed by different travellers; but it was probably first referred to by Sir David Brewster, who noticed the veins of the Mer de Glace on the 10th of September, 1814. It was also observed by General Sabine,[A] by Rendu, by Agassiz, and no doubt by many others; but the first clear description of it was given by M. Guyot, in a communication presented to the Geological Society of France in 1838. I quote the following passage from this paper:--"I saw under my feet the surface of the entire glacier covered with regular furrows from one to two inches wide, hollowed out in a half snowy mass, and separated by protruding plates of harder and more transparent ice. It was evident that the mass of the glacier here was composed of two sorts of ice, one that of the furrows, snowy and more easily melted; the other that of the plates, more perfect, crystalline, glassy, and resistant; and that the unequal resistance which the two kinds of ice presented to the atmosphere was the cause of the furrows and ridges. After having followed them for several hundreds of yards, I reached a fissure twenty or thirty feet wide, which, as it cut the plates and furrows at right angles, exposed the interior of the glacier to a depth of thirty or forty feet, and gave a beautiful transverse section of the structure. As far as my vision could reach I saw the mass of the glacier composed of layers of snowy ice, each two of which were separated by one of the plates of which I have spoken, the whole forming a regularly laminated mass, which resembled certain calcareous slates."

[Sidenote: FORBES'S RESEARCHES.]

Previous observers had mistaken the lamination for stratification; but M. Guyot not only clearly saw that they were different, but in the comparison which he makes he touches, I believe, on the true cause of the glacier-structure. He did not hazard an explanation of the phenomenon, and I believe his memoir remained unprinted. In 1841 the structure was noticed by Professor Forbes during his visit to M. Agassiz on the lower Aar Glacier, and described in a communication presented by him to the Royal Society of Edinburgh. He subsequently devoted much time to the subject, and his great merit in connexion with it consists in the significance which he ascribed to the phenomenon when he first observed it, and in the fact of his having proved it to be a constitutional feature of glaciers in general.

[Sidenote: FORBES'S THEORY.]

The first explanation given of those veins by Professor Forbes was, that they were small fissures formed in the ice by its motion; that these were filled with the water of the melted ice in summer, which froze in winter so as to form the blue veins. This is the explanation given in his 'Travels,' page 377; and in a letter published in the 'Edinburgh New Philosophical Journal,' October, 1844, it is re-affirmed in these words:--"With the abundance of blue bands before us in the direction in which the differential motion must take place (in this case sensibly parallel to the sides of the glacier), it is impossible to doubt that these infiltrated crevices (for such they undoubtedly are) have this origin." This theory was examined by Mr. Huxley and myself in our joint paper; but it has been since alleged that ours was unnecessary labour, Prof. Forbes himself having in his Thirteenth Letter renounced the theory, and substituted another in its place. The latter theory differs, so far as I can understand it, from the former in this particular, that the _freezing of the water_ in the fissures is discarded, their sides being now supposed to be united "by the simple effects of time and cohesion."[B] For a statement of the change which his opinions have undergone, I would refer to the Prefatory Note which precedes the volume of 'Occasional Papers' recently published by Prof. Forbes; but it would have diminished my difficulty had the author given, in connexion with his new volume, a more distinct statement of his present views regarding the veined structure. With many of his observations and remarks I should agree; with many others I cannot say whether I agree or not; and there are others still with which I do not think I should agree: but in hardly any case am I certain of his precise views, excepting, indeed, the cardinal one, wherein he and others agree in ascribing to the structure a different origin from stratification. Thus circumstanced, my proper course, I think, will be to state what I believe to be the cause of the structure, and leave it to the reader to decide how far our views harmonize; or to what extent either of them is a true interpretation of nature.

[Sidenote: USUAL ASPECT OF BLUE VEINS.]

Most of the earlier observers considered the structure to be due to the stratification of the mountain-snows--a view which has received later development at the hands of Mr. John Ball; and the practical difficulty of distinguishing the undoubted effects of _stratification_ from the phenomena presented by _structure_, entitles this view to the fullest consideration. The blue veins of glaciers are, however, not always, nor even generally, such as we should expect to result from stratification. The latter would furnish us with distinct planes extending parallel to each other for considerable distances through the glacier; but this, though sometimes the case, is by no means the general character of the structure. We observe blue streaks, from a few inches to several feet in length, upon the walls of the same crevasse, and varying from the fraction of an inch to several inches in thickness. In some cases the streaks are definitely bounded, giving rise to an appearance resembling the section of a lens, and hence called the "lenticular structure" by Mr. Huxley and myself; but more usually they fade away in pale washy streaks through the general mass of the whitish ice. In Fig. 39 I have given a representation of the structure as it is very commonly exhibited on the walls of crevasses. Its aspect is not that which we should expect from the consolidation of successive beds of mountain snow.

[Illustration: Fig. 39. Veined Structure of the walls of crevasses.]

Further, at the bases of ice-cascades the structural laminae are usually _vertical_: below the cascade of the Talefre, of the Noire, of the Strahleck branch of the Lower Grindelwald Glacier, of the Rhone, and other ice-falls, this is the case; and it seems extremely difficult to conceive that a mass horizontally stratified at the summit of the fall, should, in its descent, contrive to turn its strata perfectly on end.

Again, we often find a very feebly-developed structure at the central portions of a glacier, while the lateral portions are very decidedly laminated. This is the case where the inclination of the glacier is nearly uniform throughout; and where no medial moraines occur to complicate the phenomenon. But if the veins mark the bedding, there seems to be no sufficient reason for their appearance at the lateral portions of the glacier, and their absence from the centre.

[Sidenote: ILLUSTRATIVE EXPERIMENTS.]

This leads me to the point at which what I consider to be the true cause of the structure may be referred to. The theoretic researches of Mr. Hopkins have taught us a good deal regarding the pressures and tensions consequent upon glacier-motion. Aided by this knowledge, and also by a mode of experiment first introduced by Professor Forbes, I will now endeavour to explain the significance of the fact referred to in the last paragraph. If a plastic substance, such as mud, flow down a sloping canal, the lateral portions, being held back by friction, will be outstripped by the central ones. When the flow is so regulated that the velocity of a point at the centre shall not vary throughout the entire length of the canal, a coloured circle stamped upon the centre of the mud stream, near its origin, will move along with the mud, and still retain its circular form; for, inasmuch as the velocity of all points along the centre is the same, there can be no elongation of the circle longitudinally or transversely by either strain or pressure. A similar absence of longitudinal pressure may exist in a glacier, and, where it exists throughout, no central structure can, in my opinion, be developed.

But let a circle be stamped upon the mud-stream near its side, then, when the mud flows, this circle will be distorted to an oval, with its major axis oblique to the direction of motion; the cause of this is that the portion of the circle farthest from the side of the canal moves more freely than that adjacent to the side. The mechanical effect of the slower lateral motion is to squeeze the circle in one direction, and draw it out in the perpendicular one.

[Sidenote: MARGINAL STRUCTURE.]

[Illustration: Fig. 40. Figure explanatory of the Marginal Structure.]

A glance at Fig. 40 will render all that I have said intelligible. The three circles are first stamped on the mud in the same transverse line; but after they have moved downwards they will be in the same straight line no longer. The central one will be the foremost; while the lateral ones have their forms changed from circles to ovals. In a glacier of the shape of this canal exactly similar effects are produced. Now the shorter axis _m n_ of each oval is a line of squeezing or pressure; the longer axis is a line of strain or tension; and the associated glacier-phenomena are as follows:--Across the line _m n_, or perpendicular to the pressure, we have the _veined structure_ developed, while across the line of tension the glacier usually breaks and forms _marginal crevasses_. Mr. Hopkins has shown that the lines of greatest pressure and of greatest strain are at right angles to each other, and that in valleys of a uniform width they enclose an angle of forty-five degrees with the side of the glacier. To the structure thus formed I have applied the term _marginal structure_. Here, then, we see that there are mechanical agencies at work near the side of such a glacier which are absent from the centre, and we have effects developed--I believe _by the pressure_--in the lateral ice, which are not produced in the central.

I have used the term "uniform inclination" in connexion with the marginal structure, and my reason for doing so will now appear. In many glaciers the structure, instead of being confined to the margins, sweeps quite across them. This is the case, for example, on the Glacier du Geant, the structure of which is prolonged into the Mer de Glace. In passing the strait at Trelaporte, however, the curves are squeezed and their apices bruised, so that the structure is thrown into a state of confusion; and thus upon the Mer de Glace we encounter difficulty in tracing it fairly from side to side. Now the key to this transverse structure I believe to be the following: Where the inclination of the glacier suddenly changes from a steep slope to a gentler, as at the bases of the "cascades,"--the ice to a certain depth must be thrown into a state of violent longitudinal compression; and along with this we have the resistance which the gentler slope throws athwart the ice descending from the steep one. At such places a structure is developed transverse to the axis of the glacier, and likewise transverse to the pressure. The quicker flow of the centre causes this structure to bend more and more, and after a time it sweeps in vast curves across the entire glacier.

[Sidenote: STRUCTURE OF GRINDELWALD GLACIER.]

In illustration of this point I will refer, in the first place, to that tributary of the Lower Glacier of Grindelwald which descends from the Strahleck. Walking up this tributary we come at length to the base of an ice-fall. Let the observer here leave the ice, and betake himself to either side of the flanking mountain. On attaining a point which commands a view both of the fall and of the glacier below it, an inspection of the glacier will, I imagine, solve to his satisfaction the case of structure now under consideration.

It is indeed a grand experiment which Nature here submits to our inspection. The glacier descending from its _neve_ reaches the summit of the cascade, and is broken transversely as it crosses the brow; it afterwards descends the fall in a succession of cliffy ice-ridges with transverse hollows between them. In these latter the broken ice and debris collect, thus partially choking the fissures formed in the first instance. Carrying the eye downwards along the fall, we see, as we approach the base, these sharp ridges toned down; and a little below the base they dwindle into rounded protuberances which sweep in curves quite across the glacier. At the base of the fall the structure begins to appear, feebly at first, but becoming gradually more pronounced, until, at a short distance below the base of the fall, the eye can follow the fine superficial groovings from side to side; while at the same time the ice underneath the surface has become laminated in the most beautiful manner.

It is difficult to convey by writing the force of the evidence which the actual observation of this natural experiment places before the mind. The ice at the base of the fall, retarded by the gentler inclination of the valley, has to bear the thrust of the descending mass, the sudden change of inclination producing powerful longitudinal compression. The protuberances are squeezed more closely together, the hollows between them appear to wrinkle up in submission to the pressure--in short, the entire aspect of the glacier suggests the powerful operations of the latter force. At the place where _it_ is exerted the veined structure makes its appearance; and being once formed, it moves downwards, and gives a character to other portions of the glacier which had no share in its formation.

[Sidenote: BASE OF CASCADE A "STRUCTURE-MILL."]

An illustration almost as good, and equally accessible, is furnished by the Glacier of the Rhone. I have examined the grand cascade of this glacier from both sides; and an ordinary mountaineer will find little difficulty in reaching a point from which the fall and the terminal portion of the glacier are both distinctly visible. Here also he will find the cliffy ridges separated from each other by transverse chasms, becoming more and more subdued at the bottom of the fall, and disappearing entirely lower down the glacier. As in the case of the Grindelwald Glacier the squeezing of the protuberances and of the spaces between them, is quite apparent, and where this squeezing commences the transverse structure makes its appearance. All the ice that forms the lower portion of this glacier has to pass through the _structure-mill_ at the bottom of the fall, and the consequence is that _it is all laminated_.

[Sidenote: STRUCTURE OF RHONE GLACIER.]

[Illustration: Fig. 41. Plan of part of ice-fall, and of glacier below it (Glacier of the Rhone).]

[Illustration: Fig. 42. Section of part of ice-fall, and of glacier below it (Glacier of the Rhone).]

[Sidenote: TRANSVERSE STRUCTURE.]

This case of structural development will be better appreciated on reference to Figs. 41 and 42, the former of which is a plan, and the latter a section, of a part of the ice-fall and of the glacier below it; _a b e f_ is the gorge of the fall, _f b_ being the base. The transverse cliffy ice-ridges are shown crossing the cascade, being subdued at the base to protuberances which gradually disappear as they advance downwards. The structure sweeps over the glacier in the direction of the fine curved lines; and I have also endeavoured to show the direction of the radial crevasses, which, in the centre at least, are at right angles to the veins. To the manifestation of structure here considered I have, for the sake of convenient reference, applied the term _transverse structure_.

A third exhibition of the structure is now to be noticed. We sometimes find it in the _middle_ of a glacier and running _parallel_ to its length. On the centre of the ice-fall of the Talefre, for example, we have a structure of this kind which preserves itself parallel to the axis of the fall from top to bottom. But we discover its origin higher up. The structure here has been produced at the extremity of the Jardin, where the divided ice meets, and not only brings into partial parallelism the veins previously existing along the sides of the Jardin, but develops them still further by the mutual pressure of the portions of newly welded ice. Where two tributary glaciers unite, this is perhaps without exception the case. Underneath the moraine formed by the junction of the Talefre and Lechaud the structure is finely developed, and the veins run in the direction of the moraine. The same is true of the ice under the moraine formed by the junction of the Lechaud and Geant. These afterwards form the great medial moraines of the Mer de Glace, and hence the structure of the trunk-stream underneath these moraines is parallel to the direction of the glacier. This is also true of the system of moraines formed by the glaciers of Monte Rosa. It is true in an especial manner of the lower glacier of the Aar, whose medial moraine perhaps attains grander proportions than any other in the Alps, and underneath which the structure is finely developed.

[Sidenote: LONGITUDINAL STRUCTURE.]

[Illustration: Fig. 43. Figure explanatory of Longitudinal Structure.]

The manner in which I have illustrated the production of this structure will be understood from Fig. 43. B B are two wooden boxes, communicating by sluice-fronts with two branch canals, which unite to a common trunk at G. They are intended to represent respectively the trunk and tributaries of the Unteraar Glacier, the part G being the Abschwung, where the Lauteraar and Finsteraar glaciers unite to form the Unteraar. The mud is first permitted to flow beneath the two sluices until it has covered the bottom of the trough for some distance, when it is arrested. The end of a glass tube is then dipped into a mixture of rouge and water, and small circles are stamped upon the mud. The two branches are thickly covered with these circles. The sluices being again raised, the mud in the branches moves downwards, carrying with it the circles stamped upon it; and the manner in which these circles are distorted enables us to infer the strains and pressures to which the mud is subjected during its descent. The figure represents approximately what takes place. The side-circles, as might be expected, are squeezed to oblique ovals, but it is at the junction of the branches that the chief effect of pressure is produced. Here, by the mutual thrust of the branches, the circles are not only changed to elongated ellipses, but even squeezed to straight lines. In the case of the glacier this is the region at which the structure receives its main development. To this manifestation of the veins I have applied the term _longitudinal structure_.

The three main sources of the blue veins are, I think, here noted; but besides these there are many local causes which influence their production. I have seen them well formed where a glacier is opposed by the sudden bend of a valley, or by a local promontory which presents an obstacle sufficient to bring the requisite pressure into play. In the glaciers of the Tyrol and of the Oberland I have seen examples of this kind; but the three principal sources of the veins are, I think, those stated above.

[Sidenote: EFFORTS TO SOLVE QUESTION.]

It was long before I cleared my mind of doubt regarding the origin of the lamination. When on the Mer de Glace in 1857 I spared neither risk nor labour to instruct myself regarding it. I explored the Talefre basin, its cascade, and the ice beneath it. Several days were spent amid the ice humps and cliffs at the lower portion of the fall. I suppose I traversed the Glacier du Geant twenty times, and passed eight or ten days amid the confusion of its great cascade. I visited those places where, it had been affirmed, the veins were produced. I endeavoured to satisfy myself of the mutability which had been ascribed to them; but a close examination reduced the value of each particular case so much that I quitted the glacier that year with nothing more than an _opinion_ that the structure and the stratification were two different things. I, however, drew up a statement of the facts observed, with the view of presenting it to the Royal Society; but I afterwards felt that in thus

## acting I should merely swell the literature of the subject without

adding anything certain. I therefore withheld the paper, and resolved to devote another year to a search among the chief glaciers of the Oberland, of the Canton Valais, and of Savoy, for proofs which should relieve my mind of all doubt upon the subject.

[Sidenote: EXPEDITION FOR THIS PURPOSE.]

Accordingly in 1858 I visited the glaciers of Rosenlaui, Schwartzwald, Grindelwald, the Aar, the Rhone, and the Aletsch, to the examination of which latter I devoted more than a week. I afterwards went to Zermatt, and, taking up my quarters at the Riffelberg, devoted eleven days to the examination of the great system of glaciers of Monte Rosa. I explored the Goerner Glacier up almost to the Cima de Jazzi; and believed that in it I could trace the structure from portions of the glacier where it vanished, through various stages of perfection, up to its full development. I believe this still; but yet it is nothing but a belief, which the utmost labour that I could bestow did not raise to a certainty. The Western glacier of Monte Rosa, the Schwartze Glacier, the Trifti Glacier, the glacier of the little Mont Cervin, and of St. Theodule, were all examined in connexion with the great trunk-stream of the Goerner, to which they weld themselves; and though the more I pursued the subject the stronger my conviction became that pressure was the cause of the structure, a crucial case was still wanting.

In the phenomena of slaty cleavage, it is often, if not usually, found that the true cleavage _cuts_ the planes of stratification--sometimes at a very high angle. Had this not been proved by the observations of Sedgwick and others, geologists would not have been able to conclude that cleavage and bedding were two different things, and needed wholly different explanations. My aim, throughout the expedition of 1858, was to discover in the ice a parallel case to the above; to find a clear and undoubted instance where the veins and the stratification were simultaneously exhibited, cutting each other at an unmistakable angle. On the 6th of August, while engaged with Professor Ramsay upon the Great Aletsch Glacier, not far from its junction with the Middle Aletsch, I observed what appeared to me to be the lines of bedding running nearly horizontal along the wall of a great crevasse, while cutting them at a large angle was the true veined structure. I drew my friend's attention to the fact, and to him it appeared perfectly conclusive. It is from a sketch made by him at the place that Fig. 44 has been taken.

[Sidenote: CASE OF STRUCTURE ON THE ALETSCH.]

[Illustration: Fig. 44. Structure and bedding on the Great Aletsch Glacier.]

This was the only case of the kind which I observed upon the Aletsch Glacier; and as I afterwards spent day after day upon the Monte Rosa glaciers, vainly seeking a similar instance, the thought again haunted me that we might have been mistaken upon the Aletsch. In this state of mind I remained until the 18th of August, a day devoted to the examination of the Furgge Glacier, which lies at the base of the Mont Cervin.

[Sidenote: STRUCTURE OF THE FURGGE GLACIER.]

Crossing the valley of the Goerner Glacier, and taking a plunge as I passed into the Schwarze See, I reached, in good time, the object of my day's excursion. Walking up the glacier, I at length found myself opposed by a frozen cascade composed of four high terraces of ice. The highest of these was chiefly composed of ice-cliffs and _seracs_, many of which had fallen, and now stood like rocking-stones upon the edge of the second terrace. The glacier at the base of the cascade was strewn with broken ice, and some blocks two hundred cubic feet in volume had been cast to a considerable distance down the glacier.

Upon the faces of the terraces the stratification of the _neve_ was most beautifully shown, running in parallel and horizontal lines along the weathered surface. The snow-field above the cascade is a frozen plain, smooth almost as a sheltered lake. The successive snow-falls deposit themselves with great regularity, and at the summit of the cascade the sections of the _neve_ are for the first time exposed. Hence their peculiar beauty and definition.

[Sidenote: ICE TERRACE EXAMINED.]

Indeed the figure of a lake pouring itself over a rocky barrier which curves convexly upwards, thus causing the water to fall down it, not only longitudinally over the vertex of the curve, but laterally over its two arms, will convey a tolerably correct conception of the shape of the fall. Towards the centre the ice was powerfully squeezed laterally, the beds were bent, and their continuity often broken by faults. On inspecting the ice from a distance with my opera glass, I thought I saw structural groovings cutting the strata at almost a right angle. Had the question been an undisputed one, I should perhaps have felt so sure of this as not to incur the danger of pushing the inquiry further; but, under the circumstances, danger was a secondary point. Resigning, therefore, my glass to my guide, who was to watch the tottering blocks overhead, and give me warning should they move, I advanced to the base of the fall, removed with my hatchet the weathered surface of the ice, and found underneath it the true veined structure, cutting, at nearly a right angle, the planes of stratification. The superficial groovings were not uniformly distributed over the fall, but appeared most decided at those places where the ice appeared to have been most squeezed. I examined three or four of these places, and in each case found the true veins nearly vertical, while the bedding was horizontal. Having perfectly satisfied myself of these facts, I made a speedy retreat, for the ice-blocks seemed most threatening, and the sunny hour was that at which they fall most frequently.

I next tried the ascent of the glacier up a dislocated declivity to the right. The ice was much riven, but still practicable. My way for a time lay amid fissures which exposed magnificent sections, and every step I took added further demonstration to what I had observed below. The strata were perfectly distinct, the structure equally so, and one crossed the other at an angle of seventy or eighty degrees. Mr. Sorby has adduced a case of the crumpling of a bed of sandstone through which the cleavage passes: here on the glacier I had parallel cases; the beds were bent and crumpled, but the structure ran through the ice in sharp straight lines. This perhaps was the most pleasant day I ever spent upon the glaciers: my mind was relieved of a long brooding doubt, and the intellectual freedom thus obtained added a subjective grandeur to the noble scene before me. Climbing the cliffs near the base of the Matterhorn, I walked along the rocky spine which extends to the Hoernli, and afterwards descended by the valley of Zmutt to Zermatt.

A year after my return to England a remark contained in Professor Mousson's interesting little work 'Die Gletscher der Jetzzeit' caused me to refer to the atlas of M. Agassiz's 'Systeme Glaciaire,' from which I learned that this indefatigable observer had figured a case of stratification and structure cutting each other. If, however, I had seen this figure beforehand, it would not have changed my movements; for the case, as sketched, would not have convinced me. I have now no doubt that M. Agassiz has preceded me in this observation, and hence my results are to be taken as mere confirmations of his.

[Sidenote: LAMINATION AND STRATIFICATION.]

Fig. 45 represents a crumpled portion of the ice with the lines of lamination passing through the strata. Fig. 46 represents a case where a fault had occurred, the veins at both sides of the line of dislocation being inclined towards each other.

[Illustration: Fig. 45. Structure and Stratification on the Furgge glacier.]

[Illustration: Fig. 46. Structure and Stratification on the Furgge glacier.]

[Figs. 45 and 46 are from sketches made on the Furgge Glacier.--L. C. T.]

FOOTNOTES:

[A] In reply to a question in connexion with this subject, General Sabine has favoured me with the following note:--

"My dear Tyndall,

"It was in the summer of 1841, at the Lower Grindelwald Glacier, that I first saw, and was greatly impressed and interested by examining and endeavouring to understand (in which I did not succeed), the veined structure of the ice. I do not remember when I mentioned it to Forbes, but it must be before 1843, because it is noticed in his book, p. 29. I had never observed it in the glaciers of Spitzbergen or Baffin's Bay, or in the icebergs of the shores and straits of Davis or Barrow. I feel the more confident of this, because, when I first saw the veined structure in Switzerland, my Arctic experience was more fresh in my recollection, and I recollected nothing like it.

"_Veins_ are indeed not uncommon in icebergs, but they quite resemble veins in rocks, and are formed by water filling fissures and freezing into blue ice, finely contrasted with the white granular substance of the berg.

"The ice of the Grindelwald Glacier (where I examined the veined structure) was broken up into very large masses, which by pressure had been upturned, so that a very poor judgment would be formed of the direction of the veins as they existed in the glacier before it had broken up.

"Sincerely yours, "EDWARD SABINE.

"_Feb. 20, 1860_."

[B] In a letter to myself, published in the 17th volume of the 'Philosophical Magazine,' Professor Forbes writes as follows:--"In 1846, then, I abandoned no part of the theory of the veined structure, on which as you say so much labour had been expended, except the admission, always yielded with reluctance, and got rid of with satisfaction, that the congelation of water in the crevices of the glacier may extend in winter to a great depth."

THE VEINED STRUCTURE AND THE DIFFERENTIAL MOTION.

(28.)

[Sidenote: DIFFERENTIAL MOTION GREATEST AT EDGES.]

I have now to examine briefly the explanation of the structure which refers it to differential motion--to a sliding of the particles of ice past each other, which leaves the traces of its existence in the blue veins. The fact is emphatically dwelt upon by those who hold this view, that the structure is best developed nearest to the sides of the glacier, where the differential motion is greatest. Why the differential motion is at its maximum near to the sides is easily understood. Let A B, C D, Fig. 47, represent the two sides of a glacier, moving in the direction of the arrow, and let _m a b c n_ be a straight line of stakes set out across the glacier to-day. Six months hence this line, by the motion of the ice downwards, will be bent to the form _m a' b' c' n_: this curve will not be circular, it will be flattened in the middle; the points _a_ and _c_, at some distance on each side of the centre _b_, move in fact with nearly the same velocity as the centre itself. Not so with the sides:--_a'_ and _c'_ have moved considerably in advance of _m_ and _n_, and hence we say that the difference of motion, or the differential motion, of the particles of ice near to the side is a maximum.

[Illustration: Fig. 47. Diagram illustrating Differential Motion.]

During all this time the points _m a' b' c' n_ have been moving straight down the glacier; and hence it will be understood that the sliding of the parts past each other, or, in other words, the differential motion, _is parallel to the sides of the glacier_. This, indeed, is the only differential motion that experiment has ever established; and consequently, when we find the best blue veins referred to the sides of the glacier because the differential motion is there greatest, we naturally infer that the motion meant is parallel to the sides.

[Sidenote: STRUCTURE OBLIQUE TO SIDES.]

But the fact is, that this motion would not at all account for the blue veins, for they are not parallel to the sides, but _oblique_ to them. This difficulty revealed itself after a time to those who first propounded the theory of differential motion, and caused them to modify their explanation of the structure. Differential motion is still assumed to be the cause of the veins, but now a motion is meant oblique to the sides, and it is supposed to be obtained in the following way:--Through the quicker motion of the point _c'_ the ice between it and _n_ becomes distended; that is to say, the line _c' n_ is in a state of strain--there is a _drag_, it is said, oblique to the sides of the glacier; and it is therefore in this direction that the particles will be caused to slide past each other. Dr. Whewell, who advocates this view, thus expounds it. He supposes the case of an alpine valley filled with india-rubber which has been warmed until it has partially melted, or become viscous, and then asks, "What will now be the condition of the mass? The sides and bottom will still be held back by the friction; the middle and upper part will slide forwards, but not freely. This want of freedom in the motion (arising from the viscosity) will produce a drag towards the middle of the valley, where the motion is freest; hence the direction in which the filaments slide past each other will be obliquely directed towards the middle. The sliding will separate the mass according to such lines; and though new attachments will take place, the mass may be expected to retain the results of this separation in the traces of parallel fissures."[A] Nothing can be clearer than the image of the process thus placed before the mind's eye.

One fact of especial importance is to be borne in mind: the sliding of filaments which is thus supposed to take place oblique to the glacier has never been proved; it is wholly assumed. A moraine, it is admitted, will run parallel to the side of a glacier, or a block will move in the same direction from beginning to end, without being sensibly drawn towards the centre, but still it is supposed that the sliding of parts exists, though of a character so small as to render it insensible to measurement.

[Sidenote: STRUCTURE CROSSES LINES OF SLIDING.]

My chief difficulty as regards this theory may be expressed in a very few words. If the structure be produced by differential motion, why is the large and _real_ differential motion which experiments have established incompetent to produce it? And how can the veins run, as they are admitted to do, _across the lines of maximum sliding_ from their origin throughout the glacier to its end?

That a drag towards the centre of the glacier exists is undeniable, but that in consequence of the drag there is a sliding of filaments in this direction, is quite another thing. I have in another place[B] endeavoured to show experimentally that no such sliding takes place, that the drag on any point towards the centre expresses only half the conditions of the problem; being exactly neutralized by the thrust towards the sides. It has been, moreover, shown by Mr. Hopkins that the lines of maximum strain and of maximum sliding cannot coincide; indeed, if all the particles be urged by the same force, no matter how strong the pull may be, there will be no tendency of one to slide past the other.

FOOTNOTES:

[A] 'Philosophical Magazine,' Ser. III., vol. xxvi.

[B] 'Proceedings of the Royal Institution,' vol. ii. p. 324.

THE RIPPLE-THEORY OF THE VEINED STRUCTURE.

(29.)

[Sidenote: THEORY STATED.]

[Sidenote: THEORY EXAMINED.]

The assumption of oblique sliding, and the production thereby of the marginal structure, have, however, been fortified by considerations of an ingenious and very interesting kind. "How," I have asked, "can the oblique structure persist across the lines of greatest differential motion throughout the length of the glacier?" But here I am met by another question which at first sight might seem equally unanswerable--"How do ripple-marks on the surface of a flowing river, which are nothing else than lines of differential motion of a low order, cross the river from the sides obliquely, while the direction of greatest differential motion is parallel to the sides?" If I understand aright, this is the main argument of Professor Forbes in favour of his theory of the oblique marginal structure. It is first introduced in a note at page 378 of his 'Travels;' he alludes to it in a letter written the following year; in his paper in the 'Philosophical Transactions' he develops the theory. He there gives drawings of ripple-marks observed in smooth gutters after rain, and which he finds to be inclined to the course of the stream, exactly as the marginal structure is inclined to the side of the glacier. The explanation also embraces the case of an obstacle placed in the centre of a river. "A case," writes Professor Forbes, "parallel to the last mentioned, where a fixed obstacle cleaves a descending stream, and leaves its trace in a fan-shaped tail, is well known in several glaciers, as in that at Ferpecle, and the Glacier de Lys on the south side of Monte Rosa; particularly the last, where the veined structure follows the law just mentioned." In his Twelfth Letter he also refers to the ripples "as exactly corresponding to the position of the icy bands." In his letter to Dr. Whewell, published in the 'Occasional Papers,' page 58, he writes as follows:--"The same is remarkably shown in the case of a stream of water, for instance a mill-race. Although the movement of the water, as shown by floating bodies, is exceedingly nearly (for small velocities sensibly) parallel to the sides, yet the variation of the speed from the side to the centre of the stream occasions a _ripple_, or molecular discontinuity, which inclines forwards from the sides to the centre of the stream at an angle with the axis depending on the ratio of the central and lateral velocity. The veined structure of the ice corresponds to the ripple of the water, a molecular discontinuity whose measure is not comparable to the actual velocity of the ice; and therefore the general movement of the glacier, as indicated by the moraines, remains sensibly parallel to the sides." This theory opens up to us a series of interesting and novel considerations which I think will repay the reader's attention. If the ripples in the water and the veins in the ice be due to the same mechanical cause, when we develop clearly the origin of the former we are led directly to the explanation of the latter. I shall now endeavour to reduce the ripples to their mechanical elements.

The Messrs. Weber have described in their 'Wellenlehre' an effect of wave-motion which it is very easy to obtain. When a boat moves through perfectly smooth water, and the rower raises his oar out of the water, drops trickle from its blade, and each drop where it falls produces a system of concentric rings. The circular waves as they widen become depressed, and, if the drops succeed each other with sufficient speed, the rings cross each other at innumerable points. The effect of this is to blot out more or less completely all the circles, and to leave behind two straight divergent ripple-lines, which are tangents to all the external rings; being in fact formed by the intersections of the latter, as a caustic in optics is formed by the intersection of luminous rays. Fig. 48, which is virtually copied from M. Weber, will render this description at once intelligible. The boat is supposed to move in the direction of the arrow, and as it does so the rings which it leaves behind widen, and produce the divergence of the two straight resultant lines of ripple.

[Sidenote: RIPPLES DEDUCED FROM RINGS.]

[Illustration: Fig. 48. Diagram explanatory of the formation of Ripples.]

The more quickly the drops succeed each other, the more frequent will be the intersections of the rings; but as the speed of succession augments we approach the case of _a continuous vein_ of liquid; and if we suppose the continuity to be perfectly established, the ripples will still be produced with a smooth space between them as before. This experiment may indeed be made with a well-wetted oar, which on its first emergence from the water sends into it a continuous liquid vein. The same effect is produced when we substitute for the stream of liquid a solid rod--a common walking-stick for example. A water-fowl swimming in calm water produces two divergent lines of ripples of a similar kind.

We have here supposed the water of the lake to be at rest, and the liquid vein or the solid rod to move through it; but precisely the same effect is produced if we suppose the rod at rest and the liquid in motion. Let a post, for example, be fixed in the middle of a flowing river; diverging from that post right and left we shall have lines of ripples exactly as if the liquid were at rest and the post moved through it with the velocity of the river. If the same post be placed close to the bank, so that _one_ of its edges only shall act upon the water, diverging from that edge we shall have a _single_ line of ripples which will cross the river obliquely towards its centre. It is manifest that any other obstacle will produce the same effect as our hypothetical post. In the words of Professor Forbes, "the slightest prominence of any kind in the wall of such a conduit, a bit of wood or a tuft of grass, is sufficient to produce a well-marked ripple-streak from the side towards the centre."

[Sidenote: MEASURE OF DIVERGENCE OF RIPPLES.]

The foregoing considerations show that the divergence of the two lines of ripples from the central post, and of the single line in the case of the lateral post, have their mechanical element, if I may use the term, in the experiment of the Messrs. Weber. In the case of a swimming duck the connexion between the diverging lines of ripples and the propagation of rings round a disturbed point is often very prettily shown. When the creature swims with vigour the little foot with which it strikes the water often comes sufficiently near to the surface to produce an elevation,--sometimes indeed emerging from the water altogether. Round the point thus disturbed rings are immediately propagated, and the widening of those rings is _the exact measure of the divergence of the ripple lines_. The rings never cross the lines;--the lines never retreat from the rings.

[Sidenote: RIPPLES AND VEINS DUE TO DIFFERENT CAUSES.]

If we compare the mechanical actions here traced out with those which take place upon a glacier, I think it will be seen that the analogy between the ripples and the veined structure is entirely superficial. How the structure ascribed to the Glacier de Lys is to be explained I do not know, for I have never seen it; but it seems impossible that it could be produced, as ripples are, by a fixed obstacle which "cleaves a descending stream." No one surely will affirm that glacier-ice so closely resembles a fluid as to be capable of transmitting undulations, as water propagates rings round a disturbed point. The difficulty of such a supposition would be augmented by taking into account the motion of the _individual liquid particles_ which go to form a ripple; for the Messrs. Weber have shown that these move in closed curves, describing orbits more or less circular. Can it be supposed that the particles of ice execute a motion of this kind? If so, their orbital motions may be easily calculated, being deducible from the motion of the glacier compounded with the inclination of the veins. If so important a result could be established, all glacier theories would vanish in comparison with it.

[Sidenote: POSITION OF RIPPLES NOT THAT OF STRUCTURE.]

There is another interesting point involved in the passage above quoted. Professor Forbes considers that the ripple is occasioned by the variation of speed from the side to the centre of the stream, and that its _inclination_ depends on the ratio of the central and lateral velocity. If I am correct in the above analysis, this cannot be the case. The inclination of the ripple depends solely on the ratio of the river's translatory motion to the velocity of its wave-motion. Were the lateral and central velocities alike, a momentary disturbance at the side would produce a _straight_ ripple-mark, whose inclination would be compounded of the two elements just mentioned. If the motion of the water vary from side to centre, the velocity of wave-propagation remaining constant, the inclination of the ripple will also vary, that is to say, we shall have a _curved_ ripple instead of a straight one. This, of course, is the case which we find in Nature, but the curvature of such ripples is totally different from that of the veined structure. Owing to the quicker translatory movement, the ripples, as they approach the centre, tend more to parallelism with the direction of the river; and after having passed the centre, and reached the slower water near the opposite side, their inclination to the axis gradually augments. Thus the ripples from the two sides form a pair of symmetric curves, which cross each other at the centre, and possess the form _a o b_, _c o d_, shown in Fig. 49. A similar pair of curves would be produced by the reflection of these. Knowing the variation of motion from side to centre, any competent mathematician could find the equation of the ripple-curves; but it would be out of place for me to attempt it here.

[Illustration: Fig. 49. Diagram explanatory of the formation of Ripples.]

THE VEINED STRUCTURE AND PRESSURE.

(30.)

If a prism of glass be pressed by a sufficient weight, the particles in the line of pressure will be squeezed more closely together, while those at right angles to this line will be forced further apart. The existence of this state of strain may be demonstrated by the action of such squeezed glass upon polarised light. It gives rise to colours, and it is even possible to infer from the tint the precise amount of pressure to which the glass is subjected. M. Wertheim indeed has most ably applied these facts to the construction of a dynamometer, or instrument for measuring pressures, exceeding in accuracy any hitherto devised.

When the pressure applied becomes too great for the glass to sustain, it flies to pieces. But let us suppose the sides of the prism defended by an extremely strong jacket, in which the prism rests like a closely-fitting plug, and which yields only when a pressure more than sufficient to crush the glass is applied. Let the pressure be gradually augmented until this point is attained; afterwards both the glass and its jacket will shorten and widen; the jacket will yield laterally, being pushed out with extreme slowness by the glass within.

[Sidenote: POSSIBLE EXPERIMENT WITH GLASS PRISM.]

Now I believe that it would be possible to make this experiment in such a manner that the glass should be _flattened_, partly through rupture, and partly through lateral molecular yielding; the prism would change its form, and yet present a firmly coherent mass when removed from its jacket. I have never made the experiment; nobody has, as far as I know; but experiments of this kind are often made by Nature. In the Museum of the Government School of Mines, for example, we have a collection of quartz stones placed there by Mr. Salter, and which have been subjected to enormous pressure in the neighbourhood of a fault. These rigid pebbles have, in some cases, been squeezed against each other so as to produce mutual flattening and indentation. Some of them have yielded along planes passing through them, as if one half had slidden over the other; but the reattachment is very strong. Some of the larger stones, moreover, which have endured pressure at a particular point, are fissured radially around this point. In short, the whole collection is a most instructive example of the manner and extent to which one of the most rigid substances in Nature can yield on the application of a sufficient force.

[Sidenote: POSSIBLE EXPERIMENT WITH PRISM OF ICE.]

Let a prism of ice at 32 deg. be placed in a similar jacket to that which we have supposed to envelop the glass prism. The ice yields to the pressure with incomparably greater ease than the glass; and if the force be slowly applied, the lateral yielding will far more closely resemble that of a truly plastic body. Supposing such a piece of ice to be filled with numerous small air-bubbles, the tendency of the pressure would be to flatten these bubbles, and to squeeze them out of the ice. Were the substance perfectly homogeneous, this flattening and expulsion would take place uniformly throughout its entire mass; but I believe there is no such homogeneous substance in nature;--the ice will yield at different places, leaving between them spaces which are comparatively unaffected by the pressure. From the former spaces the air-bubbles will be more effectually expelled; and I have no doubt that the result of such pressure acting upon ice so protected would be to produce a laminated structure somewhat similar to that which it produces in those bodies which exhibit slaty cleavage.

[Sidenote: LAMINATION PRODUCED BY PRESSURE.]

[Sidenote: NO SLIDING OF FILAMENTS.]

I also think it certain that, in this lateral displacement of the

## particles, these must move past each other. This is an idea which I

have long entertained, as the following passage taken from the paper published by Mr. Huxley and myself will prove:--"Three principal causes may operate in producing cleavage: first, the reducing of surfaces of weak cohesion to parallel planes; second, the flattening of minute cavities; and third, the weakening of cohesion by tangential action. The third action is exemplified by the state of the rails near a station where a break is habitually applied to a locomotive. In this case, while the weight of the train presses vertically, its motion tends to cause longitudinal sliding of the particles of the rail. Tangential action does not, however, necessarily imply a force of the latter kind. When a solid cylinder an inch in height is squeezed to a vertical cake a quarter of an inch in height, it is impossible, physically speaking, that the particles situated in the same vertical line shall move laterally with the same velocity; but if they do not, the cohesion between them will be weakened or ruptured. The pressure, however, will produce new contact; and if this have a cohesive value equal to that of the old contact, no cleavage from this cause can arise. The relative capacities of different substances for cleavage appear to depend in a great measure upon their different properties in this respect. In butter, for example, the new attachments are equal, or nearly so, to the old, and the cleavage is consequently indistinct; in wax this does not appear to be the case, and hence may arise in a great degree the perfection of its cleavage. The further examination of this subject promises interesting results." I would dwell upon this point the more distinctly as the advocates of differential motion may deem it to be in their favour; but it appears to me that the mechanical conceptions implied in the above passage are totally different from theirs. If they think otherwise, then it seems to me that they should change the expressions which refer the differential motion to a "drag" towards the centre, and the structure to the sliding of "filaments" past each other in consequence of this drag. Such filamentary sliding may take place in a truly viscous body, but it does not take place in ice.

In one particular the ice resembles the butter referred to in the above quotation; for its new attachments appear to be equal to the old, and this, I think, is to be ascribed to its perfect regelation. As justly pointed out by Mr. John Ball, the veined ice of a glacier, if unweathered, shows no tendency to cleave; for though the expulsion of the air-bubbles has taken place, the reattachment of the particles is so firm as to abolish all evidence of cleavage. When the ice, on the contrary, is weathered, the plates become detached, and I have often been able to split such ice into thin tablets having an area of two or three square feet.

In his Thirteenth Letter Professor Forbes throws out a new and possibly a pregnant thought in connexion with the veins. If I understand him aright--and I confess it is usually a matter of extreme difficulty with me to make sure of this--he there refers the veins, not to the expulsion of the air from the ice, but to its redistribution. The pressure produces "_lines of tearing_ in which the air is distributed in the form of regular globules." I do not know what might be made of this idea if it were developed, but at present I do not see how the supposed action could produce the blue bands; and I agree with Professor Wm. Thomson in regarding the explanation as improbable.[A]

FOOTNOTES:

[A] For an extremely ingenious view of the origin of the veined structure, I would refer to a paper by Professor Thomson, in the 'Proceedings of the Royal Society,' April, 1858.

THE VEINED STRUCTURE AND THE LIQUEFACTION OF ICE BY PRESSURE.

(31.)

I have already noticed an important fact for which we are indebted to Mr. James Thomson, and have referred to the original communications on the subject. I shall here place the physical circumstances connected with this fact before my reader in the manner which I deem most likely to interest him.

[Sidenote: INFLUENCE OF PRESSURE ON BOILING POINT.]

When a liquid is heated, the attraction of the molecules operates against the action of the heat, which tends to tear them asunder. At a certain point the force of heat triumphs, the cohesion is overcome, and the liquid boils. But supposing we assist the attraction of the molecules by applying an external pressure, the difficulty of tearing them asunder will be increased; more heat will be required for this purpose; and hence we say that the _boiling point_ of the liquid has been _elevated_ by the pressure.

[Sidenote: INFLUENCE OF PRESSURE ON FUSING POINT.]

If molten sulphur be poured into a bullet-mould, it will be found on cooling to contract, so as to leave a large hollow space in the middle of each sphere. Cast musket-bullets are thus always found to possess a small cavity within them produced by the contraction of the lead. Conceive the bullet placed within its mould and the latter heated; to produce fusion it is necessary that the sulphur or the lead should _swell_. Here, as in the case of the heated water, the tendency to expand is opposed by the attraction of the molecules; with a certain amount of heat however this attraction is overcome and the solid _melts_. But suppose we assist the molecular attraction by a suitable force applied externally, a greater amount of heat than before will be necessary to tear them asunder; and hence we say that the _fusing point_ has been _elevated_ by the pressure. This fact has been experimentally established by Messrs. Hopkins and Fairbairn, who applied to spermaceti and other substances pressures so great as to raise their points of fusion a considerable number of degrees.

Let us now consider the case of the metal bismuth. If the molten metal be poured into a bullet-mould it will _expand_ on solidifying. I have myself filled a strong cast-iron bottle with the metal, and found its expansion on cooling sufficiently great to split the bottle from neck to bottom. Hence, in order to fuse the bismuth the substance must _contract_; and it is manifest that an external pressure which tends to squeeze the molecules more closely together here _assists_ the heat instead of opposing it. Hence, to fuse bismuth under great pressure, a less amount of heat will be required than when the pressure is removed; or, in other words, the fusing point of bismuth is _lowered_ by the pressure. Now, in passing from the solid to the liquid state, _ice_, like bismuth, contracts, and if the contraction be promoted by external pressure, as shown by the Messrs. Thomson, a less amount of heat suffices to liquefy it.

[Sidenote: EXPERIMENTS.]

These remarks will enable us to understand a singular effect first obtained by myself at the close of 1856 or in January 1857, noticed at the time in the 'Proceedings of the Royal Society,' and afterwards fully described in a paper presented to the Society in December of that year. A cylinder of clear ice two inches high and an inch in diameter was placed between two slabs of box-wood, and subjected to a gradual pressure. I watched the ice in a direction perpendicular to its length, and saw cloudy lines drawing themselves across it. As the pressure continued, these lines augmented in numbers, until finally the prism presented the appearance of a crystal of gypsum whose planes of cleavage had been forced out of optical contact. When looked at obliquely it was found that the lines were merely the sections of flat dim surfaces, which lay like laminae one over the other throughout the length of the prism. Fig. 50 represents the prism as it appeared when looked at in a direction perpendicular to its axis; Fig. 51 shows the appearance when viewed obliquely.[A]

[Illustration: Fig. 50, 51. Appearance of a prism of ice partially liquefied by Pressure.]

At first sight it might appear as if air had intruded itself between the separated surfaces of the ice, and to test this point I placed a cylinder two inches long and an inch wide upright in a copper vessel which was filled with ice-cold water. The ice cylinder rose about half an inch above the surface of the water. Placing the copper vessel on a slab of wood, and a second slab on the top of the cylinder of ice, the latter was subjected to the gradual action of a small hydraulic press. When the hazy surfaces were well developed in the portion of the ice above the water, the cylinder was removed and examined: the planes of rupture extended throughout the entire length of the cylinder, just as if it had been squeezed in air. I subsequently placed the ice in a stout vessel of glass, and squeezed it, as in the last experiment: the surfaces of discontinuity were seen forming _under the liquid_ quite as distinctly as in air.

To prove that the surfaces were due to compression and not to any tearing asunder of the mass by tension, the following experiment was made:--A cylindrical piece of ice, one of whose ends, however, was not parallel to the other, was placed between the slabs of wood, and subjected to pressure. Fig. 52 shows the disposition of the experiment. The effect upon the ice cylinder was that shown in Fig. 53, the surfaces being developed along that side which had suffered the pressure. On examining the surfaces by a pocket lens they resembled the effect produced upon a smooth cold surface by breathing on it.

[Illustration: Fig. 52, 53. Figures illustrative of compression and liquefaction of ice.]

[Sidenote: LIQUID LAYERS PRODUCED BY PRESSURE.]

The surfaces were always dim; and had the spaces been filled with air, or were they simply vacuous, the reflection of light from them would have been so copious as to render them much more brilliant than they were observed to be. To examine them more particularly I placed a concave mirror so as to throw the diffused daylight from a window full upon the cylinder. On applying the pressure dim spots were sometimes seen forming in the very middle of the ice, and these as they expanded laterally appeared to be in a state of intense motion, which followed closely the edge of each surface as it advanced through the solid ice. Once or twice I observed the hazy surfaces pioneered through the mass by dim offshoots, apparently liquid, and constituting a kind of decrystallisation. From the closest examination to which I was able to subject them, the surfaces appeared to me to be due to internal liquefaction; indeed, when the melting point of ice, having already a temperature of 32 deg., is lowered by pressure, its excess of heat must instantly be applied to produce this effect.

[Sidenote: APPLICATION TO THE VEINED STRUCTURE.]

I have already given a drawing (p. 386) showing the development of the veined structure at the base of the ice-cascade of the Rhone; and if we compare that diagram with Fig. 53 a striking similarity at once reveals itself. The ice of the glacier must undoubtedly be liquefied to some extent by the tremendous pressure to which it is here subjected. Surfaces of discontinuity will in all probability be formed, which facilitate the escape of the imprisoned air. The small quantity of water produced will be partly imbibed by the adjacent porous ice, and will be refrozen when relieved from the pressure. This action, associated with that ascribed to pressure in the last section, appears to me to furnish a complete physical explanation of the laminated structure of glacier-ice.

FOOTNOTES:

[A] This effect projected upon a screen is a most striking and instructive class experiment.

WHITE ICE-SEAMS IN THE GLACIER DU GEANT.

(32.)

[Sidenote: GENERAL APPEARANCE OF WHITE ICE-SEAMS.]

On the 28th of July, 1857, while engaged upon the Glacier du Geant, my attention was often attracted by protuberant ridges of what at first appeared to be pure white snow, but which on examination I found to be compact ice filled with innumerable round air-cells; and which, in virtue of its greater power of resistance to wasting, often rose to a height of three or four feet above the general level of the ice. As I stood amongst these ridges, they appeared detached and without order of arrangement, but looked at from a distance they were seen to sweep across the proper Glacier du Geant in a direction concentric with its dirt-bands and its veined structure. In some cases the seams were admirable indications of the relative displacement of two adjacent portions of the glacier, which were divided from each other by a crevasse. Usually the sections of a seam exposed on the opposite sides of a fissure accurately faced each other, and the direction of the seam on both sides was continuous; but at other places they demonstrated the existence of lateral faults, being shifted asunder laterally through spaces varying from a few inches to six or seven feet.

On the following day I was again upon the same glacier, and noticed in many cases the white ice-seams exquisitely honeycombed. The case was illustrative of the great difference between the absorptive power of the ice itself and of the objects which lie upon its surface. Deep cylindrical cells were produced by spots of black dirt which had been scattered upon the surface of the white ice, and which sank to a depth of several inches into the mass. I examined several sections of the veins, and in general I found that their deeper portions blended gradually with the ice on either side of them. But higher up the glacier I found that the veins penetrated only to a limited depth, and did not therefore form an integrant portion of the glacier. Figs. 54 and 55 show the sections of two of the seams which were exposed on the wall of a crevasse at some distance below the great ice-fall of the Glacier du Geant.

[Sidenote: SECTIONS OF SEAMS.]

[Illustration: Fig. 54, 55. Sections of White Ice-seams.]

[Illustration: Fig. 56. Variations in the Dip of the Veined Structure.]

It was at the base of the Talefre cascade that the explanation of these curious seams presented itself to me. In one of my earliest visits to this portion of the glacier I was struck by a singular disposition of the blue veins on the vertical wall of a crevasse. Fig. 56 will illustrate what I saw. The veins, within a short distance, dipped _backward_ and _forward_, like the junctions of stones used to turn an arch. In some cases I found this variation of the structure so great as to pass in a short distance from the vertical to the horizontal, as shown in Fig. 57.

[Sidenote: VARIATIONS IN "DIP" OF STRUCTURE.]

[Illustration: Fig. 57. Variations in the Dip of the Veined Structure.]

Further examination taught me that the glacier here is crumpled in a most singular manner; doubtless by the great pressure to which it is exposed. The following illustration will convey a notion of its aspect: Let one hand be laid flat upon a table, palm downwards, and let the fingers be bent until the space between the first joint and the ends of the fingers is vertical; one of the crumples to which I refer will then be represented. The ice seems bent like the fingers, and the crumples of the glacier are cut by crevasses, which are accurately typified by the spaces between the fingers. Let the second hand now be placed upon the first, as the latter is upon the table, so that the tops of the bent fingers of the second hand shall rest upon the roots of the first: two crumples would thus be formed; a series of such protuberances, with steep fronts, follow each other from the base of the Talefre cascade for some distance downwards.

On Saturday the 1st of August I ascended these rounded terraces in succession, and observed among them an extremely remarkable disposition of the structure. Fig. 58 is a section of a series of three of the crumples, on which the shading lines represent the direction of the blue veins. At the base of each protuberance I found a seam of white ice wedged firmly into the glacier, and _each of the seams marked a place of dislocation of the veins_. The white seams thinned off gradually, and finally vanished where the violent crumpling of the ice disappeared. In Fig. 59 I have sketched the wall of a crevasse, which represents what may be regarded as the incipient crumpling. The undulating line shows the contour of the surface, and the shading lines the veins. It will be observed that the direction of the veins yields in conformity with the undulation of the surface; and an augmentation of the effect would evidently result in the crumples shown in Fig. 58. The appearance of the white seams at those places where a dislocation occurred was, as far as I could observe, invariable; but in a few instances the seams were observed upon the platforms of the terraces, and also upon their slopes. The width of a seam was very irregular, varying from a few inches at some places to three or four feet at others.

[Sidenote: CRUMPLES OF THE TALEFRE.]

[Illustration: Fig. 58. Section of three glacier Crumples.]

[Illustration: Fig. 59. Wall of a crevasse, with incipient crumpling.]

[Sidenote: MOULDS OF WHITE ICE-SEAMS.]

On the 3rd of August I was again at the base of the Talefre cascade, and observed a fact the significance of which had previously escaped me. The rills which ran down the ice-slopes collected at the base of each protuberance into a stream, which, at the time of my visit, had hollowed out for itself a deep channel in the ice. At some places the stream widened, at others its banks of ice approached each other, and rapids were produced; in fact, _the channels of such streams appeared to be the exact moulds of the seams of white ice_.

Instructed thus far, I ascended the Glacier du Geant on the 5th of August, and then observed on the wrinkles of this glacier the same leaning backwards and forwards of the blue veins as I had previously observed upon the Talefre. I also noticed on this day that a seam of white ice would sometimes open out into two branches, which, after remaining for some distance separate, would reunite and thus enclose a little glacier-island. At other places lateral branches were thrown off from the principal seam, thus suggesting the form of a glacier-rivulet which had been fed by tributary branches. On the 7th of August I hunted the seams still farther up the glacier; and found them at one place descending a steep ice-hill, being crossed by other similar bands, which however were far less white and compact. I followed these new bands to their origin, and found it to be a system of crevasses formed at the summit of the hill, some of which were filled with snow. Lower down the crevasses closed, and the snow thus jammed between their walls was converted into white ice. These seams, however, never attained the compactness and prominence of the larger ones which had their origin far higher up. I singled out one of the best of the latter, and traced it through all the dislocation and confusion of the ice, until I found it to terminate in a cavity filled with snow.

This was near the base of the _seracs_, and the streams here were abundant. Comparing the shapes of some of them with that of the ice-bands lower down the glacier, a striking resemblance was observed. Fig. 60 is the plan of a deep-cut channel through which a stream flowed on the day to which I now refer. Fig. 61 is the plan of a seam of white ice sketched on the same day, low down upon the glacier. Instances of this kind might be multiplied; and the result, I think, renders it certain that the white ice-seams referred to are due to the filling up of the channels of glacier-streams by snow during winter, and the subsequent compression of the mass to ice during the descent of the glacier. I have found such seams at the bases of all cascades that I have visited; and in all cases they appear to be due to the same cause. The depth to which they penetrate the glacier must be profound, or the _ablation_ of the ice must be less than what is generally supposed; for the seams formed so high up on the Glacier du Geant may be traced low down upon the trunk-stream of the Mer de Glace.[A]

[Sidenote: STREAMS AND SEAMS.]

[Illustration: Fig. 60. Plan of a Stream on the Glacier du Geant.]

[Illustration: Fig. 61. Plan of a Seam of White Ice on the Glacier du Geant.]

[Sidenote: SCALING OFF BY PRESSURE.]

These observations on the white ice-seams enable us to add an important supplement to what has been stated regarding the origin of the dirt-bands of the Mer de Glace; The protuberances at the base of the cascade are due not only to the toning down of the ridges produced by the transverse fracture of the glacier at the summit of the fall, but they undergo modifications by the pressure locally exerted at its base. The state of things represented in Fig. 57 is plainly due to the partial pushing of one crumple over that next in advance of it. There seems to be a differential motion of the parts of the glacier in the same longitudinal line; showing that upon the general motion of the glacier smaller local motions are superposed. The occurrence of the seams upon the faces of the slopes seems also to prove that the pressure is competent, in some cases, to cause the bases of the protuberances to swell, so that what was once the base of a crumple may subsequently form a portion of its slope. Another interesting fact is also observed where the pressure is violent: the crumples _scale off_, bows of ice being thus formed which usually span the crumples over their most violently compressed portions. I have found this scaling off at the bases of all the cascades which I have visited, and it is plainly due to the pressure exerted at such places upon the ice.

FOOTNOTES:

[A] The more permanent seams may possibly be due to the filling of the profound crevasses of the cascade.

(33.)

[Sidenote: COMPRESSION OF GLACIER DU GEANT.]

Not only at the base of its great cascade, but throughout the greater part of its length, the Glacier du Geant is in a state of longitudinal compression. The meaning of this term will be readily understood: Let two points, for example, be marked upon the axis of the glacier; if these during its descent were drawn wider apart, it would show that the glacier was in a state of longitudinal strain or tension; if they remained at the same distance apart, it would indicate that neither strain nor pressure was exerted; whereas, if the two points approached each other, which could only be by the quicker motion of the hinder one, the existence of longitudinal compression would be thereby demonstrated.

Taking "Le Petit Balmat" with me, to carry my theodolite, I ascended the Glacier du Geant until I came near the place where it is joined by the Glacier des Periades, and whence I observed a patch of fresh green grass upon the otherwise rocky mountain-side. To this point I climbed, and made it the station for my instrument. Choosing a well-defined object at the opposite side of the glacier, I set, on the 9th of August, in the line between this object and the theodolite, three stakes, one in the centre of the glacier, and the other two at opposite sides of the centre and about 100 yards from it. This done, I descended for a quarter of a mile, when I again climbed the flanking rocks, placing my theodolite in a couloir, down which stones are frequently discharged from the end of a secondary glacier which hangs upon the heights above. Here, as before, I fixed three stakes, chiselled a mark upon the granite, so as to enable me to find the place, and regained the ice without accident. A day or two previously we had set out a third line at some distance lower down, and I was thus furnished with a succession of points along the glacier, the relative motions of which would decide whether it was _pressed_ or _stretched_ in the direction of its length. On the 10th of August Mr. Huxley joined us; and on the following day we all set out for the Glacier du Geant, to measure the progress of the stakes which I had fixed there. Hirst remained upon the glacier to measure the displacements; I shouldered the theodolite; and Huxley was my guide to the mountain-side, sounding in advance of me the treacherous-looking snow over which we had to pass.

Calling the central stake of the highest line No. 1, that of the middle line No. 2, and that of the line nearest the Tacul No. 3, the following are the spaces moved over by these three points in twenty-four hours:

Inches. Distances asunder.

No. 1 20.55 } 545 yards. No. 2 15.43 } 487 yards. No. 3 12.75

Here we have the fact which the aspect of the glacier suggested. The first stake moves five inches a day more than the second, and the second nearly three inches a day more than the third. As surmised, therefore, the glacier is in a state of longitudinal compression, whereby a portion of it 1000 yards in length is shortened at the rate of eight inches a day.

[Sidenote: STRUCTURE IN WHITE ICE-SEAMS.]

In accordance with this result, the transverse undulations of the Glacier du Geant, described in the chapter upon Dirt-Bands, _shorten_ as they descend. A series of three of them measured along the axis of the glacier on the 6th of August, 1857, gave the following respective lengths:--955 links, 855 links, 770 links, the shortest undulation being the farthest from the origin of the undulations. This glacier then constitutes a vast ice-press, and enables us to test the explanation which refers the veined structure of the ice to pressure. The glacier itself is transversely laminated, as already stated; and in many cases a structure of extreme definition and beauty is developed in the compressed snow, which constitutes the seams of white ice. In 1857 I discovered a well-developed lenticular structure in some of these seams. In 1858 I again examined them. Clearing away the superficial portions with my axe, I found, drawn through the body of the seams, long lines of blue ice of exquisite definition; in fact, I had never seen the structure so delicately exhibited. The seams, moreover, were developed in portions of the white ice which were near the _centre_ of the glacier, and where consequently filamentous sliding was entirely out of the question.

[Sidenote: PARTIAL SUMMARY.]

## PARTIAL SUMMARY.

1. Glaciers are derived from mountain snow, which has been consolidated to ice by pressure.

2. That pressure is competent to convert snow into ice has been proved by experiment.

3. The power of yielding to pressure diminishes as the mass becomes more compact; but it does not cease even when the substance has attained the compactness which would entitle it to be called ice.

4. When a sufficient depth of snow collects upon the earth's surface, the lower portions are squeezed out by the pressure of the superincumbent mass. If it rests upon a slope it will yield principally in the direction of the slope, and move downwards.

5. In addition to this, the whole mass slides bodily along its inclined bed, and leaves the traces of its sliding on the rocks over which it passes, grinding off their asperities, and marking them with grooves and scratches in the direction of the motion.

6. In this way the deposit of consolidated and unconsolidated snow which covers the higher portions of lofty mountains moves slowly down into an adjacent valley, through which it descends as a true glacier, partly by sliding and partly by the yielding of the mass itself.

7. Several valleys thus filled may unite in a single valley, the tributary glaciers welding themselves together to form a trunk-glacier.

8. Both the main valley and its tributaries are often sinuous, and the tributaries must change their direction to form the trunk; the width of the valley often varies. The glacier is forced through narrow gorges, widening after it has passed them; the centre of the glacier moves more quickly than the sides, and the surface more quickly than the bottom; the point of swiftest motion follows the same law as that observed in the flow of rivers, shifting from one side of the centre to the other as the flexure of the valley changes.

9. These various effects may be reproduced by experiments on small masses of ice. The substance may moreover be moulded into vases and statuettes. Straight bars of it may be bent into rings, or even coiled into knots.

10. Ice, capable of being thus moulded, is practically incapable of being stretched. The condition essential to success is that the

## particles of the ice operated on shall be kept in close contact, so that

when old attachments have been severed new ones may be established.

11. The nearer the ice is to its melting point in temperature, the more easily are the above results obtained; when ice is many degrees below its freezing point it is crushed by pressure to a white powder, and is not capable of being moulded as above.

12. Two pieces of ice at 32 deg. Fahr., with moist surfaces, when placed in contact freeze together to a rigid mass; this is called Regelation.

13. When the attachments of pressed ice are broken, the continuity of the mass is restored by the regelation of the new contiguous surfaces. Regelation also enables two tributary glaciers to weld themselves to form a continuous trunk; thus also the crevasses are mended, and the dislocations of the glacier consequent on descending cascades are repaired. This healing of ruptures extends to the smallest particles of the mass, and it enables us to account for the continued compactness of the ice during the descent of the glacier.

14. The quality of viscosity is practically absent in glacier-ice. Where pressure comes into play the phenomena are suggestive of viscosity, but where tension comes into play the analogy with a viscous body breaks down. When subjected to strain the glacier does not yield by stretching, but by breaking; this is the origin of the crevasses.

15. The crevasses are produced by the mechanical strains to which the glacier is subjected. They are divided into marginal, transverse, and longitudinal crevasses; the first produced by the oblique strain consequent on the quicker motion of the centre; the second by the passage of the glacier over the summit of an incline; the third by pressure from behind and resistance in front, which causes the mass to split at right angles to the pressure [strain?].

16. The moulins are formed by deep cracks intersecting glacier rivulets. The water in descending such cracks scoops out for itself a shaft, sometimes many feet wide, and some hundreds of feet deep, into which the cataract plunges with a sound like thunder. The supply of water is periodically cut off from the moulins by fresh cracks, in which new moulins are formed.

17. The lateral moraines are formed from the debris which loads the glacier along its edges; the medial moraines are formed on a trunk-glacier by the union of the lateral moraines of its tributaries; the terminal moraines are formed from the debris carried by the glacier to its terminus, and there deposited. The number of medial moraines on a trunk glacier is always one less than the number of tributaries.

18. When ordinary lake-ice is intersected by a strong sunbeam it liquefies so as to form flower-shaped figures within the mass; each flower consists of six petals with a vacuous space at the centre; the flowers are always formed parallel to the planes of freezing, and depend on the crystallization of the substance.

19. Innumerable liquid disks, with vacuous spots, are also formed by the solar beams in glacier-ice. These empty spaces have been hitherto mistaken for air-bubbles, the flat form of the disks being erroneously regarded as the result of pressure.

20. These disks are indicators of the intimate constitution of glacier-ice, and they teach us that it is composed of an aggregate of parts with surfaces of crystallization in all possible planes.

21. There are also innumerable small cells in glacier-ice holding air and water; such cells also occur in lake-ice; and here they are due to the melting of the ice in contact with the bubble of air. Experiments are needed on glacier-ice in reference to this point.

22. At a free surface within or without, ice melts with more ease than in the centre of a compact mass. The motion which we call heat is less controlled at a free surface, and it liberates the molecules from the solid condition sooner than when the atoms are surrounded on all sides by other atoms which impede the molecular motion. Regelation is the complementary effect to the above; for here the superficial portions of a mass of ice are made virtually central by the contact of a second mass.

23. The dirt-bands have their origin in the ice-cascades. The glacier, in passing the brow, is transversely fractured; ridges are formed with hollows between them; these transverse hollows are the principal receptacles of the fine debris scattered over the glacier; and after the ridges have been melted away, the dirt remains in successive stripes upon the glacier.

24. The ice of many glaciers is laminated, and when weathered may be cloven into thin plates. In the sound ice the lamination manifests itself in blue stripes drawn through the general whitish mass of the glacier; these blue veins representing portions of ice from which the air-bubbles have been more completely expelled. This is the veined structure of the ice. It is divided into marginal, transverse, and longitudinal structure; which may be regarded as complementary to marginal, longitudinal, and transverse crevasses. The latter are produced by tension, the former by pressure, which acts in two different ways: firstly, the pressure acts upon the ice as it has acted upon rocks which exhibit the lamination technically called cleavage; secondly, it produces partial liquefaction of the ice. The liquid spaces thus formed help the escape of the air from the glacier; and the water produced, being refrozen when the pressure is relieved, helps to form the blue veins.

APPENDIX.

COMPARATIVE VIEW OF THE CLEAVAGE OF CRYSTALS AND SLATE-ROCKS.

A LECTURE DELIVERED AT THE ROYAL INSTITUTION, ON FRIDAY EVENING THE 6TH OF JUNE, 1856.[A]

When the student of physical science has to investigate the character of any natural force, his first care must be to purify it from the mixture of other forces, and thus study its simple action. If, for example, he wishes to know how a mass of water would shape itself, supposing it to be at liberty to follow the bent of its own molecular forces, he must see that these forces have free and undisturbed exercise. We might perhaps refer him to the dew-drop for a solution of the question; but here we have to do, not only with the action of the molecules of the liquid upon each other, but also with the action of gravity upon the mass, which pulls the drop downwards and elongates it. If he would examine the problem in its purity, he must do as Plateau has done, withdraw the liquid mass from the action of gravity, and he would then find the shape of the mass to be perfectly spherical. Natural processes come to us in a mixed manner, and to the uninstructed mind are a mass of unintelligible confusion. Suppose half-a-dozen of the best musical performers to be placed in the same room, each playing his own instrument to perfection: though each individual instrument might be a well-spring of melody, still the mixture of all would produce mere noise. Thus it is with the processes of nature. In nature, mechanical and molecular laws mingle, and create apparent confusion. Their mixture constitutes what may be called the _noise_ of natural laws, and it is the vocation of the man of science to resolve this noise into its components, and thus to detect the "music" in which the foundations of nature are laid.

The necessity of this detachment of one force from all other forces is nowhere more strikingly exhibited than in the phenomena of crystallization. I have here a solution of sulphate of soda. Prolonging the mental vision beyond the boundaries of sense, we see the atoms of that liquid, like squadrons under the eye of an experienced general, arranging themselves into battalions, gathering round a central standard, and forming themselves into solid masses, which after a time assume the visible shape of the crystal which I here hold in my hand. I may, like an ignorant meddler wishing to hasten matters, introduce confusion into this order. I do so by plunging this glass rod into the vessel. The consequent action is not the pure expression of the crystalline forces; the atoms rush together with the confusion of an unorganized mob, and not with the steady accuracy of a disciplined host. Here, also, in this mass of bismuth we have an example of this confused crystallization; but in the crucible behind me a slower process is going on: here there is an architect at work "who makes no chips, no din," and who is now building the particles into crystals, similar in shape and structure to those beautiful masses which we see upon the table. By permitting alum to crystallize in this slow way, we obtain these perfect octahedrons; by allowing carbonate of lime to crystallize, nature produces these beautiful rhomboids; when silica crystallizes, we have formed these hexagonal prisms capped at the ends by pyramids; by allowing saltpetre to crystallize, we have these prismatic masses; and when carbon crystallizes, we have the diamond. If we wish to obtain a perfect crystal, we must allow the molecular forces free play: if the crystallizing mass be permitted to rest upon a surface it will be flattened, and to prevent this a small crystal must be so suspended as to be surrounded on all sides by the liquid, or, if it rest upon the surface, it must be turned daily so as to present all its faces in succession to the working builder. In this way the scientific man nurses these children of his intellect, watches over them with a care worthy of imitation, keeps all influences away which might possibly invade the strict morality of crystalline laws, and finally sees them developed into forms of symmetry and beauty which richly reward the care bestowed upon them.

In building up crystals, these little atomic bricks often arrange themselves into layers which are perfectly parallel to each other, and which can be separated by mechanical means; this is called the cleavage of the crystal. I have here a crystallized mass which has thus far escaped the abrading and disintegrating forces which, sooner or later, determine the fate of sugar-candy. If I am skilful enough, I shall discover that this crystal of sugar cleaves with peculiar facility in one direction. Here, again, I have a mass of rock-salt: I lay my knife upon it, and with a blow cleave it in this direction; but I find on further examining this substance that it cleaves in more directions than one. Laying my knife at right angles to its former position, the crystal cleaves again; and, finally placing the knife at right angles to the two former positions, the mass cleaves again. Thus rock-salt cleaves in three directions, and the resulting solid is this perfect cube, which may be broken up into any number of smaller cubes. Here is a mass of Iceland spar, which also cleaves in three directions, not at right angles, but obliquely to each other, the resulting solid being a rhomboid. In each of these cases the mass cleaves with equal facility in all three directions. For the sake of completeness, I may say that many substances cleave with unequal facility in different directions, and the heavy spar I hold in my hand presents an example of this kind of cleavage.

Turn we now to the consideration of some other phenomena to which the term cleavage may be applied. This piece of beech-wood cleaves with facility parallel to the fibre, and if our experiments were fine enough we should discover that the cleavage is most perfect when the edge of the axe is laid across the rings which mark the growth of the tree. The fibres of the wood lie side by side, and a comparatively small force is sufficient to separate them. If you look at this mass of hay severed from a rick, you will see a sort of cleavage developed in it also; the stalks lie in parallel planes, and only a small force is required to separate them laterally. But we cannot regard the cleavage of the tree as the same in character as the cleavage of the hayrick. In the one case it is the atoms arranging themselves according to organic laws which produce a cleavable structure; in the other case the easy separation in a certain direction is due to the mechanical arrangement of the coarse sensible masses of stalks of hay.

In like manner I find that this piece of sandstone cleaves parallel to the planes of bedding. This rock was once a powder, more or less coarse, held in mechanical suspension by water. The powder was composed of two distinct parts, fine grains of sand and small plates of mica. Imagine a wide strand covered by a tide which holds such powder in suspension:[B] how will it sink? The rounded grains of sand will reach the bottom first, the mica afterwards, and when the tide recedes we have the little plates shining like spangles upon the surface of the sand. Each successive tide brings its charge of mixed powder, deposits its duplex layer day after day, and finally masses of immense thickness are thus piled up, which, by preserving the alternations of sand and mica, tell the tale of their formation. I do not wish you to accept this without proof. Take the sand and mica, mix them together in water, and allow them to subside, they will arrange themselves in the manner I have indicated; and by repeating the process you can actually build up a sandstone mass which shall be the exact counterpart of that presented by nature, as I have done in this glass jar. Now this structure cleaves with readiness along the planes in which the particles of mica are strewn. Here is a mass of such a rock sent to me from Halifax: here are other masses from the quarries of Over Darwen in Lancashire. With a hammer and chisel you see I can cleave them into flags; indeed these flags are made use of for roofing purposes in the districts from which the specimens have come, and receive the name of "slate-stone." But you will discern, without a word from me, that this cleavage is not a crystalline cleavage any more than that of a hayrick is. It is not an arrangement produced by molecular forces; indeed it would be just as reasonable to suppose that in this jar of sand and mica the particles arranged themselves into layers by the forces of crystallization, instead of by the simple force of gravity, as to imagine that such a cleavage as this could be the product of crystallization.

This, so far as I am aware of, has never been imagined, and it has been agreed among geologists not to call such splitting as this cleavage at all, but to restrict the term to a class of phenomena which I shall now proceed to consider.

Those who have visited the slate quarries of Cumberland and North Wales will have witnessed the phenomena to which I refer. We have long drawn our supply of roofing-slates from such quarries; schoolboys ciphered on these slates, they were used for tombstones in churchyards, and for billiard-tables in the metropolis; but not until a comparatively late period did men begin to inquire how their wonderful structure was produced. What is the agency which enables us to split Honister Crag, or the cliffs of Snowdon, into laminae from crown to base? This question is at the present moment one of the greatest difficulties of geologists, and occupies their attention perhaps more than any other. You may wonder at this. Looking into the quarry of Penrhyn, you may be disposed to explain the question as I heard it explained two years ago. "These planes of cleavage," said a friend who stood beside me on the quarry's edge, "are the planes of stratification which have been lifted by some convulsion into an almost vertical position." But this was a great mistake, and indeed here lies the grand difficulty of the problem. These planes of cleavage stand in most cases at a high angle to the bedding. Thanks to Sir Roderick Murchison, who has kindly permitted me the use of specimens from the Museum of Practical Geology (and here I may be permitted to express my acknowledgments to the distinguished staff of that noble establishment, who, instead of considering me an intruder, have welcomed me as a brother), I am able to place the proof of this before you. Here is a mass of slate in which the planes of bedding are distinctly marked; here are the planes of cleavage, and you see that one of them makes a large angle with the other. The cleavage of slates is therefore not a question of stratification, and the problem which we have now to consider is, "By what cause has this cleavage been produced?"

In an able and elaborate essay on this subject in 1835, Professor Sedgwick proposed the theory that cleavage is produced by the action of crystalline or polar forces after the mass has been consolidated. "We may affirm," he says, "that no retreat of the parts, no contraction of dimensions in passing to a solid state can explain such phenomena. They appear to me only resolvable on the supposition that crystalline or polar forces acted upon the whole mass simultaneously in one direction and with adequate force." And again, in another place: "Crystalline forces have rearranged whole mountain-masses, producing a beautiful crystalline cleavage, passing alike through all the strata."[C] The utterance of such a man struck deep, as was natural, into the minds of geologists, and at the present day there are few who do not entertain this view either in whole or in part.[D] The magnificence of the theory, indeed, has in some cases caused speculation to run riot, and we have books published, aye and largely sold, on the action of polar forces and geologic magnetism, which rather astonish those who know something about the subject. According to the theory referred to, miles and miles of the districts of North Wales and Cumberland, comprising huge mountain-masses, are neither more nor less than the parts of a gigantic crystal. These masses of slate were originally fine mud; this mud is composed of the broken and abraded particles of older rocks. It contains silica, alumina, iron, potash, soda, and mica, mixed in sensible masses mechanically together. In the course of ages the mass became consolidated, and the theory before us assumes that afterwards a process of crystallization rearranged the particles and developed in the mass a single plane of crystalline cleavage. With reference to this hypothesis, I will only say that it is a bold stretch of analogies; but still it has done good service: it has drawn attention to the question; right or wrong, a theory thus thoughtfully uttered has its value; it is a dynamic power which operates against intellectual stagnation; and, even by provoking opposition, is eventually of service to the cause of truth. It would, however, have been remarkable, if, among the ranks of geologists themselves, men were not found to seek an explanation of the phenomena in question, which involved a less hardy spring on the part of the speculative faculty than the view to which I have just referred.

The first step in an inquiry of this kind is to put oneself into contact with nature, to seek facts. This has been done, and the labours of Sharpe (the late President of the Geological Society, who, to the loss of science and the sorrow of all who knew him, has so suddenly been taken away from us), Sorby, and others, have furnished us with a body of evidence which reveals to us certain important physical phenomena, associated with the appearance of slaty cleavage, if they have not produced it. The nature of this evidence we will now proceed to consider.

Fossil shells are found in these slate-rocks. I have here several specimens of such shells, occupying various positions with regard to the cleavage planes. They are squeezed, distorted, and crushed. In some cases a flattening of the convex shell occurs, in others the valves are pressed by a force which acted in the plane of their junction, but in all cases the distortion is such as leads to the inference that the rock which contains these shells has been subjected to enormous pressure in a direction at right angles to the planes of cleavage; the shells are all flattened and spread out upon these planes. I hold in my hand a fossil trilobite of normal proportions. Here is a series of fossils of the same creature which have suffered distortion. Some have lain across, some along, and some oblique to the cleavage of the slate in which they are found; in all cases the nature of the distortion is such as required for its production a compressing force acting at right angles to the planes of cleavage. As the creatures lay in the mud in the manner indicated, the jaws of a gigantic vice appear to have closed upon them and squeezed them into the shape you see. As further evidence of the exertion of pressure, let me introduce to your notice a case of contortion which has been adduced by Mr. Sorby. The bedding of the rock shown in this figure[E] was once horizontal; at A we have a deep layer of mud, and at _m n_ a layer of comparatively unyielding gritty material; below that again, at B, we have another layer of the fine mud of which slates are formed. This mass cleaves along the shading lines of the diagram; but look at the shape of the intermediate bed: it is contorted into a serpentine form, and leads irresistibly to the conclusion that the mass has been pressed together at right angles to the planes of cleavage. This action can be experimentally imitated, and I have here a piece of clay in which this is done and the same result produced on a small scale. The amount of compression, indeed, might be roughly estimated by supposing this contorted bed _m n_ to be stretched out, its length measured and compared with the distance _c d_; we find in this way that the yielding of the mass has been considerable.

Let me now direct your attention to another proof of pressure. You see the varying colours which indicate the bedding on this mass of slate. The dark portion, as I have stated, is gritty, and composed of comparatively coarse particles, which, owing to their size, shape, and gravity, sink first and constitute the bottom of each layer. Gradually from bottom to top the coarseness diminishes, and near the upper surface of each layer we have a mass of comparatively fine clean mud. Sometimes this fine mud forms distinct layers in a mass of slate-rock, and it is the mud thus consolidated from which are derived the German razor-stones, so much prized for the sharpening of surgical instruments. I have here an example of such a stone. When a bed is thin, the clean white mud is permitted to rest, as in this case, upon a slab of the coarser slate in contact with it: when the bed is thick, it is cut into slices which are cemented to pieces of ordinary slate, and thus rendered stronger. The mud thus deposited sometimes in layers is, as might be expected, often rolled up into nodular masses, carried forward, and deposited by the rivers from which the slate-mud has subsided. Here, indeed, are such nodules enclosed in sandstone. Everybody who has ciphered upon a school-slate must remember the whitish-green spots which sometimes dotted the surface of the slate; he will remember how his slate-pencil usually slid over such spots as if they were greasy. Now these spots are composed of the finer mud, and they could not, on account of their fineness, _bite_ the pencil like the surrounding gritty portions of the slate. Here is a beautiful example of the spots: you observe them on the cleavage surface in broad patches; but if this mass has been compressed at right angles to the planes of cleavage, ought we to expect the same marks when we look at the edge of the slab? The nodules will be flattened by such pressure, and we ought to see evidence of this flattening when we turn the slate edgeways. Here it is. The section of a nodule is a sharp ellipse with its major axis parallel to the cleavage. There are other examples of the same nature on the table; I have made excursions to the quarries of Wales and Cumberland, and to many of the slate-yards of London, but the same fact invariably appears, and thus we elevate a common experience of our boyhood into evidence of the highest significance as regards one of the most important problems of geology. In examining the magnetism of these slates, I was led to infer that these spots would contain a less amount of iron than the surrounding dark slate. The analysis was made for me by Mr. Hambly in the laboratory of Dr. Percy at the School of Mines. The result which is stated in this Table justifies the conclusion to which I have referred.

_Analysis of Slate._

Purple Slate. Two Analyses. 1. Percentage of iron 5.85 2. " " 6.13 Mean 5.99

Greenish Slate. 1. Percentage of iron 3.24 2. " " 3.12 Mean 3.18

The quantity of iron in the dark slate immediately adjacent to the greenish spot is, according to these analyses, nearly double of the quantity contained in the spot itself. This is about the proportion which the magnetic experiments suggested.

Let me now remind you that the facts which I have brought before you are typical facts--each is the representative of a class. We have seen shells crushed, the unhappy trilobites squeezed, beds contorted, nodules of greenish marl flattened; and all these sources of independent testimony point to one and the same conclusion, namely, that slate-rocks have been subjected to enormous pressure in a direction at right angles to the planes of cleavage.[F]

In reference to Mr. Sorby's contorted bed, I have said that by supposing it to be stretched out and its length measured, it would give us an idea of the amount of yielding of the mass above and below the bed. Such a measurement, however, would not quite give the amount of yielding; and here I would beg your attention to a point, the significance of which has, so far as I am aware of, hitherto escaped attention. I hold in my hand a specimen of slate, with its bedding marked upon it; the lower portions of each bed are composed of a comparatively coarse gritty material, something like what you may suppose this contorted bed to be composed of. Well, I find that the cleavage takes a bend in crossing these gritty portions, and that the tendency of these portions is to cleave more at right angles to the bedding. Look to this diagram: when the forces commenced to act, this intermediate bed, which though comparatively unyielding is not entirely so, suffered longitudinal pressure; as it bent, the pressure became gradually more lateral, and the direction of its cleavage is exactly such as you would infer from a force of this kind--it is neither quite across the bed, nor yet in the same direction as the cleavage of the slate above and below it, but intermediate between the two. Supposing the cleavage to be at right angles to the pressure, this is the direction which it ought to take across these more unyielding strata.

Thus we have established the concurrence of the phenomena of cleavage and pressure--that they accompany each other; but the question still remains, Is this pressure of itself sufficient to account for the cleavage? A single geologist, as far as I am aware, answers boldly in the affirmative. This geologist is Sorby, who has attacked the question in the true spirit of a physical investigator. You remember the cleavage of the flags of Halifax and Over Darwen, which is caused by the interposition of plates of mica between the layers. Mr. Sorby examines the structure of slate-rock, and finds plates of mica to be a constituent. He asks himself, what will be the effect of pressure upon a mass containing such plates confusedly mixed up in it? It will be, he argues--and he argues rightly--to place the plates with their flat surfaces more or less perpendicular to the direction in which the pressure is exerted. He takes scales of the oxide of iron, mixes them with a fine powder, and, on squeezing the mass, finds that the tendency of the scales is to set themselves at right angles to the line of pressure. Now the planes in which these plates arrange themselves will, he contends, be those along which the mass cleaves.

I could show you, by tests of a totally different character from those applied by Mr. Sorby, how true his conclusion is, that the effect of pressure on elongated particles or plates will be such as he describes it. Nevertheless, while knowing this fact, and admiring the ability with which Mr. Sorby has treated this question, I cannot accept his explanation of slate-cleavage. I believe that even if these plates of mica were wholly absent, the cleavage of slate-rocks would be much the same as it is at present.

I will not dwell here upon minor facts,--I will not urge that the perfection of the cleavage bears no relation to the quantity of mica present; but I will come at once to a case which to my mind completely upsets the notion that such plates are a necessary element in the production of cleavage.

Here is a mass of pure white wax: there are no mica particles here; there are no scales of iron, or anything analogous mixed up with the mass. Here is the self-same substance submitted to pressure. I would invite the attention of the eminent geologists whom I see before me to the structure of this mass. No slate ever exhibited so clean a cleavage; it splits into laminae of surpassing tenuity, and proves at a single stroke that pressure is sufficient to produce cleavage, and that this cleavage is independent of the intermixed plates of mica assumed in Mr. Sorby's theory. I have purposely mixed this wax with elongated

## particles, and am unable to say at the present moment that the cleavage

is sensibly affected by their presence,--if anything, I should say they rather impair its fineness and clearness than promote it.

The finer the slate the more perfect will be the resemblance of its cleavage to that of the wax. Compare the surface of the wax with the surface of this slate from Borrodale in Cumberland. You have precisely the same features in both: you see flakes clinging to the surfaces of each, which have been partially torn away by the cleavage of the mass: I entertain the conviction that if any close observer compares these two effects, he will be led to the conclusion that they are the product of a common cause.[G]

But you will ask, how, according to my view, does pressure produce this remarkable result? This may be stated in a very few words.

Nature is everywhere imperfect! The eye is not perfectly achromatic, the colours of the rose and tulip are not pure colours, and the freshest air of our hills has a bit of poison in it. In like manner there is no such thing in nature as a body of perfectly homogeneous structure. I break this clay which seems so intimately mixed, and find that the fracture presents to my eyes innumerable surfaces along which it has given way, and it has yielded along these surfaces because in them the cohesion of the mass is less than elsewhere. I break this marble, and even this wax, and observe the same result: look at the mud at the bottom of a dried pond; look to some of the ungravelled walks in Kensington Gardens on drying after rain,--they are cracked and split, and other circumstances being equal, they crack and split where the cohesion of the mass is least. Take then a mass of partially consolidated mud. Assuredly such a mass is divided and subdivided by surfaces along which the cohesion is comparatively small. Penetrate the mass, and you will see it composed of numberless irregular nodules bounded by surfaces of weak cohesion. Figure to your mind's eye such a mass subjected to pressure,--the mass yields and spreads out in the direction of least resistance;[H] the little nodules become converted into laminae, separated from each other by surfaces of weak cohesion, and the infallible result will be that such a mass will cleave at right angles to the line in which the pressure is exerted.

Further, a mass of dried mud is full of cavities and fissures. If you break dried pipe-clay you see them in great numbers, and there are multitudes of them so small that you cannot see them. I have here a piece of glass in which a bubble was enclosed; by the compression of the glass the bubble is flattened, and the sides of the bubble approach each other so closely as to exhibit the colours of thin plates. A similar flattening of the cavities must take place in squeezed mud, and this must materially facilitate the cleavage of the mass in the direction already indicated.

Although the time at my disposal has not permitted me to develop this thought as far as I could wish, yet for the last twelve months the subject has presented itself to me almost daily under one aspect or another. I have never eaten a biscuit during this period in which an intellectual joy has not been superadded to the more sensual pleasure, for I have remarked in all such cases cleavage developed in the mass by the rolling-pin of the pastrycook or confectioner. I have only to break these cakes, and to look at the fracture, to see the laminated structure of the mass; nay, I have the means of pushing the analogy further: I have here some slate which was subjected to a high temperature during the conflagration of Mr. Scott Russell's premises. I invite you to compare this structure with that of a biscuit; air or vapour within the mass has caused it to swell, and the mechanical structure it reveals is precisely that of a biscuit. I have gone a little into the mysteries of baking while conducting my inquiries on this subject, and have received much instruction from a lady-friend in the manufacture of puff-paste. Here is some paste baked in this house under my own superintendence. The cleavage of our hills is accidental cleavage, but this is cleavage with intention. The volition of the pastrycook has entered into the formation of the mass, and it has been his aim to preserve a series of surfaces of structural weakness, along which the dough divides into layers. Puff-paste must not be handled too much, for then the continuity of the surfaces is broken; it ought to be rolled on a cold slab, to prevent the butter from melting and diffusing itself through the mass, thus rendering it more homogeneous and less liable to split. This is the whole philosophy of puff-paste; it is a grossly exaggerated case of slaty cleavage.

As time passed on, cases multiplied, illustrating the influence of pressure in producing lamination. Mr. Warren De la Rue informs me that he once wished to obtain white-lead in a fine granular state, and to accomplish this he first compressed the mass: the mould was conical, and permitted the mass to spread a little laterally under the pressure. The lamination was as perfect as that of slate, and quite defeated him in his effort to obtain a granular powder. Mr. Brodie, as you are aware, has recently discovered a new kind of graphite: here is the substance in powder, of exquisite fineness. This powder has the peculiarity of clinging together in little confederacies; it cannot be shaken asunder like lycopodium; and when the mass is squeezed, these groups of

## particles flatten, and a perfect cleavage is produced. Mr. Brodie

himself has been kind enough to furnish me with specimens for this evening's lecture. I will cleave them before you: you see they split up into plates which are perpendicular to the line in which the pressure was exerted. This testimony is all the more valuable, as the facts were obtained without any reference whatever to the question of cleavage.

I have here a mass of that singular substance Boghead Cannel. This was once a mass of mud, more or less resembling this one, which I have obtained from a bog in Lancashire. I feel some hesitation in bringing this substance before you, for, as in other cases, so in regard to Boghead Cannel, science--not science, let me not libel it, but the quibbling, litigious, money-loving portion of human nature speaking through the mask of science--has so contrived to split hairs as to render the qualities of the substance somewhat mythical. I shall therefore content myself with showing you how it cleaves, and with expressing my conviction that pressure had a great share in the production of this cleavage.

The principle which I have enunciated is so simple as to be almost trivial; nevertheless, it embraces not only the cases I have mentioned, but, if time permitted, I think I could show you that it takes a much wider range. When iron is taken from the puddling furnace, it is a more or less spongy mass: it is at a welding heat, and at this temperature is submitted to the process of rolling: bright smooth bars such as this are the result of this rolling. But I have said that the mass is more or less spongy or nodular, and, notwithstanding the high heat, these nodules do not perfectly incorporate with their neighbours: what then? You would say that the process of rolling must draw the nodules into fibres--it does so; and here is a mass acted upon by dilute sulphuric acid, which exhibits in a striking manner this fibrous structure. The experiment was made by my friend Dr. Percy, without any reference to the question of cleavage.

Here are other cases of fibrous iron. This fibrous structure is the result of mechanical treatment. Break a mass of ordinary iron and you have a granular fracture; beat the mass, you elongate these granules, and finally render the mass fibrous. Here are pieces of rails along which the wheels of locomotives have slidden; the granules have yielded and become plates; they exfoliate or come off in leaves. All these effects belong, I believe, to the great class of phenomena of which slaty cleavage forms the most prominent example.[I]

Thus, ladies and gentlemen, we have reached the termination of our task. I commenced by exhibiting to you some of the phenomena of crystallization. I have placed before you the facts which are found to be associated with the cleavage of slate-rocks. These facts, as finely expressed by Helmholtz, are so many telescopes to our spiritual vision, by which we can see backward through the night of antiquity, and discern the forces which have been in operation upon the earth's surface

"Ere the lion roared, Or the eagle soared."

From evidence of the most independent and trustworthy character, we come to the conclusion that these slaty masses have been subjected to enormous pressure, and by the sure method of experiment we have shown--and this is the only really new point which has been brought before you--how the pressure is sufficient to produce the cleavage. Expanding our field of view, we find the self-same law, whose footsteps we trace amid the crags of Wales and Cumberland, stretching its ubiquitous fingers into the domain of the pastrycook and ironfounder; nay, a wheel cannot roll over the half-dried mud of our streets without revealing to us more or less of the features of this law. I would say, in conclusion, that the spirit in which this problem has been attacked by geologists indicates the dawning of a new day for their science. The great intellects who have laboured at geology, and who have raised it to its present pitch of grandeur, were compelled to deal with the subject in mass; they had no time to look after details. But the desire for more exact knowledge is increasing; facts are flowing in, which, while they leave untouched the intrinsic wonders of geology, are gradually supplanting by solid truths the uncertain speculations which beset the subject in its infancy. Geologists now aim to imitate, as far as possible, the conditions of nature, and to produce her results; they are approaching more and more to the domain of physics; and I trust the day will soon come when we shall interlace our friendly arms across the common boundary of our sciences, and pursue our respective tasks in a spirit of mutual helpfulness, encouragement, and good-will.

FOOTNOTES:

[A] Referred to in the Introduction.

[B] I merely use this as an illustration; the deposition may have really been due to sediment carried down by rivers. But the action must have been periodic, and the powder duplex.

[C] 'Transactions of the Geological Society,' Ser. ii. vol. iii. p. 477.

[D] In a letter to Sir Charles Lyell, dated from the Cape of Good Hope, February 20, 1836, Sir John Herschel writes as follows:--"If rocks have been so heated as to allow of a commencement of crystallization, that is to say, if they have been heated to a point at which the particles can begin to move amongst themselves, or at least on their own axes, some general law must then determine the position in which these particles will rest on cooling. Probably that position will have some relation to the direction in which the heat escapes. Now when all or a majority of

## particles of the same nature have a general tendency to one position,

that must of course determine a cleavage plane."

[E] Omitted here.

[F] While to my mind the evidence in proof of pressure seems perfectly irresistible, I by no means assert that the manner in which I stated it is incapable of modification. All that I deem important is the fact that pressure has been exerted; and provided this remain firm, the fate of any minor portion of the evidence by which it is here established is of comparatively little moment.

[G] I have usually softened the wax by warming it, kneaded it with the fingers, and pressed it between thick plates of glass previously wetted. At the ordinary summer-temperature the wax is soft, and tears rather than cleaves; on this account I cool my compressed specimens in a mixture of pounded ice and salt, and when thus cooled they split beautifully.

[H] It is scarcely necessary to say that if the mass were squeezed equally in _all_ directions no laminated structure could be produced; it must have room to yield in a lateral direction.

[I] An eminent authority informs me that he believes these surfaces of weak cohesion to be due to the interposition of films of graphite, and not to any tendency of the iron itself to become fibrous: this of course does not in any way militate against the theory which I have ventured to propose. All that the theory requires is surfaces of weak cohesion, however produced, and a change of shape of such surfaces consequent on pressure or rolling.

INDEX.

AEggischhorn, 100, 105.

Agassiz on glacier motion, 270, 310.

Air-bubbles, 359, 376.

Aletsch Glacier, 101. -- --, bedding and structure observed on, 120, 391.

Aletschhorn, cloud iridescences on, 100, 238.

Allalein Glacier, 162.

Alpine climbers, suggestions to, 169.

Alps, winter temperature of, 168.

Altmann's theory of glacier motion, 296.

Ancient glaciers, action of, 99, 141.

Arveiron, arch of, 38, 217.

Atmosphere, permeability of, to radiant heat, 105, 243-247.

Atmospheric refraction, 35.

Avalanche at Saas, 164. --, sound of, explained, 12, 14.

Bakewell, Mr., on motion of Glacier des Bossons, 337.

Balmat, Auguste, 169, 188.

Bedding, lines of, 391.

Bennen, Johann Joseph, 104, 118.

Bergschrund, 98, 325.

"Blower," glacier, 87.

Blue colour of ice, 256. -- -- -- snow, 29, 83, 132, 203. -- -- -- water, 33, 253, 259-262.

Blueness of sky, 22, 174, 257-261.

Blue veins, 376, 381.

Boiling-point, influence of pressure on, 408. -- -- at different altitudes, 105, 106, 113, 120, 129, 175, 190.

Bois, Glacier des, 39, 275, 368.

Brevent, ascent of, 172.

Brocken, Spirit of the, 22, 238.

Bubbles, in ice, 44, 147, 359, 425. -- in snow, 18, 251.

Capillaries of glacier, 335-339.

Cave of ice, 135.

Cavities in ice, 163, 356, 424.

Cells in ice, 147, see Bubbles.

Chamouni, 37. --, difficulties at, 170, 192. -- in winter, 198, 336.

Charmoz, view from, 45, 68, 368.

Charpentier's theory of glacier motion, 296.

Chemical action, rays producing, 240.

Chromatic effects, 235.

Cleavage, 406. -- and stratification distinct, 2, 390, 431. -- caused by pressure, 6, 436. --, contortions of, 9, 59. -- of crystals and slate rocks, lecture on, 427. -- of glaciers, 26, 393, 425-426. -- -- ice, 352, 407. -- -- slate, &c., 1, 430.

"Cleft station," the, 47, 369.

Clouds, formation and dissipation of, 22, 97, 137, 146. --, iridescent, 100, 105, 147, 154, 238. -- on Mont Blanc, 82. -- on Monte Rosa, 124. --, winter, at Montanvert, 208.

Colour answers to pitch, 227.

Colours of sky, 257. --, subjective, 37.

Comet, discovery of, 186.

Compass affected by rocks, 140.

Crepitation of glaciers, 44, 357.

Crevasses, 315 (_marginal_, 318; _transverse_, 320; _longitudinal_, 322), 424. --, first opening of, 317, 327.

Crumples in ice, 174, 415, 419.

Crystallization of ice, 353.

Crystals, cleavage of, 3, 428. -- of snow, 130, 205, 212.

Deafness, artificial, 167.

Differential motion, 395. -- --, Dr. Whewell on, 396.

Diffraction, explanation of, 237.

Dirt-bands, 45, 46, 68, 95, 367, 373. -- --, maps of, 367, 368, 369. -- --, Forbes on, 371. -- --, source of, 369, 425.

Disks in ice, planes of, 163, 358, 425.

Dollfuss, M., hut of, 18, 112.

Dome du Gouter, 68, 75.

Donny, M., on cohesion of liquids, 355.

Echoes, theory of, 15.

Eismeer, the, 13, 362.

Expedition of 1856, Oberland and Tyrol, 9-32. -- -- 1857, Montanvert and Mer de Glace, 33-91. -- -- 1858, Oberland, Valais, and Monte Rosa district, 92-192. -- -- 1859, winter, Chamouni, and Mer de Glace, 195-219.

Faraday, Prof., on Regelation, 351.

Faulberg, cave of, 107.

Fee, glacier of, 165.

Fend, 32.

Finsteraarhorn, 104, 110. --, summit of, 112.

Flowers, liquid, in ice, 147, 354-358, 424.

Forbes, Prof., comparison of glacier to river, 306, 308. -- --, on glacier motion, 272, 304, 308. -- --, on magnetism of rocks, 145. -- --, on veined structure, 379. -- --, viscous theory, 311, 327, 333, 335.

Freezing, planes of, 163, 358, 424.

Frost-bites, 191.

Frozen flowers, 130, 212.

Furgge glacier, structure crossing strata on, 160, 392-394.

Gases, passage of heat through, 243.

Geant, Col du, 50, 173.

Geant, glacier du, 53-57, 280, 369-373. --, measurements on, 419-421. --, motion of, 281, 286. --, white ice seams of, 56, 413.

Gebatsch Alp, 23. --, glacier of, 24, 26.

Geneva, Lake of, 33, 259-262.

Glaciers, ancient, action of, 99, 163. -- "blower," 87. --, capillaries of, 335-339. --, crepitation of, 44, 357. -- d'ecoulement, 301. -- de Lechaud, see Lechaud. -- des Bois, 39, 275, 368. -- du Geant, see Geant. -- du Talefre, see Talefre. --, groovings on, 20, 56, 377. --, measurement of, 276. -- motion, 52, 269-295, 422. -- --, earlier theories of, 296-314. -- --, pressure theory of, 346. --, origin of, 248-252. -- reservoirs, 301. --, ridges on, 42, 55. --, structure of, 136, 148, see Veined structure. -- tables, 44, 265. --, veins of, 54, 376, 381. --, wrinkles on, 370.

Goethe's theory of colours, 258.

Goerner glacier, 120, 138.

Goerner grat, 137, 145.

Goernerhorn glacier, 147, 149.

Grand Plateau, 187.

Grands Mulets, 73, 185.

Graun, 29.

Grimsel, the, 18, 99.

Grindelwald, lower glacier of, 13, 92, 321, 384.

Groovings on glaciers, 20, 56, 377.

Gruener's theory of glacier motion, 296.

Guides of Chamouni, rules of, 60, 170, 192. -- lost in crevasse, 76.

Guyot, M., on veined structure, 378.

Hailstones, conical, 31. --, spherical, 65.

Handeck, waterfall of, 17.

Hasli, valley of, 17, 99.

Heat and light, 223, 239, 241. -- -- work, 328. --, luminous, 241-247. --, mechanical equivalent of, 329. --, obscure, 240. --, passage through gases, 243-245. --, radiant, 239. -- --, permeability of atmosphere to, 105, 243-247. --, radiated, 242. --, specific, 331.

Heisse Platte, the, 13.

Hirst, Mr., measurements on Mer de Glace, 38, 46, 275, 283, 289, 313, 420.

Hochjoch, 32.

Hoechste Spitze of Monte Rosa, 128.

Hopkins, Mr., on crevasses, 318, 383.

Hotel des Neufchatelois, 19, 112, 270.

Hugi on glacier motion, 270.

Huxley, Mr., on glacier capillaries, 338. -- --, on water-cells, 251, 359.

Hydrogen, effect on rays, 253.

Ice, blue colour of, 256. -- cascades, 94, 384, 391. -- cave, 135. -- cells, 147, see Bubbles. -- cones, 266. --, cracking of, 317, 326. --, crystallization of, 353. --, effects of pressure on, 405, 409. --, experiments on, 346. --, friability of, 333. --, liquefaction of, 353, 408. --, liquid flowers in, 354-358, 424. --, Thomson's theory of plasticity of, 340. --, softening of, 333. --, structure of, 136, 148. --, temperature of, 241, 332. --, white, seams of, 56, 413, 421.

Illumination of trees, &c., at sunrise and sunset, 178, 238.

Interference rings, 229. -- spectra, 76, 178, 235, 238.

Iridescent clouds, 100, 105, 147, 154, 238.

Jardin, the, 61, 174.

Joch, the passage of a, 28.

Joule, M., on heat and work, 328.

Jungfrau, the, 11. --, evening near, 106.

Laminated structure, 376, 378, 426.

Lechaud, glacier de, 53, 387. -- -- --, motion of, 60, 286-288.

Lenticular structure, 381.

Light and heat, 223, 239, 241. --, undulation theory of, 224.

Linth, M. Escher de la, 271.

Liquefaction of ice, 353, 408.

Liquid flowers, 147, 354-358, 424.

Magnetic force, 144.

Magnetism of rocks, 140, 143, 145.

Maerjelen See, 101, 119.

Mastic, Bruecke's solution of, 259.

Mattmark See, 162.

Maximum motion, locus of point of, 285, 323.

Mayenwand, summit of, 20, 100, 323.

Mayer, on connexion of heat with work, 328.

Measurement of glaciers, 276.

Mer de Glace, 42-67, 86-90, 173. -- -- --, dirt-bands of the, 367 (seen from Charmoz, 45, 368; from Cleft station, 47, 369; from the Flegere, 367). -- -- --, map of, 53, 264. -- -- --, motion of, 275-293. -- -- --, winter motion of, 294, 343. -- -- --, winter visit to, 195, 206-218.

Milk, cause of blueness of, 261.

Mirage, 36.

Montanvert, 40, 89, 173. -- in winter, 204.

Mont Blanc, first ascent of, 68. -- --, second ascent of, 177. -- --, summit of, 81, 189.

Monte Rosa, first ascent of, 122. -- --, second ascent of, 151. -- --, summit of, 128, 156. -- --, western glacier of, 138, 147. -- --, zones of colour, 154, 238.

Moraines, 263. -- of Talefre, 54, 63, 267, 387.

Motion of glaciers, 52, 269-295, 422.

Moulins, 362, 424. --, depth of, 365. --, motion of, 364.

Necker, letter from, 178.

Neufchatelois, Hotel des, 19, 112, 270.

Neve ice, 249, 251.

Oberland, the, visited, 9-22; 92-120; 390.

Oils, effect of films of, 236.

Person, M., on softening of ice, 333.

Pistol fired on summit of Mont Blanc, 82, 83, 224.

Pitch of musical sounds, 225.

Planes of freezing, 163, 358, 424.

Plasticity of ice, Thomson's theory of, 340.

Polar forces, 4.

Pressure and cleavage, see Cleavage. -- and liquefaction of ice, 340, 408. -- -- veined structure, 404; 147-149, 382-394, 412, 425-426. --, effects of, on boiling point, 408. -- -- -- -- ice, 405, 409. -- theory of glacier motion, 346.

Radiant heat, 105, 239.

Rays, calorific, 240. --, transmission of, 242.

Redness of sunset, 175.

Refraction on lake of Geneva, 35.

Regelation, 347, 351.

Reichenbach fall, 17.

Rendu, comparison of glacier to river, 306. --, measurements of glaciers, 304. --, notice of regelation, 301. -- on conversion of snow into ice, 301. -- on ductility, 298. -- on law of circulation, 300. -- on motion of glaciers, 305. -- on veined structure, 301. -- theory of glaciers, 299.

Rhone at lake of Geneva, 34, 261. -- glacier, 20, 100, 323, 386. -- --, chromatic effects, 21, 238.

Ridges on glaciers, 42, 55.

Riffelhorn, the, 133, 141-145.

Rings, interference, 229. -- round sun, 21, 238.

Ripples deduced from rings, 400.

Ripple theory, Forbes on, 398. -- -- of veined structure, 398. -- waves, movement of, 232.

River and glacier, analogies between, 281-285, 423; 368.

Rocks, magnetism of, 140, 143, 145.

Saas, avalanche at, 164.

Sabine, Gen., on veined structure, 378.

Sand-cones, 266.

Saussure's theory of glacier motion, 52, 296.

Scheuchzer's theory of glacier motion, 296.

Seams, white, in ice, 56, 88, 413, 421.

Sedgwick, Prof., on cleavage, 2-5, 390, 431.

Seracs, 51, 75.

Serpentine, boulders of, 161.

Shadows, coloured, 38.

Sharpe, on slaty cleavage, 5, 432.

Silberhorn, the, 11.

Sky, blueness of, 22, 174, 175. --, colours of, explained, 257.

Slate, cleavage of, 1, 430.

Snow, blue colour of, 29, 132, 203. -- crystals, 130, 205, 212. --, dry, 250. -- line, 29, 248. --, perpetual, 248. --, sound of breaking, 202. -- storm, sound through, 215. --, whiteness of, explained, 250.

Sorby, Mr., on slaty cleavage, 5, 435.

Sound in a vacuum, 224. --, intensity of, 83. --, rate of motion of, 226.

Spectra, interference, 76, 178, 235, 238.

Spectrum, rays of, 240.

Stars, twinkling of, 72, 238.

Stelvio, pass of, 29.

Storm on Grands Mulets, 185. -- -- Mer de Glace, 210.

Strahleck, glacier of, 94, 384. --, passage of, 93, 97.

Strata of ice, 136.

Stratification of neve, 392. -- -- slate, 1, 430.

Structure, doubts regarding, 44, 92, 389. -- of ice, 136, 148, see Veined structure.

Subjective colours, 37.

Summary of glacier theory, 422.

Sun, rings round, 21, 238.

Sunrise at Chamouni, 39. -- and sunset, illumination of trees, &c., at, 178, 238.

Sunset, gorgeous, 184.

Tables, glacier, 44, 265-266.

Tacul, motion of ice-wall at, 289.

Talefre, glacier of, 43, 61-62, 87. --, moraines of, 54, 63, 267, 387.

Temperature, winter, of Alps, 168.

Theodolite, use of, 275.

Theory of cleavage, 5.

Thermometer at Jardin, 174. -- buried on Mont Blanc, 190. -- on Finsteraarhorn, 113.

Thomson, Prof., theory of plasticity, 340. -- -- -- -- regelation, 352.

Twinkling of stars, 72, 238.

Tyrol, the, 23.

Undulation theory of light, 224.

Unteraar, glacier of, 18, 265, 388.

Vacuum in ice-cavities, 163, 356.

Veined structure, 376 (_marginal_, 383; _transverse_, 384; _longitudinal_, 387), 395, 404, 408. -- --, experiments on, 382, 388. -- -- caused by pressure, 147-149, 382-389, 412, 425-426. -- -- crossing strata, 389-394. -- --, Forbes on, 379. -- --, Gen. Sabine on, 378. -- --, M. Guyot on, 378. -- --, ripple theory of, 398.

Viesch, glacier of, 109, 118.

Viscosity, 312, 325, 334, 350, 423.

Water absorbs red rays, 254. --, blue colour of, 254; 33, 259, 261. --, rippling waves of, 232.

Waves, frozen, 43, 55. --, interference of, 231. -- motion, Weber on, 232, 399. -- of sound, 225.

Wengern Alp, 9.

Wetterhorn, echoes of, 15.

White ice, seams of, 56, 57, 88, 413, 421.

Whiteness of ice, 250, 268, 376.

Winter motion of Mer de Glace, 294.

Wrinkles on glacier, 370.

Young, Thomas, theory of light, 224.

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Transcriber's Notes.

The titles from the List of Illustrations were copied to the captions of the figures that otherwise had no caption, for the convenience of the reader.

The "sidenotes" in the main body of the text were originally page headers. They have been moved to a place more fitting for the flow, typically to the head of the appropriate paragraph.

Spelling variants where there was no obviously preferred choice were retained. These include: "Cleft-Station" and "Cleft Station," plus variants; "Cima di Jazzi" and "Cima de Jazzi;" "fanlike" and "fan-like;" "firewood" and "fire-wood;" "Flegere" and "Flegere;" "foreshorten(ed)" and "fore-shorten(ing);" "generalisation" and "generalization;" "judgment" and "judgement;" "Kumm" and "Kumme," which may be the same as "Kamm;" "lime light" and "lime-light;" "realize" and "realise(d);" "recognise" and "recognize(d);" "rearranged" and "re-arranged;" "refrozen" and "re-frozen;" "self-same" and "selfsame;" "semifluid" and "semi-fluid;" "sundial" and "sun-dial;" "Trift" and "Trifti," probably the same glacier; "weatherworn" and "weather-worn."

In the Latin-1 encoded text version, the oe-ligature was replaced by the two separate characters, "oe."

Changed "Hockjoch" to "Hochjoch" on page xi: "passage of the Hochjoch."

Changed "39" to "239" on page xvii, as the page number for chapter 2.

Changed "icefall" to "ice-fall" on page xxvi: "part of ice-fall."

Changed "havresack" to "haversack" on page 71: "my waterproof haversack."

Changed "affluent" to "affluent" on page 98: "Finsteraar affluent."

Changed "184 deg.92" to "184.92 deg." on page 129.

Changed "gulleys" to "gullies" on page 143: "fissures and gullies."

Changed "SNOWSTORM" to "SNOW-STORM" in the sidenote from page 215: "SOUND THROUGH THE SNOW-STORM."

Changed "neutralise" to "neutralize" on page 231: "oppose and neutralize."

Moved the semi-colon inside the double quotes on page 285, around: "corresponding points."

Changed "THOMPSON'S" to "THOMSON'S" in the chapter heading on page 340: "THOMSON'S THEORY."

Changed "last" to "least" in the footnote to page 292: "at least as anxious."

Changed "I" to "It" on page 377: "It was also."

"Die Gletscher der Jetzzeit" on page 393 should probably be "Die Gletscher der Jetztzeit," but was not changed.

Inserted a comma in the index entry for "Aletsch Glacier:" "-- --, bedding."

Inserted a comma in the index entry for "Dirt-bands:" "-- --, maps of."

Changed "Gouter" to "Gouter" in the index entry for "Dome du Gouter."

Changed "Hoch-joch" to "Hochjoch" in its index entry.

Inserted second em-dash in the index entry for "Mont Blanc:" "-- --, second ascent of."

Inserted a comma in the index entry for "Rays:" "--, transmission of."

Inserted a comma in the index entry for "Strahleck:" "--, passage of."

End of Project Gutenberg's The Glaciers of the Alps, by John Tyndall