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

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

have long entertained, as the following passage taken from the paper

published by Mr. Huxley and myself will prove:--"Three principal causes

may operate in producing cleavage: first, the reducing of surfaces of

weak cohesion to parallel planes; second, the flattening of minute

cavities; and third, the weakening of cohesion by tangential action. The

third action is exemplified by the state of the rails near a station

where a break is habitually applied to a locomotive. In this case, while

the weight of the train presses vertically, its motion tends to cause

longitudinal sliding of the particles of the rail. Tangential action

does not, however, necessarily imply a force of the latter kind. When a

solid cylinder an inch in height is squeezed to a vertical cake a

quarter of an inch in height, it is impossible, physically speaking,

that the particles situated in the same vertical line shall move

laterally with the same velocity; but if they do not, the cohesion

between them will be weakened or ruptured. The pressure, however, will

produce new contact; and if this have a cohesive value equal to that of

the old contact, no cleavage from this cause can arise. The relative

capacities of different substances for cleavage appear to depend in a

great measure upon their different properties in this respect. In

butter, for example, the new attachments are equal, or nearly so, to the

old, and the cleavage is consequently indistinct; in wax this does not

appear to be the case, and hence may arise in a great degree the

perfection of its cleavage. The further examination of this subject

promises interesting results." I would dwell upon this point the more

distinctly as the advocates of differential motion may deem it to be in

their favour; but it appears to me that the mechanical conceptions

implied in the above passage are totally different from theirs. If they

think otherwise, then it seems to me that they should change the

expressions which refer the differential motion to a "drag" towards the

centre, and the structure to the sliding of "filaments" past each other

in consequence of this drag. Such filamentary sliding may take place in

a truly viscous body, but it does not take place in ice.

In one particular the ice resembles the butter referred to in the above

quotation; for its new attachments appear to be equal to the old, and

this, I think, is to be ascribed to its perfect regelation. As justly

pointed out by Mr. John Ball, the veined ice of a glacier, if

unweathered, shows no tendency to cleave; for though the expulsion of

the air-bubbles has taken place, the reattachment of the particles is so

firm as to abolish all evidence of cleavage. When the ice, on the

contrary, is weathered, the plates become detached, and I have often

been able to split such ice into thin tablets having an area of two or

three square feet.

In his Thirteenth Letter Professor Forbes throws out a new and possibly

a pregnant thought in connexion with the veins. If I understand him

aright--and I confess it is usually a matter of extreme difficulty with

me to make sure of this--he there refers the veins, not to the expulsion

of the air from the ice, but to its redistribution. The pressure

produces "_lines of tearing_ in which the air is distributed in the form

of regular globules." I do not know what might be made of this idea if

it were developed, but at present I do not see how the supposed action

could produce the blue bands; and I agree with Professor Wm. Thomson in

regarding the explanation as improbable.[A]

FOOTNOTES:

[A] For an extremely ingenious view of the origin of the veined

structure, I would refer to a paper by Professor Thomson, in the

'Proceedings of the Royal Society,' April, 1858.

THE VEINED STRUCTURE AND THE LIQUEFACTION OF ICE BY PRESSURE.

(31.)

I have already noticed an important fact for which we are indebted to

Mr. James Thomson, and have referred to the original communications on

the subject. I shall here place the physical circumstances connected

with this fact before my reader in the manner which I deem most likely

to interest him.

[Sidenote: INFLUENCE OF PRESSURE ON BOILING POINT.]

When a liquid is heated, the attraction of the molecules operates

against the action of the heat, which tends to tear them asunder. At a

certain point the force of heat triumphs, the cohesion is overcome, and

the liquid boils. But supposing we assist the attraction of the

molecules by applying an external pressure, the difficulty of tearing

them asunder will be increased; more heat will be required for this

purpose; and hence we say that the _boiling point_ of the liquid has

been _elevated_ by the pressure.

[Sidenote: INFLUENCE OF PRESSURE ON FUSING POINT.]

If molten sulphur be poured into a bullet-mould, it will be found on

cooling to contract, so as to leave a large hollow space in the middle

of each sphere. Cast musket-bullets are thus always found to possess a

small cavity within them produced by the contraction of the lead.

Conceive the bullet placed within its mould and the latter heated; to

produce fusion it is necessary that the sulphur or the lead should

_swell_. Here, as in the case of the heated water, the tendency to

expand is opposed by the attraction of the molecules; with a certain

amount of heat however this attraction is overcome and the solid

_melts_. But suppose we assist the molecular attraction by a suitable

force applied externally, a greater amount of heat than before will be

necessary to tear them asunder; and hence we say that the _fusing

point_ has been _elevated_ by the pressure. This fact has been

experimentally established by Messrs. Hopkins and Fairbairn, who applied

to spermaceti and other substances pressures so great as to raise their

points of fusion a considerable number of degrees.

Let us now consider the case of the metal bismuth. If the molten metal

be poured into a bullet-mould it will _expand_ on solidifying. I have

myself filled a strong cast-iron bottle with the metal, and found its

expansion on cooling sufficiently great to split the bottle from neck to

bottom. Hence, in order to fuse the bismuth the substance must

_contract_; and it is manifest that an external pressure which tends to

squeeze the molecules more closely together here _assists_ the heat

instead of opposing it. Hence, to fuse bismuth under great pressure, a

less amount of heat will be required than when the pressure is removed;

or, in other words, the fusing point of bismuth is _lowered_ by the

pressure. Now, in passing from the solid to the liquid state, _ice_,

like bismuth, contracts, and if the contraction be promoted by external

pressure, as shown by the Messrs. Thomson, a less amount of heat

suffices to liquefy it.

[Sidenote: EXPERIMENTS.]

These remarks will enable us to understand a singular effect first

obtained by myself at the close of 1856 or in January 1857, noticed at

the time in the 'Proceedings of the Royal Society,' and afterwards fully

described in a paper presented to the Society in December of that year.

A cylinder of clear ice two inches high and an inch in diameter was

placed between two slabs of box-wood, and subjected to a gradual

pressure. I watched the ice in a direction perpendicular to its length,

and saw cloudy lines drawing themselves across it. As the pressure

continued, these lines augmented in numbers, until finally the prism

presented the appearance of a crystal of gypsum whose planes of cleavage

had been forced out of optical contact. When looked at obliquely it was

found that the lines were merely the sections of flat dim surfaces,

which lay like laminae one over the other throughout the length of the

prism. Fig. 50 represents the prism as it appeared when looked at in a

direction perpendicular to its axis; Fig. 51 shows the appearance when

viewed obliquely.[A]

[Illustration: Fig. 50, 51. Appearance of a prism of ice partially

liquefied by Pressure.]

At first sight it might appear as if air had intruded itself between the

separated surfaces of the ice, and to test this point I placed a

cylinder two inches long and an inch wide upright in a copper vessel

which was filled with ice-cold water. The ice cylinder rose about half

an inch above the surface of the water. Placing the copper vessel on a

slab of wood, and a second slab on the top of the cylinder of ice, the

latter was subjected to the gradual action of a small hydraulic press.

When the hazy surfaces were well developed in the portion of the ice

above the water, the cylinder was removed and examined: the planes of

rupture extended throughout the entire length of the cylinder, just as

if it had been squeezed in air. I subsequently placed the ice in a stout

vessel of glass, and squeezed it, as in the last experiment: the

surfaces of discontinuity were seen forming _under the liquid_ quite as

distinctly as in air.

To prove that the surfaces were due to compression and not to any

tearing asunder of the mass by tension, the following experiment was

made:--A cylindrical piece of ice, one of whose ends, however, was not

parallel to the other, was placed between the slabs of wood, and

subjected to pressure. Fig. 52 shows the disposition of the experiment.

The effect upon the ice cylinder was that shown in Fig. 53, the surfaces

being developed along that side which had suffered the pressure. On

examining the surfaces by a pocket lens they resembled the effect

produced upon a smooth cold surface by breathing on it.

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

liquefaction of ice.]

[Sidenote: LIQUID LAYERS PRODUCED BY PRESSURE.]

The surfaces were always dim; and had the spaces been filled with air,

or were they simply vacuous, the reflection of light from them would

have been so copious as to render them much more brilliant than they

were observed to be. To examine them more particularly I placed a

concave mirror so as to throw the diffused daylight from a window full

upon the cylinder. On applying the pressure dim spots were sometimes

seen forming in the very middle of the ice, and these as they expanded

laterally appeared to be in a state of intense motion, which followed

closely the edge of each surface as it advanced through the solid ice.

Once or twice I observed the hazy surfaces pioneered through the mass by

dim offshoots, apparently liquid, and constituting a kind of

decrystallisation. From the closest examination to which I was able to

subject them, the surfaces appeared to me to be due to internal

liquefaction; indeed, when the melting point of ice, having already a

temperature of 32 deg., is lowered by pressure, its excess of heat must

instantly be applied to produce this effect.

[Sidenote: APPLICATION TO THE VEINED STRUCTURE.]

I have already given a drawing (p. 386) showing the development of the

veined structure at the base of the ice-cascade of the Rhone; and if we

compare that diagram with Fig. 53 a striking similarity at once reveals

itself. The ice of the glacier must undoubtedly be liquefied to some

extent by the tremendous pressure to which it is here subjected.

Surfaces of discontinuity will in all probability be formed, which

facilitate the escape of the imprisoned air. The small quantity of water

produced will be partly imbibed by the adjacent porous ice, and will be

refrozen when relieved from the pressure. This action, associated with

that ascribed to pressure in the last section, appears to me to furnish

a complete physical explanation of the laminated structure of

glacier-ice.

FOOTNOTES:

[A] This effect projected upon a screen is a most striking and

instructive class experiment.

WHITE ICE-SEAMS IN THE GLACIER DU GEANT.

(32.)

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

On the 28th of July, 1857, while engaged upon the Glacier du Geant, my

attention was often attracted by protuberant ridges of what at first

appeared to be pure white snow, but which on examination I found to be

compact ice filled with innumerable round air-cells; and which, in

virtue of its greater power of resistance to wasting, often rose to a

height of three or four feet above the general level of the ice. As I

stood amongst these ridges, they appeared detached and without order of

arrangement, but looked at from a distance they were seen to sweep

across the proper Glacier du Geant in a direction concentric with its

dirt-bands and its veined structure. In some cases the seams were

admirable indications of the relative displacement of two adjacent

portions of the glacier, which were divided from each other by a

crevasse. Usually the sections of a seam exposed on the opposite sides

of a fissure accurately faced each other, and the direction of the seam

on both sides was continuous; but at other places they demonstrated the

existence of lateral faults, being shifted asunder laterally through

spaces varying from a few inches to six or seven feet.

On the following day I was again upon the same glacier, and noticed in

many cases the white ice-seams exquisitely honeycombed. The case was

illustrative of the great difference between the absorptive power of the

ice itself and of the objects which lie upon its surface. Deep

cylindrical cells were produced by spots of black dirt which had been

scattered upon the surface of the white ice, and which sank to a depth

of several inches into the mass. I examined several sections of the

veins, and in general I found that their deeper portions blended

gradually with the ice on either side of them. But higher up the glacier

I found that the veins penetrated only to a limited depth, and did not

therefore form an integrant portion of the glacier. Figs. 54 and 55 show

the sections of two of the seams which were exposed on the wall of a

crevasse at some distance below the great ice-fall of the Glacier du

Geant.

[Sidenote: SECTIONS OF SEAMS.]

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

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

It was at the base of the Talefre cascade that the explanation of these

curious seams presented itself to me. In one of my earliest visits to

this portion of the glacier I was struck by a singular disposition of

the blue veins on the vertical wall of a crevasse. Fig. 56 will

illustrate what I saw. The veins, within a short distance, dipped

_backward_ and _forward_, like the junctions of stones used to turn an

arch. In some cases I found this variation of the structure so great as

to pass in a short distance from the vertical to the horizontal, as

shown in Fig. 57.

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

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

Further examination taught me that the glacier here is crumpled in a

most singular manner; doubtless by the great pressure to which it is

exposed. The following illustration will convey a notion of its aspect:

Let one hand be laid flat upon a table, palm downwards, and let the

fingers be bent until the space between the first joint and the ends of

the fingers is vertical; one of the crumples to which I refer will then

be represented. The ice seems bent like the fingers, and the crumples of

the glacier are cut by crevasses, which are accurately typified by the

spaces between the fingers. Let the second hand now be placed upon the

first, as the latter is upon the table, so that the tops of the bent

fingers of the second hand shall rest upon the roots of the first: two

crumples would thus be formed; a series of such protuberances, with

steep fronts, follow each other from the base of the Talefre cascade for

some distance downwards.

On Saturday the 1st of August I ascended these rounded terraces in

succession, and observed among them an extremely remarkable disposition

of the structure. Fig. 58 is a section of a series of three of the

crumples, on which the shading lines represent the direction of the blue

veins. At the base of each protuberance I found a seam of white ice

wedged firmly into the glacier, and _each of the seams marked a place of

dislocation of the veins_. The white seams thinned off gradually, and

finally vanished where the violent crumpling of the ice disappeared. In

Fig. 59 I have sketched the wall of a crevasse, which represents what

may be regarded as the incipient crumpling. The undulating line shows

the contour of the surface, and the shading lines the veins. It will be

observed that the direction of the veins yields in conformity with the

undulation of the surface; and an augmentation of the effect would

evidently result in the crumples shown in Fig. 58. The appearance of the

white seams at those places where a dislocation occurred was, as far as

I could observe, invariable; but in a few instances the seams were

observed upon the platforms of the terraces, and also upon their slopes.

The width of a seam was very irregular, varying from a few inches at

some places to three or four feet at others.

[Sidenote: CRUMPLES OF THE TALEFRE.]

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

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

[Sidenote: MOULDS OF WHITE ICE-SEAMS.]

On the 3rd of August I was again at the base of the Talefre cascade, and

observed a fact the significance of which had previously escaped me. The

rills which ran down the ice-slopes collected at the base of each

protuberance into a stream, which, at the time of my visit, had hollowed

out for itself a deep channel in the ice. At some places the stream

widened, at others its banks of ice approached each other, and rapids

were produced; in fact, _the channels of such streams appeared to be the

exact moulds of the seams of white ice_.

Instructed thus far, I ascended the Glacier du Geant on the 5th of

August, and then observed on the wrinkles of this glacier the same

leaning backwards and forwards of the blue veins as I had previously

observed upon the Talefre. I also noticed on this day that a seam of

white ice would sometimes open out into two branches, which, after

remaining for some distance separate, would reunite and thus enclose a

little glacier-island. At other places lateral branches were thrown off

from the principal seam, thus suggesting the form of a glacier-rivulet

which had been fed by tributary branches. On the 7th of August I hunted

the seams still farther up the glacier; and found them at one place

descending a steep ice-hill, being crossed by other similar bands, which

however were far less white and compact. I followed these new bands to

their origin, and found it to be a system of crevasses formed at the

summit of the hill, some of which were filled with snow. Lower down the

crevasses closed, and the snow thus jammed between their walls was

converted into white ice. These seams, however, never attained the

compactness and prominence of the larger ones which had their origin far

higher up. I singled out one of the best of the latter, and traced it

through all the dislocation and confusion of the ice, until I found it

to terminate in a cavity filled with snow.

This was near the base of the _seracs_, and the streams here were

abundant. Comparing the shapes of some of them with that of the

ice-bands lower down the glacier, a striking resemblance was observed.

Fig. 60 is the plan of a deep-cut channel through which a stream flowed

on the day to which I now refer. Fig. 61 is the plan of a seam of white

ice sketched on the same day, low down upon the glacier. Instances of

this kind might be multiplied; and the result, I think, renders it

certain that the white ice-seams referred to are due to the filling up

of the channels of glacier-streams by snow during winter, and the

subsequent compression of the mass to ice during the descent of the

glacier. I have found such seams at the bases of all cascades that I

have visited; and in all cases they appear to be due to the same cause.

The depth to which they penetrate the glacier must be profound, or the

_ablation_ of the ice must be less than what is generally supposed; for

the seams formed so high up on the Glacier du Geant may be traced low

down upon the trunk-stream of the Mer de Glace.[A]

[Sidenote: STREAMS AND SEAMS.]

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

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

Geant.]

[Sidenote: SCALING OFF BY PRESSURE.]

These observations on the white ice-seams enable us to add an important

supplement to what has been stated regarding the origin of the

dirt-bands of the Mer de Glace; The protuberances at the base of the

cascade are due not only to the toning down of the ridges produced by

the transverse fracture of the glacier at the summit of the fall, but

they undergo modifications by the pressure locally exerted at its base.

The state of things represented in Fig. 57 is plainly due to the partial

pushing of one crumple over that next in advance of it. There seems to

be a differential motion of the parts of the glacier in the same

longitudinal line; showing that upon the general motion of the glacier

smaller local motions are superposed. The occurrence of the seams upon

the faces of the slopes seems also to prove that the pressure is

competent, in some cases, to cause the bases of the protuberances to

swell, so that what was once the base of a crumple may subsequently form

a portion of its slope. Another interesting fact is also observed where

the pressure is violent: the crumples _scale off_, bows of ice being

thus formed which usually span the crumples over their most violently

compressed portions. I have found this scaling off at the bases of all

the cascades which I have visited, and it is plainly due to the pressure

exerted at such places upon the ice.

FOOTNOTES:

[A] The more permanent seams may possibly be due to the filling of the

profound crevasses of the cascade.

(33.)

[Sidenote: COMPRESSION OF GLACIER DU GEANT.]

Not only at the base of its great cascade, but throughout the greater

part of its length, the Glacier du Geant is in a state of longitudinal

compression. The meaning of this term will be readily understood: Let

two points, for example, be marked upon the axis of the glacier; if

these during its descent were drawn wider apart, it would show that the

glacier was in a state of longitudinal strain or tension; if they

remained at the same distance apart, it would indicate that neither

strain nor pressure was exerted; whereas, if the two points approached

each other, which could only be by the quicker motion of the hinder

one, the existence of longitudinal compression would be thereby

demonstrated.

Taking "Le Petit Balmat" with me, to carry my theodolite, I ascended the

Glacier du Geant until I came near the place where it is joined by the

Glacier des Periades, and whence I observed a patch of fresh green grass

upon the otherwise rocky mountain-side. To this point I climbed, and

made it the station for my instrument. Choosing a well-defined object at

the opposite side of the glacier, I set, on the 9th of August, in the

line between this object and the theodolite, three stakes, one in the

centre of the glacier, and the other two at opposite sides of the centre

and about 100 yards from it. This done, I descended for a quarter of a

mile, when I again climbed the flanking rocks, placing my theodolite in

a couloir, down which stones are frequently discharged from the end of a

secondary glacier which hangs upon the heights above. Here, as before, I

fixed three stakes, chiselled a mark upon the granite, so as to enable

me to find the place, and regained the ice without accident. A day or

two previously we had set out a third line at some distance lower down,

and I was thus furnished with a succession of points along the glacier,

the relative motions of which would decide whether it was _pressed_ or

_stretched_ in the direction of its length. On the 10th of August Mr.

Huxley joined us; and on the following day we all set out for the

Glacier du Geant, to measure the progress of the stakes which I had

fixed there. Hirst remained upon the glacier to measure the

displacements; I shouldered the theodolite; and Huxley was my guide to

the mountain-side, sounding in advance of me the treacherous-looking

snow over which we had to pass.

Calling the central stake of the highest line No. 1, that of the middle

line No. 2, and that of the line nearest the Tacul No. 3, the following

are the spaces moved over by these three points in twenty-four hours:

Inches. Distances asunder.

No. 1 20.55

} 545 yards.

No. 2 15.43

} 487 yards.

No. 3 12.75

Here we have the fact which the aspect of the glacier suggested. The

first stake moves five inches a day more than the second, and the second

nearly three inches a day more than the third. As surmised, therefore,

the glacier is in a state of longitudinal compression, whereby a portion

of it 1000 yards in length is shortened at the rate of eight inches a

day.

[Sidenote: STRUCTURE IN WHITE ICE-SEAMS.]

In accordance with this result, the transverse undulations of the

Glacier du Geant, described in the chapter upon Dirt-Bands, _shorten_ as

they descend. A series of three of them measured along the axis of the

glacier on the 6th of August, 1857, gave the following respective

lengths:--955 links, 855 links, 770 links, the shortest undulation being

the farthest from the origin of the undulations. This glacier then

constitutes a vast ice-press, and enables us to test the explanation

which refers the veined structure of the ice to pressure. The glacier

itself is transversely laminated, as already stated; and in many cases a

structure of extreme definition and beauty is developed in the

compressed snow, which constitutes the seams of white ice. In 1857 I

discovered a well-developed lenticular structure in some of these seams.

In 1858 I again examined them. Clearing away the superficial portions

with my axe, I found, drawn through the body of the seams, long lines of

blue ice of exquisite definition; in fact, I had never seen the

structure so delicately exhibited. The seams, moreover, were developed

in portions of the white ice which were near the _centre_ of the

glacier, and where consequently filamentous sliding was entirely out of

the question.

[Sidenote: PARTIAL SUMMARY.]

PARTIAL SUMMARY.

1. Glaciers are derived from mountain snow, which has been consolidated

to ice by pressure.

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

by experiment.

3. The power of yielding to pressure diminishes as the mass becomes more

compact; but it does not cease even when the substance has attained the

compactness which would entitle it to be called ice.

4. When a sufficient depth of snow collects upon the earth's surface,

the lower portions are squeezed out by the pressure of the

superincumbent mass. If it rests upon a slope it will yield principally

in the direction of the slope, and move downwards.

5. In addition to this, the whole mass slides bodily along its inclined

bed, and leaves the traces of its sliding on the rocks over which it

passes, grinding off their asperities, and marking them with grooves and

scratches in the direction of the motion.

6. In this way the deposit of consolidated and unconsolidated snow which

covers the higher portions of lofty mountains moves slowly down into an

adjacent valley, through which it descends as a true glacier, partly by

sliding and partly by the yielding of the mass itself.

7. Several valleys thus filled may unite in a single valley, the

tributary glaciers welding themselves together to form a trunk-glacier.

8. Both the main valley and its tributaries are often sinuous, and the

tributaries must change their direction to form the trunk; the width of

the valley often varies. The glacier is forced through narrow gorges,

widening after it has passed them; the centre of the glacier moves more

quickly than the sides, and the surface more quickly than the bottom;

the point of swiftest motion follows the same law as that observed in

the flow of rivers, shifting from one side of the centre to the other as

the flexure of the valley changes.

9. These various effects may be reproduced by experiments on small

masses of ice. The substance may moreover be moulded into vases and

statuettes. Straight bars of it may be bent into rings, or even coiled

into knots.

10. Ice, capable of being thus moulded, is practically incapable of

being stretched. The condition essential to success is that the

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

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

11. The nearer the ice is to its melting point in temperature, the more

easily are the above results obtained; when ice is many degrees below

its freezing point it is crushed by pressure to a white powder, and is

not capable of being moulded as above.

12. Two pieces of ice at 32 deg. Fahr., with moist surfaces, when placed in

contact freeze together to a rigid mass; this is called Regelation.

13. When the attachments of pressed ice are broken, the continuity of

the mass is restored by the regelation of the new contiguous surfaces.

Regelation also enables two tributary glaciers to weld themselves to

form a continuous trunk; thus also the crevasses are mended, and the

dislocations of the glacier consequent on descending cascades are

repaired. This healing of ruptures extends to the smallest particles of

the mass, and it enables us to account for the continued compactness of

the ice during the descent of the glacier.

14. The quality of viscosity is practically absent in glacier-ice. Where

pressure comes into play the phenomena are suggestive of viscosity, but

where tension comes into play the analogy with a viscous body breaks

down. When subjected to strain the glacier does not yield by stretching,

but by breaking; this is the origin of the crevasses.

15. The crevasses are produced by the mechanical strains to which the

glacier is subjected. They are divided into marginal, transverse, and

longitudinal crevasses; the first produced by the oblique strain

consequent on the quicker motion of the centre; the second by the

passage of the glacier over the summit of an incline; the third by

pressure from behind and resistance in front, which causes the mass to

split at right angles to the pressure [strain?].

16. The moulins are formed by deep cracks intersecting glacier rivulets.

The water in descending such cracks scoops out for itself a shaft,

sometimes many feet wide, and some hundreds of feet deep, into which the

cataract plunges with a sound like thunder. The supply of water is

periodically cut off from the moulins by fresh cracks, in which new

moulins are formed.

17. The lateral moraines are formed from the debris which loads the

glacier along its edges; the medial moraines are formed on a

trunk-glacier by the union of the lateral moraines of its tributaries;

the terminal moraines are formed from the debris carried by the glacier

to its terminus, and there deposited. The number of medial moraines on a

trunk glacier is always one less than the number of tributaries.

18. When ordinary lake-ice is intersected by a strong sunbeam it

liquefies so as to form flower-shaped figures within the mass; each

flower consists of six petals with a vacuous space at the centre; the

flowers are always formed parallel to the planes of freezing, and depend

on the crystallization of the substance.

19. Innumerable liquid disks, with vacuous spots, are also formed by the

solar beams in glacier-ice. These empty spaces have been hitherto

mistaken for air-bubbles, the flat form of the disks being erroneously

regarded as the result of pressure.

20. These disks are indicators of the intimate constitution of

glacier-ice, and they teach us that it is composed of an aggregate of

parts with surfaces of crystallization in all possible planes.

21. There are also innumerable small cells in glacier-ice holding air

and water; such cells also occur in lake-ice; and here they are due to

the melting of the ice in contact with the bubble of air. Experiments

are needed on glacier-ice in reference to this point.

22. At a free surface within or without, ice melts with more ease than

in the centre of a compact mass. The motion which we call heat is less

controlled at a free surface, and it liberates the molecules from the

solid condition sooner than when the atoms are surrounded on all sides

by other atoms which impede the molecular motion. Regelation is the

complementary effect to the above; for here the superficial portions of

a mass of ice are made virtually central by the contact of a second

mass.

23. The dirt-bands have their origin in the ice-cascades. The glacier,

in passing the brow, is transversely fractured; ridges are formed with

hollows between them; these transverse hollows are the principal

receptacles of the fine debris scattered over the glacier; and after the

ridges have been melted away, the dirt remains in successive stripes

upon the glacier.

24. The ice of many glaciers is laminated, and when weathered may be

cloven into thin plates. In the sound ice the lamination manifests

itself in blue stripes drawn through the general whitish mass of the

glacier; these blue veins representing portions of ice from which the

air-bubbles have been more completely expelled. This is the veined

structure of the ice. It is divided into marginal, transverse, and

longitudinal structure; which may be regarded as complementary to

marginal, longitudinal, and transverse crevasses. The latter are

produced by tension, the former by pressure, which acts in two different

ways: firstly, the pressure acts upon the ice as it has acted upon rocks

which exhibit the lamination technically called cleavage; secondly, it

produces partial liquefaction of the ice. The liquid spaces thus formed

help the escape of the air from the glacier; and the water produced,

being refrozen when the pressure is relieved, helps to form the blue

veins.

APPENDIX.

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

A LECTURE DELIVERED AT THE ROYAL INSTITUTION, ON FRIDAY EVENING THE 6TH

OF JUNE, 1856.[A]

When the student of physical science has to investigate the character of

any natural force, his first care must be to purify it from the mixture

of other forces, and thus study its simple action. If, for example, he

wishes to know how a mass of water would shape itself, supposing it to

be at liberty to follow the bent of its own molecular forces, he must

see that these forces have free and undisturbed exercise. We might

perhaps refer him to the dew-drop for a solution of the question; but

here we have to do, not only with the action of the molecules of the

liquid upon each other, but also with the action of gravity upon the

mass, which pulls the drop downwards and elongates it. If he would

examine the problem in its purity, he must do as Plateau has done,

withdraw the liquid mass from the action of gravity, and he would then

find the shape of the mass to be perfectly spherical. Natural processes

come to us in a mixed manner, and to the uninstructed mind are a mass of

unintelligible confusion. Suppose half-a-dozen of the best musical

performers to be placed in the same room, each playing his own

instrument to perfection: though each individual instrument might be a

well-spring of melody, still the mixture of all would produce mere

noise. Thus it is with the processes of nature. In nature, mechanical

and molecular laws mingle, and create apparent confusion. Their mixture

constitutes what may be called the _noise_ of natural laws, and it is

the vocation of the man of science to resolve this noise into its

components, and thus to detect the "music" in which the foundations of

nature are laid.

The necessity of this detachment of one force from all other forces is

nowhere more strikingly exhibited than in the phenomena of

crystallization. I have here a solution of sulphate of soda. Prolonging

the mental vision beyond the boundaries of sense, we see the atoms of

that liquid, like squadrons under the eye of an experienced general,

arranging themselves into battalions, gathering round a central

standard, and forming themselves into solid masses, which after a time

assume the visible shape of the crystal which I here hold in my hand. I

may, like an ignorant meddler wishing to hasten matters, introduce

confusion into this order. I do so by plunging this glass rod into the

vessel. The consequent action is not the pure expression of the

crystalline forces; the atoms rush together with the confusion of an

unorganized mob, and not with the steady accuracy of a disciplined host.

Here, also, in this mass of bismuth we have an example of this confused

crystallization; but in the crucible behind me a slower process is going

on: here there is an architect at work "who makes no chips, no din," and

who is now building the particles into crystals, similar in shape and

structure to those beautiful masses which we see upon the table. By

permitting alum to crystallize in this slow way, we obtain these perfect

octahedrons; by allowing carbonate of lime to crystallize, nature

produces these beautiful rhomboids; when silica crystallizes, we have

formed these hexagonal prisms capped at the ends by pyramids; by

allowing saltpetre to crystallize, we have these prismatic masses; and

when carbon crystallizes, we have the diamond. If we wish to obtain a

perfect crystal, we must allow the molecular forces free play: if the

crystallizing mass be permitted to rest upon a surface it will be

flattened, and to prevent this a small crystal must be so suspended as

to be surrounded on all sides by the liquid, or, if it rest upon the

surface, it must be turned daily so as to present all its faces in

succession to the working builder. In this way the scientific man nurses

these children of his intellect, watches over them with a care worthy of

imitation, keeps all influences away which might possibly invade the

strict morality of crystalline laws, and finally sees them developed

into forms of symmetry and beauty which richly reward the care bestowed

upon them.

In building up crystals, these little atomic bricks often arrange

themselves into layers which are perfectly parallel to each other, and

which can be separated by mechanical means; this is called the cleavage

of the crystal. I have here a crystallized mass which has thus far

escaped the abrading and disintegrating forces which, sooner or later,

determine the fate of sugar-candy. If I am skilful enough, I shall

discover that this crystal of sugar cleaves with peculiar facility in

one direction. Here, again, I have a mass of rock-salt: I lay my knife

upon it, and with a blow cleave it in this direction; but I find on

further examining this substance that it cleaves in more directions than

one. Laying my knife at right angles to its former position, the crystal

cleaves again; and, finally placing the knife at right angles to the

two former positions, the mass cleaves again. Thus rock-salt cleaves in

three directions, and the resulting solid is this perfect cube, which

may be broken up into any number of smaller cubes. Here is a mass of

Iceland spar, which also cleaves in three directions, not at right

angles, but obliquely to each other, the resulting solid being a

rhomboid. In each of these cases the mass cleaves with equal facility in

all three directions. For the sake of completeness, I may say that many

substances cleave with unequal facility in different directions, and the

heavy spar I hold in my hand presents an example of this kind of

cleavage.

Turn we now to the consideration of some other phenomena to which the

term cleavage may be applied. This piece of beech-wood cleaves with

facility parallel to the fibre, and if our experiments were fine enough

we should discover that the cleavage is most perfect when the edge of

the axe is laid across the rings which mark the growth of the tree. The

fibres of the wood lie side by side, and a comparatively small force is

sufficient to separate them. If you look at this mass of hay severed

from a rick, you will see a sort of cleavage developed in it also; the

stalks lie in parallel planes, and only a small force is required to

separate them laterally. But we cannot regard the cleavage of the tree

as the same in character as the cleavage of the hayrick. In the one case

it is the atoms arranging themselves according to organic laws which

produce a cleavable structure; in the other case the easy separation in

a certain direction is due to the mechanical arrangement of the coarse

sensible masses of stalks of hay.

In like manner I find that this piece of sandstone cleaves parallel to

the planes of bedding. This rock was once a powder, more or less coarse,

held in mechanical suspension by water. The powder was composed of two

distinct parts, fine grains of sand and small plates of mica. Imagine a

wide strand covered by a tide which holds such powder in suspension:[B]

how will it sink? The rounded grains of sand will reach the bottom

first, the mica afterwards, and when the tide recedes we have the little

plates shining like spangles upon the surface of the sand. Each

successive tide brings its charge of mixed powder, deposits its duplex

layer day after day, and finally masses of immense thickness are thus

piled up, which, by preserving the alternations of sand and mica, tell

the tale of their formation. I do not wish you to accept this without

proof. Take the sand and mica, mix them together in water, and allow

them to subside, they will arrange themselves in the manner I have

indicated; and by repeating the process you can actually build up a

sandstone mass which shall be the exact counterpart of that presented by

nature, as I have done in this glass jar. Now this structure cleaves

with readiness along the planes in which the particles of mica are

strewn. Here is a mass of such a rock sent to me from Halifax: here are

other masses from the quarries of Over Darwen in Lancashire. With a

hammer and chisel you see I can cleave them into flags; indeed these

flags are made use of for roofing purposes in the districts from which

the specimens have come, and receive the name of "slate-stone." But you

will discern, without a word from me, that this cleavage is not a

crystalline cleavage any more than that of a hayrick is. It is not an

arrangement produced by molecular forces; indeed it would be just as

reasonable to suppose that in this jar of sand and mica the particles

arranged themselves into layers by the forces of crystallization,

instead of by the simple force of gravity, as to imagine that such a

cleavage as this could be the product of crystallization.

This, so far as I am aware of, has never been imagined, and it has been

agreed among geologists not to call such splitting as this cleavage at

all, but to restrict the term to a class of phenomena which I shall now

proceed to consider.

Those who have visited the slate quarries of Cumberland and North Wales

will have witnessed the phenomena to which I refer. We have long drawn

our supply of roofing-slates from such quarries; schoolboys ciphered on

these slates, they were used for tombstones in churchyards, and for

billiard-tables in the metropolis; but not until a comparatively late

period did men begin to inquire how their wonderful structure was

produced. What is the agency which enables us to split Honister Crag, or

the cliffs of Snowdon, into laminae from crown to base? This question is

at the present moment one of the greatest difficulties of geologists,

and occupies their attention perhaps more than any other. You may wonder

at this. Looking into the quarry of Penrhyn, you may be disposed to

explain the question as I heard it explained two years ago. "These

planes of cleavage," said a friend who stood beside me on the quarry's

edge, "are the planes of stratification which have been lifted by some

convulsion into an almost vertical position." But this was a great

mistake, and indeed here lies the grand difficulty of the problem. These

planes of cleavage stand in most cases at a high angle to the bedding.

Thanks to Sir Roderick Murchison, who has kindly permitted me the use of

specimens from the Museum of Practical Geology (and here I may be

permitted to express my acknowledgments to the distinguished staff of

that noble establishment, who, instead of considering me an intruder,

have welcomed me as a brother), I am able to place the proof of this

before you. Here is a mass of slate in which the planes of bedding are

distinctly marked; here are the planes of cleavage, and you see that one

of them makes a large angle with the other. The cleavage of slates is

therefore not a question of stratification, and the problem which we

have now to consider is, "By what cause has this cleavage been

produced?"

In an able and elaborate essay on this subject in 1835, Professor

Sedgwick proposed the theory that cleavage is produced by the action of

crystalline or polar forces after the mass has been consolidated. "We

may affirm," he says, "that no retreat of the parts, no contraction of

dimensions in passing to a solid state can explain such phenomena. They

appear to me only resolvable on the supposition that crystalline or

polar forces acted upon the whole mass simultaneously in one direction

and with adequate force." And again, in another place: "Crystalline

forces have rearranged whole mountain-masses, producing a beautiful

crystalline cleavage, passing alike through all the strata."[C] The

utterance of such a man struck deep, as was natural, into the minds of

geologists, and at the present day there are few who do not entertain

this view either in whole or in part.[D] The magnificence of the theory,

indeed, has in some cases caused speculation to run riot, and we have

books published, aye and largely sold, on the action of polar forces and

geologic magnetism, which rather astonish those who know something about

the subject. According to the theory referred to, miles and miles of the

districts of North Wales and Cumberland, comprising huge

mountain-masses, are neither more nor less than the parts of a gigantic

crystal. These masses of slate were originally fine mud; this mud is

composed of the broken and abraded particles of older rocks. It contains

silica, alumina, iron, potash, soda, and mica, mixed in sensible masses

mechanically together. In the course of ages the mass became

consolidated, and the theory before us assumes that afterwards a process

of crystallization rearranged the particles and developed in the mass a

single plane of crystalline cleavage. With reference to this hypothesis,

I will only say that it is a bold stretch of analogies; but still it has

done good service: it has drawn attention to the question; right or

wrong, a theory thus thoughtfully uttered has its value; it is a dynamic

power which operates against intellectual stagnation; and, even by

provoking opposition, is eventually of service to the cause of truth. It

would, however, have been remarkable, if, among the ranks of geologists

themselves, men were not found to seek an explanation of the phenomena

in question, which involved a less hardy spring on the part of the

speculative faculty than the view to which I have just referred.

The first step in an inquiry of this kind is to put oneself into contact

with nature, to seek facts. This has been done, and the labours of

Sharpe (the late President of the Geological Society, who, to the loss

of science and the sorrow of all who knew him, has so suddenly been

taken away from us), Sorby, and others, have furnished us with a body of

evidence which reveals to us certain important physical phenomena,

associated with the appearance of slaty cleavage, if they have not

produced it. The nature of this evidence we will now proceed to

consider.

Fossil shells are found in these slate-rocks. I have here several

specimens of such shells, occupying various positions with regard to the

cleavage planes. They are squeezed, distorted, and crushed. In some

cases a flattening of the convex shell occurs, in others the valves are

pressed by a force which acted in the plane of their junction, but in

all cases the distortion is such as leads to the inference that the rock

which contains these shells has been subjected to enormous pressure in a

direction at right angles to the planes of cleavage; the shells are all

flattened and spread out upon these planes. I hold in my hand a fossil

trilobite of normal proportions. Here is a series of fossils of the same

creature which have suffered distortion. Some have lain across, some

along, and some oblique to the cleavage of the slate in which they are

found; in all cases the nature of the distortion is such as required for

its production a compressing force acting at right angles to the planes

of cleavage. As the creatures lay in the mud in the manner indicated,

the jaws of a gigantic vice appear to have closed upon them and squeezed

them into the shape you see. As further evidence of the exertion of

pressure, let me introduce to your notice a case of contortion which

has been adduced by Mr. Sorby. The bedding of the rock shown in this

figure[E] was once horizontal; at A we have a deep layer of mud, and at

_m n_ a layer of comparatively unyielding gritty material; below that

again, at B, we have another layer of the fine mud of which slates are

formed. This mass cleaves along the shading lines of the diagram; but

look at the shape of the intermediate bed: it is contorted into a

serpentine form, and leads irresistibly to the conclusion that the mass

has been pressed together at right angles to the planes of cleavage.

This action can be experimentally imitated, and I have here a piece of

clay in which this is done and the same result produced on a small

scale. The amount of compression, indeed, might be roughly estimated by

supposing this contorted bed _m n_ to be stretched out, its length

measured and compared with the distance _c d_; we find in this way that

the yielding of the mass has been considerable.

Let me now direct your attention to another proof of pressure. You see

the varying colours which indicate the bedding on this mass of slate.

The dark portion, as I have stated, is gritty, and composed of

comparatively coarse particles, which, owing to their size, shape, and

gravity, sink first and constitute the bottom of each layer. Gradually

from bottom to top the coarseness diminishes, and near the upper surface

of each layer we have a mass of comparatively fine clean mud. Sometimes

this fine mud forms distinct layers in a mass of slate-rock, and it is

the mud thus consolidated from which are derived the German

razor-stones, so much prized for the sharpening of surgical instruments.

I have here an example of such a stone. When a bed is thin, the clean

white mud is permitted to rest, as in this case, upon a slab of the

coarser slate in contact with it: when the bed is thick, it is cut into

slices which are cemented to pieces of ordinary slate, and thus rendered

stronger. The mud thus deposited sometimes in layers is, as might be

expected, often rolled up into nodular masses, carried forward, and

deposited by the rivers from which the slate-mud has subsided. Here,

indeed, are such nodules enclosed in sandstone. Everybody who has

ciphered upon a school-slate must remember the whitish-green spots which

sometimes dotted the surface of the slate; he will remember how his

slate-pencil usually slid over such spots as if they were greasy. Now

these spots are composed of the finer mud, and they could not, on

account of their fineness, _bite_ the pencil like the surrounding gritty

portions of the slate. Here is a beautiful example of the spots: you

observe them on the cleavage surface in broad patches; but if this mass

has been compressed at right angles to the planes of cleavage, ought we

to expect the same marks when we look at the edge of the slab? The

nodules will be flattened by such pressure, and we ought to see evidence

of this flattening when we turn the slate edgeways. Here it is. The

section of a nodule is a sharp ellipse with its major axis parallel to

the cleavage. There are other examples of the same nature on the table;

I have made excursions to the quarries of Wales and Cumberland, and to

many of the slate-yards of London, but the same fact invariably appears,

and thus we elevate a common experience of our boyhood into evidence of

the highest significance as regards one of the most important problems

of geology. In examining the magnetism of these slates, I was led to

infer that these spots would contain a less amount of iron than the

surrounding dark slate. The analysis was made for me by Mr. Hambly in

the laboratory of Dr. Percy at the School of Mines. The result which is

stated in this Table justifies the conclusion to which I have referred.

_Analysis of Slate._

Purple Slate. Two Analyses.

1. Percentage of iron 5.85

2. " " 6.13

Mean 5.99

Greenish Slate.

1. Percentage of iron 3.24

2. " " 3.12

Mean 3.18

The quantity of iron in the dark slate immediately adjacent to the

greenish spot is, according to these analyses, nearly double of the

quantity contained in the spot itself. This is about the proportion

which the magnetic experiments suggested.

Let me now remind you that the facts which I have brought before you are

typical facts--each is the representative of a class. We have seen

shells crushed, the unhappy trilobites squeezed, beds contorted, nodules

of greenish marl flattened; and all these sources of independent

testimony point to one and the same conclusion, namely, that slate-rocks

have been subjected to enormous pressure in a direction at right angles

to the planes of cleavage.[F]

In reference to Mr. Sorby's contorted bed, I have said that by

supposing it to be stretched out and its length measured, it would give

us an idea of the amount of yielding of the mass above and below the

bed. Such a measurement, however, would not quite give the amount of

yielding; and here I would beg your attention to a point, the

significance of which has, so far as I am aware of, hitherto escaped

attention. I hold in my hand a specimen of slate, with its bedding

marked upon it; the lower portions of each bed are composed of a

comparatively coarse gritty material, something like what you may

suppose this contorted bed to be composed of. Well, I find that the

cleavage takes a bend in crossing these gritty portions, and that the

tendency of these portions is to cleave more at right angles to the

bedding. Look to this diagram: when the forces commenced to act, this

intermediate bed, which though comparatively unyielding is not entirely

so, suffered longitudinal pressure; as it bent, the pressure became

gradually more lateral, and the direction of its cleavage is exactly

such as you would infer from a force of this kind--it is neither quite

across the bed, nor yet in the same direction as the cleavage of the

slate above and below it, but intermediate between the two. Supposing

the cleavage to be at right angles to the pressure, this is the

direction which it ought to take across these more unyielding strata.

Thus we have established the concurrence of the phenomena of cleavage

and pressure--that they accompany each other; but the question still

remains, Is this pressure of itself sufficient to account for the

cleavage? A single geologist, as far as I am aware, answers boldly in

the affirmative. This geologist is Sorby, who has attacked the question

in the true spirit of a physical investigator. You remember the cleavage

of the flags of Halifax and Over Darwen, which is caused by the

interposition of plates of mica between the layers. Mr. Sorby examines

the structure of slate-rock, and finds plates of mica to be a

constituent. He asks himself, what will be the effect of pressure upon a

mass containing such plates confusedly mixed up in it? It will be, he

argues--and he argues rightly--to place the plates with their flat

surfaces more or less perpendicular to the direction in which the

pressure is exerted. He takes scales of the oxide of iron, mixes them

with a fine powder, and, on squeezing the mass, finds that the tendency

of the scales is to set themselves at right angles to the line of

pressure. Now the planes in which these plates arrange themselves will,

he contends, be those along which the mass cleaves.

I could show you, by tests of a totally different character from those

applied by Mr. Sorby, how true his conclusion is, that the effect of

pressure on elongated particles or plates will be such as he describes

it. Nevertheless, while knowing this fact, and admiring the ability with

which Mr. Sorby has treated this question, I cannot accept his

explanation of slate-cleavage. I believe that even if these plates of

mica were wholly absent, the cleavage of slate-rocks would be much the

same as it is at present.

I will not dwell here upon minor facts,--I will not urge that the

perfection of the cleavage bears no relation to the quantity of mica

present; but I will come at once to a case which to my mind completely

upsets the notion that such plates are a necessary element in the

production of cleavage.

Here is a mass of pure white wax: there are no mica particles here;

there are no scales of iron, or anything analogous mixed up with the

mass. Here is the self-same substance submitted to pressure. I would

invite the attention of the eminent geologists whom I see before me to

the structure of this mass. No slate ever exhibited so clean a cleavage;

it splits into laminae of surpassing tenuity, and proves at a single

stroke that pressure is sufficient to produce cleavage, and that this

cleavage is independent of the intermixed plates of mica assumed in Mr.

Sorby's theory. I have purposely mixed this wax with elongated

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

is sensibly affected by their presence,--if anything, I should say they

rather impair its fineness and clearness than promote it.

The finer the slate the more perfect will be the resemblance of its

cleavage to that of the wax. Compare the surface of the wax with the

surface of this slate from Borrodale in Cumberland. You have precisely

the same features in both: you see flakes clinging to the surfaces of

each, which have been partially torn away by the cleavage of the mass: I

entertain the conviction that if any close observer compares these two

effects, he will be led to the conclusion that they are the product of a

common cause.[G]

But you will ask, how, according to my view, does pressure produce this

remarkable result? This may be stated in a very few words.

Nature is everywhere imperfect! The eye is not perfectly achromatic, the

colours of the rose and tulip are not pure colours, and the freshest air

of our hills has a bit of poison in it. In like manner there is no such

thing in nature as a body of perfectly homogeneous structure. I break

this clay which seems so intimately mixed, and find that the fracture

presents to my eyes innumerable surfaces along which it has given way,

and it has yielded along these surfaces because in them the cohesion of

the mass is less than elsewhere. I break this marble, and even this wax,

and observe the same result: look at the mud at the bottom of a dried

pond; look to some of the ungravelled walks in Kensington Gardens on

drying after rain,--they are cracked and split, and other circumstances

being equal, they crack and split where the cohesion of the mass is

least. Take then a mass of partially consolidated mud. Assuredly such a

mass is divided and subdivided by surfaces along which the cohesion is

comparatively small. Penetrate the mass, and you will see it composed of

numberless irregular nodules bounded by surfaces of weak cohesion.

Figure to your mind's eye such a mass subjected to pressure,--the mass

yields and spreads out in the direction of least resistance;[H] the

little nodules become converted into laminae, separated from each other

by surfaces of weak cohesion, and the infallible result will be that

such a mass will cleave at right angles to the line in which the

pressure is exerted.

Further, a mass of dried mud is full of cavities and fissures. If you

break dried pipe-clay you see them in great numbers, and there are

multitudes of them so small that you cannot see them. I have here a

piece of glass in which a bubble was enclosed; by the compression of the

glass the bubble is flattened, and the sides of the bubble approach each

other so closely as to exhibit the colours of thin plates. A similar

flattening of the cavities must take place in squeezed mud, and this

must materially facilitate the cleavage of the mass in the direction

already indicated.

Although the time at my disposal has not permitted me to develop this

thought as far as I could wish, yet for the last twelve months the

subject has presented itself to me almost daily under one aspect or

another. I have never eaten a biscuit during this period in which an

intellectual joy has not been superadded to the more sensual pleasure,

for I have remarked in all such cases cleavage developed in the mass by

the rolling-pin of the pastrycook or confectioner. I have only to break

these cakes, and to look at the fracture, to see the laminated structure

of the mass; nay, I have the means of pushing the analogy further: I

have here some slate which was subjected to a high temperature during

the conflagration of Mr. Scott Russell's premises. I invite you to

compare this structure with that of a biscuit; air or vapour within the

mass has caused it to swell, and the mechanical structure it reveals is

precisely that of a biscuit. I have gone a little into the mysteries of

baking while conducting my inquiries on this subject, and have received

much instruction from a lady-friend in the manufacture of puff-paste.

Here is some paste baked in this house under my own superintendence. The

cleavage of our hills is accidental cleavage, but this is cleavage with

intention. The volition of the pastrycook has entered into the formation

of the mass, and it has been his aim to preserve a series of surfaces of

structural weakness, along which the dough divides into layers.

Puff-paste must not be handled too much, for then the continuity of the

surfaces is broken; it ought to be rolled on a cold slab, to prevent the

butter from melting and diffusing itself through the mass, thus

rendering it more homogeneous and less liable to split. This is the

whole philosophy of puff-paste; it is a grossly exaggerated case of

slaty cleavage.

As time passed on, cases multiplied, illustrating the influence of

pressure in producing lamination. Mr. Warren De la Rue informs me that

he once wished to obtain white-lead in a fine granular state, and to

accomplish this he first compressed the mass: the mould was conical, and

permitted the mass to spread a little laterally under the pressure. The

lamination was as perfect as that of slate, and quite defeated him in

his effort to obtain a granular powder. Mr. Brodie, as you are aware,

has recently discovered a new kind of graphite: here is the substance in

powder, of exquisite fineness. This powder has the peculiarity of

clinging together in little confederacies; it cannot be shaken asunder

like lycopodium; and when the mass is squeezed, these groups of

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

himself has been kind enough to furnish me with specimens for this

evening's lecture. I will cleave them before you: you see they split up

into plates which are perpendicular to the line in which the pressure

was exerted. This testimony is all the more valuable, as the facts were

obtained without any reference whatever to the question of cleavage.

I have here a mass of that singular substance Boghead Cannel. This was

once a mass of mud, more or less resembling this one, which I have

obtained from a bog in Lancashire. I feel some hesitation in bringing

this substance before you, for, as in other cases, so in regard to

Boghead Cannel, science--not science, let me not libel it, but the

quibbling, litigious, money-loving portion of human nature speaking

through the mask of science--has so contrived to split hairs as to

render the qualities of the substance somewhat mythical. I shall

therefore content myself with showing you how it cleaves, and with

expressing my conviction that pressure had a great share in the

production of this cleavage.

The principle which I have enunciated is so simple as to be almost

trivial; nevertheless, it embraces not only the cases I have mentioned,

but, if time permitted, I think I could show you that it takes a much

wider range. When iron is taken from the puddling furnace, it is a more

or less spongy mass: it is at a welding heat, and at this temperature is

submitted to the process of rolling: bright smooth bars such as this are

the result of this rolling. But I have said that the mass is more or

less spongy or nodular, and, notwithstanding the high heat, these

nodules do not perfectly incorporate with their neighbours: what then?

You would say that the process of rolling must draw the nodules into

fibres--it does so; and here is a mass acted upon by dilute sulphuric

acid, which exhibits in a striking manner this fibrous structure. The

experiment was made by my friend Dr. Percy, without any reference to the

question of cleavage.

Here are other cases of fibrous iron. This fibrous structure is the

result of mechanical treatment. Break a mass of ordinary iron and you

have a granular fracture; beat the mass, you elongate these granules,

and finally render the mass fibrous. Here are pieces of rails along

which the wheels of locomotives have slidden; the granules have yielded

and become plates; they exfoliate or come off in leaves. All these

effects belong, I believe, to the great class of phenomena of which

slaty cleavage forms the most prominent example.[I]

Thus, ladies and gentlemen, we have reached the termination of our

task. I commenced by exhibiting to you some of the phenomena of

crystallization. I have placed before you the facts which are found to

be associated with the cleavage of slate-rocks. These facts, as finely

expressed by Helmholtz, are so many telescopes to our spiritual vision,

by which we can see backward through the night of antiquity, and discern

the forces which have been in operation upon the earth's surface

"Ere the lion roared,

Or the eagle soared."

From evidence of the most independent and trustworthy character, we come

to the conclusion that these slaty masses have been subjected to

enormous pressure, and by the sure method of experiment we have

shown--and this is the only really new point which has been brought

before you--how the pressure is sufficient to produce the cleavage.

Expanding our field of view, we find the self-same law, whose footsteps

we trace amid the crags of Wales and Cumberland, stretching its

ubiquitous fingers into the domain of the pastrycook and ironfounder;

nay, a wheel cannot roll over the half-dried mud of our streets without

revealing to us more or less of the features of this law. I would say,

in conclusion, that the spirit in which this problem has been attacked

by geologists indicates the dawning of a new day for their science. The

great intellects who have laboured at geology, and who have raised it to

its present pitch of grandeur, were compelled to deal with the subject

in mass; they had no time to look after details. But the desire for more

exact knowledge is increasing; facts are flowing in, which, while they

leave untouched the intrinsic wonders of geology, are gradually

supplanting by solid truths the uncertain speculations which beset the

subject in its infancy. Geologists now aim to imitate, as far as

possible, the conditions of nature, and to produce her results; they are

approaching more and more to the domain of physics; and I trust the day

will soon come when we shall interlace our friendly arms across the

common boundary of our sciences, and pursue our respective tasks in a

spirit of mutual helpfulness, encouragement, and good-will.

FOOTNOTES:

[A] Referred to in the Introduction.

[B] I merely use this as an illustration; the deposition may have really

been due to sediment carried down by rivers. But the action must have

been periodic, and the powder duplex.

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

[D] In a letter to Sir Charles Lyell, dated from the Cape of Good Hope,

February 20, 1836, Sir John Herschel writes as follows:--"If rocks have

been so heated as to allow of a commencement of crystallization, that is

to say, if they have been heated to a point at which the particles can

begin to move amongst themselves, or at least on their own axes, some

general law must then determine the position in which these particles

will rest on cooling. Probably that position will have some relation to

the direction in which the heat escapes. Now when all or a majority of

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

that must of course determine a cleavage plane."

[E] Omitted here.

[F] While to my mind the evidence in proof of pressure seems perfectly

irresistible, I by no means assert that the manner in which I stated it

is incapable of modification. All that I deem important is the fact that

pressure has been exerted; and provided this remain firm, the fate of

any minor portion of the evidence by which it is here established is of

comparatively little moment.

[G] I have usually softened the wax by warming it, kneaded it with the

fingers, and pressed it between thick plates of glass previously wetted.

At the ordinary summer-temperature the wax is soft, and tears rather

than cleaves; on this account I cool my compressed specimens in a

mixture of pounded ice and salt, and when thus cooled they split

beautifully.

[H] It is scarcely necessary to say that if the mass were squeezed

equally in _all_ directions no laminated structure could be produced; it

must have room to yield in a lateral direction.

[I] An eminent authority informs me that he believes these surfaces of

weak cohesion to be due to the interposition of films of graphite, and

not to any tendency of the iron itself to become fibrous: this of course

does not in any way militate against the theory which I have ventured to

propose. All that the theory requires is surfaces of weak cohesion,

however produced, and a change of shape of such surfaces consequent on

pressure or rolling.

INDEX.

AEggischhorn, 100, 105.

Agassiz on glacier motion, 270, 310.

Air-bubbles, 359, 376.

Aletsch Glacier, 101.

-- --, bedding and structure observed on, 120, 391.

Aletschhorn, cloud iridescences on, 100, 238.

Allalein Glacier, 162.

Alpine climbers, suggestions to, 169.

Alps, winter temperature of, 168.

Altmann's theory of glacier motion, 296.

Ancient glaciers, action of, 99, 141.

Arveiron, arch of, 38, 217.

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

Atmospheric refraction, 35.

Avalanche at Saas, 164.

--, sound of, explained, 12, 14.

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

Balmat, Auguste, 169, 188.

Bedding, lines of, 391.

Bennen, Johann Joseph, 104, 118.

Bergschrund, 98, 325.

"Blower," glacier, 87.

Blue colour of ice, 256.

-- -- -- snow, 29, 83, 132, 203.

-- -- -- water, 33, 253, 259-262.

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

Blue veins, 376, 381.

Boiling-point, influence of pressure on, 408.

-- -- at different altitudes, 105, 106, 113, 120, 129, 175, 190.

Bois, Glacier des, 39, 275, 368.

Brevent, ascent of, 172.

Brocken, Spirit of the, 22, 238.

Bubbles, in ice, 44, 147, 359, 425.

-- in snow, 18, 251.

Capillaries of glacier, 335-339.

Cave of ice, 135.

Cavities in ice, 163, 356, 424.

Cells in ice, 147, see Bubbles.

Chamouni, 37.

--, difficulties at, 170, 192.

-- in winter, 198, 336.

Charmoz, view from, 45, 68, 368.

Charpentier's theory of glacier motion, 296.

Chemical action, rays producing, 240.

Chromatic effects, 235.

Cleavage, 406.

-- and stratification distinct, 2, 390, 431.

-- caused by pressure, 6, 436.

--, contortions of, 9, 59.

-- of crystals and slate rocks, lecture on, 427.

-- of glaciers, 26, 393, 425-426.

-- -- ice, 352, 407.

-- -- slate, &c., 1, 430.

"Cleft station," the, 47, 369.

Clouds, formation and dissipation of, 22, 97, 137, 146.

--, iridescent, 100, 105, 147, 154, 238.

-- on Mont Blanc, 82.

-- on Monte Rosa, 124.

--, winter, at Montanvert, 208.

Colour answers to pitch, 227.

Colours of sky, 257.

--, subjective, 37.

Comet, discovery of, 186.

Compass affected by rocks, 140.

Crepitation of glaciers, 44, 357.

Crevasses, 315

(_marginal_, 318;

_transverse_, 320;

_longitudinal_, 322), 424.

--, first opening of, 317, 327.

Crumples in ice, 174, 415, 419.

Crystallization of ice, 353.

Crystals, cleavage of, 3, 428.

-- of snow, 130, 205, 212.

Deafness, artificial, 167.

Differential motion, 395.

-- --, Dr. Whewell on, 396.

Diffraction, explanation of, 237.

Dirt-bands, 45, 46, 68, 95, 367, 373.

-- --, maps of, 367, 368, 369.

-- --, Forbes on, 371.

-- --, source of, 369, 425.

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

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

Dome du Gouter, 68, 75.

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

Echoes, theory of, 15.

Eismeer, the, 13, 362.

Expedition of 1856, Oberland and Tyrol, 9-32.

-- -- 1857, Montanvert and Mer de Glace, 33-91.

-- -- 1858, Oberland, Valais, and Monte Rosa district, 92-192.

-- -- 1859, winter, Chamouni, and Mer de Glace, 195-219.

Faraday, Prof., on Regelation, 351.

Faulberg, cave of, 107.

Fee, glacier of, 165.

Fend, 32.

Finsteraarhorn, 104, 110.

--, summit of, 112.

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

Forbes, Prof., comparison of glacier to river, 306, 308.

-- --, on glacier motion, 272, 304, 308.

-- --, on magnetism of rocks, 145.

-- --, on veined structure, 379.

-- --, viscous theory, 311, 327, 333, 335.

Freezing, planes of, 163, 358, 424.

Frost-bites, 191.

Frozen flowers, 130, 212.

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

Gases, passage of heat through, 243.

Geant, Col du, 50, 173.

Geant, glacier du, 53-57, 280, 369-373.

--, measurements on, 419-421.

--, motion of, 281, 286.

--, white ice seams of, 56, 413.

Gebatsch Alp, 23.

--, glacier of, 24, 26.

Geneva, Lake of, 33, 259-262.

Glaciers, ancient, action of, 99, 163.

-- "blower," 87.

--, capillaries of, 335-339.

--, crepitation of, 44, 357.

-- d'ecoulement, 301.

-- de Lechaud, see Lechaud.

-- des Bois, 39, 275, 368.

-- du Geant, see Geant.

-- du Talefre, see Talefre.

--, groovings on, 20, 56, 377.

--, measurement of, 276.

-- motion, 52, 269-295, 422.

-- --, earlier theories of, 296-314.

-- --, pressure theory of, 346.

--, origin of, 248-252.

-- reservoirs, 301.

--, ridges on, 42, 55.

--, structure of, 136, 148, see Veined structure.

-- tables, 44, 265.

--, veins of, 54, 376, 381.

--, wrinkles on, 370.

Goethe's theory of colours, 258.

Goerner glacier, 120, 138.

Goerner grat, 137, 145.

Goernerhorn glacier, 147, 149.

Grand Plateau, 187.

Grands Mulets, 73, 185.

Graun, 29.

Grimsel, the, 18, 99.

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

Groovings on glaciers, 20, 56, 377.

Gruener's theory of glacier motion, 296.

Guides of Chamouni, rules of, 60, 170, 192.

-- lost in crevasse, 76.

Guyot, M., on veined structure, 378.

Hailstones, conical, 31.

--, spherical, 65.

Handeck, waterfall of, 17.

Hasli, valley of, 17, 99.

Heat and light, 223, 239, 241.

-- -- work, 328.

--, luminous, 241-247.

--, mechanical equivalent of, 329.

--, obscure, 240.

--, passage through gases, 243-245.

--, radiant, 239.

-- --, permeability of atmosphere to, 105, 243-247.

--, radiated, 242.

--, specific, 331.

Heisse Platte, the, 13.

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

420.

Hochjoch, 32.

Hoechste Spitze of Monte Rosa, 128.

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

Hotel des Neufchatelois, 19, 112, 270.

Hugi on glacier motion, 270.

Huxley, Mr., on glacier capillaries, 338.

-- --, on water-cells, 251, 359.

Hydrogen, effect on rays, 253.

Ice, blue colour of, 256.

-- cascades, 94, 384, 391.

-- cave, 135.

-- cells, 147, see Bubbles.

-- cones, 266.

--, cracking of, 317, 326.

--, crystallization of, 353.

--, effects of pressure on, 405, 409.

--, experiments on, 346.

--, friability of, 333.

--, liquefaction of, 353, 408.

--, liquid flowers in, 354-358, 424.

--, Thomson's theory of plasticity of, 340.

--, softening of, 333.

--, structure of, 136, 148.

--, temperature of, 241, 332.

--, white, seams of, 56, 413, 421.

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

Interference rings, 229.

-- spectra, 76, 178, 235, 238.

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

Jardin, the, 61, 174.

Joch, the passage of a, 28.

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

Jungfrau, the, 11.

--, evening near, 106.

Laminated structure, 376, 378, 426.

Lechaud, glacier de, 53, 387.

-- -- --, motion of, 60, 286-288.

Lenticular structure, 381.

Light and heat, 223, 239, 241.

--, undulation theory of, 224.

Linth, M. Escher de la, 271.

Liquefaction of ice, 353, 408.

Liquid flowers, 147, 354-358, 424.

Magnetic force, 144.

Magnetism of rocks, 140, 143, 145.

Maerjelen See, 101, 119.

Mastic, Bruecke's solution of, 259.

Mattmark See, 162.

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

Mayenwand, summit of, 20, 100, 323.

Mayer, on connexion of heat with work, 328.

Measurement of glaciers, 276.

Mer de Glace, 42-67, 86-90, 173.

-- -- --, dirt-bands of the, 367

(seen from Charmoz, 45, 368;

from Cleft station, 47, 369;

from the Flegere, 367).

-- -- --, map of, 53, 264.

-- -- --, motion of, 275-293.

-- -- --, winter motion of, 294, 343.

-- -- --, winter visit to, 195, 206-218.

Milk, cause of blueness of, 261.

Mirage, 36.

Montanvert, 40, 89, 173.

-- in winter, 204.

Mont Blanc, first ascent of, 68.

-- --, second ascent of, 177.

-- --, summit of, 81, 189.

Monte Rosa, first ascent of, 122.

-- --, second ascent of, 151.

-- --, summit of, 128, 156.

-- --, western glacier of, 138, 147.

-- --, zones of colour, 154, 238.

Moraines, 263.

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

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

Moulins, 362, 424.

--, depth of, 365.

--, motion of, 364.

Necker, letter from, 178.

Neufchatelois, Hotel des, 19, 112, 270.

Neve ice, 249, 251.

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

Oils, effect of films of, 236.

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

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

Pitch of musical sounds, 225.

Planes of freezing, 163, 358, 424.

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

Polar forces, 4.

Pressure and cleavage, see Cleavage.

-- and liquefaction of ice, 340, 408.

-- -- veined structure, 404; 147-149, 382-394, 412, 425-426.

--, effects of, on boiling point, 408.

-- -- -- -- ice, 405, 409.

-- theory of glacier motion, 346.

Radiant heat, 105, 239.

Rays, calorific, 240.

--, transmission of, 242.

Redness of sunset, 175.

Refraction on lake of Geneva, 35.

Regelation, 347, 351.

Reichenbach fall, 17.

Rendu, comparison of glacier to river, 306.

--, measurements of glaciers, 304.

--, notice of regelation, 301.

-- on conversion of snow into ice, 301.

-- on ductility, 298.

-- on law of circulation, 300.

-- on motion of glaciers, 305.

-- on veined structure, 301.

-- theory of glaciers, 299.

Rhone at lake of Geneva, 34, 261.

-- glacier, 20, 100, 323, 386.

-- --, chromatic effects, 21, 238.

Ridges on glaciers, 42, 55.

Riffelhorn, the, 133, 141-145.

Rings, interference, 229.

-- round sun, 21, 238.

Ripples deduced from rings, 400.

Ripple theory, Forbes on, 398.

-- -- of veined structure, 398.

-- waves, movement of, 232.

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

Rocks, magnetism of, 140, 143, 145.

Saas, avalanche at, 164.

Sabine, Gen., on veined structure, 378.

Sand-cones, 266.

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

Scheuchzer's theory of glacier motion, 296.

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

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

Seracs, 51, 75.

Serpentine, boulders of, 161.

Shadows, coloured, 38.

Sharpe, on slaty cleavage, 5, 432.

Silberhorn, the, 11.

Sky, blueness of, 22, 174, 175.

--, colours of, explained, 257.

Slate, cleavage of, 1, 430.

Snow, blue colour of, 29, 132, 203.

-- crystals, 130, 205, 212.

--, dry, 250.

-- line, 29, 248.

--, perpetual, 248.

--, sound of breaking, 202.

-- storm, sound through, 215.

--, whiteness of, explained, 250.

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

Sound in a vacuum, 224.

--, intensity of, 83.

--, rate of motion of, 226.

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

Spectrum, rays of, 240.

Stars, twinkling of, 72, 238.

Stelvio, pass of, 29.

Storm on Grands Mulets, 185.

-- -- Mer de Glace, 210.

Strahleck, glacier of, 94, 384.

--, passage of, 93, 97.

Strata of ice, 136.

Stratification of neve, 392.

-- -- slate, 1, 430.

Structure, doubts regarding, 44, 92, 389.

-- of ice, 136, 148, see Veined structure.

Subjective colours, 37.

Summary of glacier theory, 422.

Sun, rings round, 21, 238.

Sunrise at Chamouni, 39.

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

Sunset, gorgeous, 184.

Tables, glacier, 44, 265-266.

Tacul, motion of ice-wall at, 289.

Talefre, glacier of, 43, 61-62, 87.

--, moraines of, 54, 63, 267, 387.

Temperature, winter, of Alps, 168.

Theodolite, use of, 275.

Theory of cleavage, 5.

Thermometer at Jardin, 174.

-- buried on Mont Blanc, 190.

-- on Finsteraarhorn, 113.

Thomson, Prof., theory of plasticity, 340.

-- -- -- -- regelation, 352.

Twinkling of stars, 72, 238.

Tyrol, the, 23.

Undulation theory of light, 224.

Unteraar, glacier of, 18, 265, 388.

Vacuum in ice-cavities, 163, 356.

Veined structure, 376

(_marginal_, 383;

_transverse_, 384;

_longitudinal_, 387), 395, 404, 408.

-- --, experiments on, 382, 388.

-- -- caused by pressure, 147-149, 382-389, 412, 425-426.

-- -- crossing strata, 389-394.

-- --, Forbes on, 379.

-- --, Gen. Sabine on, 378.

-- --, M. Guyot on, 378.

-- --, ripple theory of, 398.

Viesch, glacier of, 109, 118.

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

Water absorbs red rays, 254.

--, blue colour of, 254; 33, 259, 261.

--, rippling waves of, 232.

Waves, frozen, 43, 55.

--, interference of, 231.

-- motion, Weber on, 232, 399.

-- of sound, 225.

Wengern Alp, 9.

Wetterhorn, echoes of, 15.

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

Whiteness of ice, 250, 268, 376.

Winter motion of Mer de Glace, 294.

Wrinkles on glacier, 370.

Young, Thomas, theory of light, 224.

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Transcriber's Notes.

The titles from the List of Illustrations were copied to the captions of

the figures that otherwise had no caption, for the convenience of the

reader.

The "sidenotes" in the main body of the text were originally page

headers. They have been moved to a place more fitting for the flow,

typically to the head of the appropriate paragraph.

Spelling variants where there was no obviously preferred choice were

retained. These include: "Cleft-Station" and "Cleft Station," plus

variants; "Cima di Jazzi" and "Cima de Jazzi;" "fanlike" and "fan-like;"

"firewood" and "fire-wood;" "Flegere" and "Flegere;" "foreshorten(ed)"

and "fore-shorten(ing);" "generalisation" and "generalization;"

"judgment" and "judgement;" "Kumm" and "Kumme," which may be the same as

"Kamm;" "lime light" and "lime-light;" "realize" and "realise(d);"

"recognise" and "recognize(d);" "rearranged" and "re-arranged;"

"refrozen" and "re-frozen;" "self-same" and "selfsame;" "semifluid" and

"semi-fluid;" "sundial" and "sun-dial;" "Trift" and "Trifti," probably

the same glacier; "weatherworn" and "weather-worn."

In the Latin-1 encoded text version, the oe-ligature was replaced by

the two separate characters, "oe."

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

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

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

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

haversack."

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

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

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

Changed "SNOWSTORM" to "SNOW-STORM" in the sidenote from page 215:

"SOUND THROUGH THE SNOW-STORM."

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

neutralize."

Moved the semi-colon inside the double quotes on page 285, around:

"corresponding points."

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

"THOMSON'S THEORY."

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

anxious."

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

"Die Gletscher der Jetzzeit" on page 393 should probably be "Die

Gletscher der Jetztzeit," but was not changed.

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

bedding."

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

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

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

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

second ascent of."

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

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

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