The explanation which I have given involves three considerations:--The transverse breaking of the glacier on the cascade, and the gradual acc.u.mulation 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, const.i.tute the residue of the air originally entrapped in the interstices of the snow from which it has been derived. Through the general whitish ma.s.s, 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 ma.s.s, 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 creva.s.ses, 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 const.i.tution 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 c.u.mberland, 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 pa.s.sage 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 creva.s.ses 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 ma.s.s.

[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 Aga.s.siz, 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 pa.s.sage 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 ma.s.s, and separated by protruding plates of harder and more transparent ice. It was evident that the ma.s.s 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, gla.s.sy, 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 ma.s.s 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 ma.s.s, 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. Aga.s.siz 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 const.i.tutional 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 subst.i.tuted 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 circ.u.mstanced, 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_, ent.i.tles 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 creva.s.se, 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 ma.s.s 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 creva.s.ses. Its aspect is not that which we should expect from the consolidation of successive beds of mountain snow.

[Ill.u.s.tration: Fig. 39. Veined Structure of the walls of creva.s.ses.]

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 ma.s.s 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: ILl.u.s.tRATIVE 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 ca.n.a.l, 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 ca.n.a.l, 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 ca.n.a.l 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.]

[Ill.u.s.tration: 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 ca.n.a.l 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 a.s.sociated 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 creva.s.ses_. 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 pa.s.sing 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 ill.u.s.tration 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 p.r.o.nounced, 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, r.e.t.a.r.ded by the gentler inclination of the valley, has to bear the thrust of the descending ma.s.s, 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 ill.u.s.tration 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 s.p.a.ces 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 pa.s.s through the _structure-mill_ at the bottom of the fall, and the consequence is that _it is all laminated_.

[Sidenote: STRUCTURE OF RHONE GLACIER.]

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

[Ill.u.s.tration: 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 creva.s.ses, 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.]

[Ill.u.s.tration: Fig. 43. Figure explanatory of Longitudinal Structure.]

The manner in which I have ill.u.s.trated the production of this structure will be understood from Fig. 43. B B are two wooden boxes, communicating by sluice-fronts with two branch ca.n.a.ls, 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.