Another difficulty is the following:--Mr. Thomson will have seen that I

have recorded certain winter measurements made on the Mer de Glace, and

that these measurements show not only that the ice moves at that period

of the year, but that it exhibits those characteristics of motion from

which its plasticity has been inferred; the velocity of the central

portions of the glacier being in round numbers double the velocity of

those near the sides. Had there been any necessity for it, this ratio

might have been augmented by placing the side-stakes closer to the walls

of the glacier. Considering the extreme coldness of the weather which

preceded these measurements, it is a moderate estimate to set down the

temperature of the ice in which my stakes were fixed at 5 deg. Cent. below

zero.

[Sidenote: REQUISITE PRESSURE CALCULATED.]

Let us now endeavour to estimate the pressure existing at the portion of

the glacier where these measurements were made. The height of the

Montanvert above the sea-level is, according to Prof. Forbes, 6300 feet;

that of the Col du Geant, which is the summit of the principal tributary

of the Mer de Glace, is 11,146 feet: deducting the former from the

latter, we find the height of the Col du Geant above the Montanvert to

be 4846 feet.

Now, according to Mr. Thomson's theory and his brother's experiments,

the melting point of ice is lowered .0075 deg. Centigrade for every

atmosphere of pressure; and one atmosphere being equivalent to the

pressure of about thirty-three feet of water, we shall not be over the

truth if we take the height of an equivalent column of glacier-ice, of a

compactness the mean of those which it exhibits upon the Col du Geant

and at the Montanvert respectively, at forty feet. The compactness of

glacier ice is, of course, affected by the air-bubbles contained within

it.

[Sidenote: ACTUAL PRESSURE INSUFFICIENT.]

If, then, the pressure of forty feet of ice lower the melting point

.0075 deg. Centigrade, it follows that the pressure of a column 4846 feet

high will lower it nine-tenths of a degree Centigrade. Supposing, then,

the _unimpeded thrust of the whole glacier, from the Col du Geant

downwards_, to be exerted on the ice at the Montanvert; or, in other

words, supposing the bed of the glacier to be absolutely smooth and

every trace of friction abolished, the utmost the pressure thus obtained

could perform would be to lower the melting point of the Montanvert ice

by the quantity above mentioned. Taking into account the actual state of

things, the friction of the glacier against its sides and bed, the

opposition which the three tributaries encounter in the neck of the

valley at Trelaporte, the resistance encountered in the sinuous valley

through which it passes; and finally, bearing in mind the comparatively

short length of the glacier, which has to bear the thrust, and oppose

the latter by its own friction merely;--I think it will appear evident

that the ice at the Montanvert cannot possibly have its melting point

lowered by pressure more than a small fraction of a degree.

The ice in which my stakes were fixed being -5 deg. Centigrade, according to

Mr. Thomson's calculation and his brother's experiments, it would

require 667 atmospheres of pressure to liquefy it; in other words, it

would require the unimpeded pressure of a column of glacier-ice 26,680

feet high. Did Mont Blanc rise to two and a half times its present

height above the Montanvert, and were the latter place connected with

the summit of the mountain by a continuous glacier with its bed

absolutely smooth, the pressure at the Montanvert would be rather under

that necessary to liquefy the ice on which my winter observations were

made.

[Sidenote: MEASUREMENTS APPLY TO SURFACE.]

If it be urged that, though the temperature near the surface may be

several degrees below the freezing point, the great body of the glacier

does not share this temperature, but is, in all probability, near to

32 deg., my reply is simple. I did not measure the motion of the ice in the

body of the glacier; nobody ever did; my measurements refer to the ice

at and near the surface, and it is this ice which showed the plastic

deportment which the measurements reveal.

Such, then, are some of the considerations which prevent me from

accepting the theory of Mr. Thomson, and I trust they will acquit me of

all desire, to make his theory co-operate with my views. I am, however,

far from considering his deduction the less important because of its

failing to account for the phenomena of glacier motion.

THE PRESSURE-THEORY OF GLACIER-MOTION.

(22.)

[Sidenote: POSSIBLE MOULDING OF ICE.]

Broadly considered, two classes of facts are presented to the

glacier-observer; the one suggestive of viscosity, and the other of the

reverse. The former are seen where _pressure_ comes into play, the

latter where _tension_ is operative. By pressure ice can be moulded to

any shape, while the same ice snaps sharply asunder if subjected to

tension. Were the result worth the labour, ice might be moulded into

vases or statuettes, bent into spiral bars, and, I doubt not, by the

proper application of pressure, a _rope_ of ice might be formed and

coiled into a _knot_. But not one of these experiments, though they

might be a thousandfold more striking than any ever made upon a glacier,

would in the least demonstrate that ice is really a viscous body.

[Illustration: Fig. 30. Moulds used in experiments with ice.]

I have here stated what I believe to be feasible. Let me now refer to

the experiments which have been actually made in illustration of this

point. Two pieces of seasoned box-wood had corresponding cavities

hollowed in them, so that, when one was placed upon the other, a

lenticular space was enclosed. A and B, Fig. 30, represent the pieces

of box-wood with the cavities in plan: C represents their section when

they are placed upon each other.

[Sidenote: ACTUAL MOULDING OF ICE.]

A _sphere_ of ice rather more than sufficient to fill the lenticular

space was placed between the pieces of wood and subjected to the action

of a small hydraulic press. The ice was crushed, but the crushed

fragments soon reattached themselves, and, in a few seconds, a lens of

compact ice was taken from the mould.

[Illustration: Fig. 31. Moulds used in experiments with ice.]

This lens was placed in a cylindrical cavity hollowed out in another

piece of box-wood, and represented at C, Fig. 31; and a flat piece of

the wood was placed over the lens as a cover, as at D. On subjecting the

whole to pressure, the lens broke, as the sphere had done, but the

crushed mass soon re-established its continuity, and in less than half a

minute a compact cake of ice was taken from the mould.

[Illustration: Fig. 32. Moulds used in experiments with ice.]

In the following experiment the ice was subjected to a still severer

test:--A hemispherical cavity was formed in one block of box-wood, and

upon a second block a hemispherical protuberance was turned, smaller

than the cavity, so that, when the latter was placed in the former, a

space of a quarter of an inch existed between the two. Fig. 32

represents a section of the two pieces of box-wood; the brass pins _a_,

_b_, fixed in the slab G H, and entering suitable apertures in the mould

I K, being intended to keep the two surfaces concentric. A lump of ice

being placed in the cavity, the protuberance was brought down upon it,

and the mould subjected to hydraulic pressure: after a short interval

the ice was taken from the mould as a smooth compact _cup_, its crushed

particles having reunited, and established their continuity.

[Sidenote: ICE MOULDED TO CUPS AND RINGS.]

[Illustration: Fig. 33. Moulds used in experiments with ice.]

To make these results more applicable to the bending of glacier-ice, the

following experiments were made:--A block of box-wood, M, Fig. 33, 4

inches long, 3 wide, and 3 deep, had its upper surface slightly curved,

and a groove an inch wide, and about an inch deep, worked into it. A

corresponding plate was prepared, having its under surface part of a

convex cylinder, of the same curvature as the concave surface of the

former piece. When the one slab was placed upon the other, they

presented the appearance represented in section at N. A straight prism

of ice 4 inches long, an inch wide, and a little more than an inch in

depth, was placed in the groove; the upper slab was placed upon it, and

the whole was subjected to the hydraulic press. The prism broke, but,

the quantity of ice being rather more than sufficient to fill the

groove, the pressure soon brought the fragments together and

re-established the continuity of the ice. After a few seconds it was

taken from the mould a bent bar of ice. This bar was afterwards passed

through three other moulds of gradually augmenting curvature, and was

taken from the last of them a _semi-ring_ of compact ice.

The ice, in changing its form from that of one mould to that of another,

was in every instance broken and crushed by the pressure; but suppose

that instead of three moulds three thousand had been used; or, better

still, suppose the curvature of a single mould to change by extremely

slow degrees; the ice would then so gradually change its form that no

rude rupture would be apparent. Practically the ice would behave as a

_plastic_ substance; and indeed this plasticity has been contended for

by M. Agassiz, in opposition to the idea of viscosity. As already

stated, the ice, bruised, and flattened, and bent in the above

experiments, was incapable of being sensibly stretched; it was plastic

to pressure but not to tension.

A quantity of water was always squeezed out of the crushed ice in the

above experiments, and the bruised fragments were intermixed with this

and with air. Minute quantities of both remained in the moulded ice, and

thus rendered it in some degree turbid. Its character, however, as to

continuity may be inferred from the fact that the ice-cup, moulded as

described, held water without the slightest visible leakage.

[Sidenote: SOFTNESS OF ICE DEFINED.]

[Sidenote: PRESSURE AND TENSION.]

Ice at 32 deg. may, as already stated, be crushed with extreme facility, and

glacier-ice with still more readiness than lake-ice: it may also be

scraped with a knife with even greater facility than some kinds of

chalk. In comparison with ice at 100 deg. below the freezing point, it might

be popularly called _soft_. But its softness is not that of paste, or

wax, or treacle, or lava, or honey, or tar. It is the softness of

calcareous spar in comparison with that of rock-crystal; and although

the latter is incomparably harder than the former, I think it will be

conceded that the term viscous would be equally inapplicable to both. My

object here is clearly to define terms, and not permit physical error to

lurk beneath them. How far this ice, with a softness thus defined, when

subjected to the gradual pressures exerted in a glacier, is bruised and

broken, and how far the motion of its parts may approach to that of a

truly viscous body under pressure, I do not know. The critical point

here is that the ice changes its form, and preserves its continuity,

during its motion, in virtue of _external_ force. It remains continuous

whilst it moves, because its particles are kept in juxtaposition by

pressure, and when this external prop is removed, and the ice, subjected

to tension, has to depend solely upon the mobility of its own particles

to preserve its continuity, the analogy with a viscous body instantly

breaks down.[A]

FOOTNOTES:

[A] "Imagine," writes Professor Forbes, "a long narrow trough or canal,

stopped at both ends and filled to a considerable depth with treacle,

honey, tar, or any such viscid fluid. Imagine one end of the trough to

give way, the bottom still remaining horizontal: if the friction of the

fluid against the bottom be greater than the friction against its own

particles, the upper strata will roll over the lower ones, and protrude

in a convex slope, which will be propagated backwards towards the other

or closed end of the trough. Had the matter been quite fluid the whole

would have run out, and spread itself on a level: as it is, it assumes

precisely the conditions which we suppose to exist in a glacier." This

is perfectly definite, and my equally definite opinion is that no

glacier ever exhibited the mechanical effects implied by this

experiment.

REGELATION.

(23.)

[Sidenote: FARADAY'S FIRST EXPERIMENT.]

I was led to the foregoing results by reflecting on an experiment

performed by Mr. Faraday, at a Friday evening meeting of the Royal

Institution, on the 7th of June, 1850, and described in the 'Athenaeum'

and 'Literary Gazette' for the same month. Mr. Faraday then showed that

when two pieces of ice, with moistened surfaces, were placed in contact,

they became cemented together by the freezing of the film of water

between them, while, when the ice was below 32 deg. Fahr., and therefore

_dry_, no effect of the kind could be produced. The freezing was also

found to take place under water; and indeed it occurs even when the

water in which the ice is plunged is as hot as the hand can bear.

A generalisation from this interesting fact led me to conclude that a

bruised mass of ice, if closely confined, must re-cement itself when its

particles are brought into contact by pressure; in fact, the whole of

the experiments above recorded immediately suggested themselves to my

mind as natural deductions from the principle established by Faraday. A

rough preliminary experiment assured me that the deductions would stand

testing; and the construction of the box-wood moulds was the

consequence. We could doubtless mould many solid substances to any

extent by suitable pressure, breaking the attachment of their particles,

and re-establishing a certain continuity by the mere force of cohesion.

With such substances, to which we should never think of applying the

term viscous, we might also imitate the changes of form to which

glaciers are subject: but, superadded to the mere cohesion which here

comes into play, we have, in the case of ice, the actual regelation of

the severed surfaces, and consequently a more perfect solid. In the

Introduction to this book I have referred to the production of slaty

cleavage by pressure; and at a future page I hope to show that the

lamination of the ice of glaciers is due to the same cause; but, as

justly observed by Mr. John Ball, there is no tendency to cleave in the

_sound_ ice of glaciers; in fact, this tendency is obliterated by the

perfect regelation of the severed surfaces.

[Sidenote: RECENT EXPERIMENTS OF FARADAY.]

Mr. Faraday has recently placed pieces of ice, in water, under the

strain of forces tending to pull them apart. When two such pieces touch

at a single point they adhere and move together as a rigid piece; but a

little lateral force carefully applied breaks up this union with a

crackling noise, and a new adhesion occurs which holds the pieces

together in opposition to the force which tends to divide them. Mr.

James Thomson had referred regelation to the cold produced by the

liquefaction of the pressed ice; but in the above experiment all

pressure is not only taken away, but is replaced by tension. Mr. Thomson

also conceives that, when pieces of ice are simply placed together

without intentional pressure, the capillary attraction brings the

pressure of the atmosphere into play; but Mr. Faraday finds that

regelation takes place _in vacuo_. A true viscidity on the part of ice

Mr. Faraday never has observed, and he considers that his recent

experiments support the view originally propounded by himself, namely,

that a particle of water on a surface of ice becomes solid when placed

between two surfaces, because of the increased influence due to their

joint action.

CRYSTALLIZATION AND INTERNAL LIQUEFACTION.

(24.)

[Sidenote: HOW CRYSTALS ARE "NURSED."]

In the Introduction to this 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.]