to come together; they freeze together when they touch. The parts
surrounding the place of contact melt away, but the pieces continue
for a time united by a narrow bridge of ice. The bridge finally
melts, and the pieces for a moment are separated. But capillary
attraction immediately draws them together, and regelation sets in
once more. A new bridge is formed, which in its turn is dissolved,
the separated pieces again closing up. A kind of pulsation is thus
established between the two pieces of ice. They touch, they freeze,
a bridge is formed and melted; and thus the rhythmic action continues
until the ice disappears.
443. According to Professor James Thomson's theory, pressure is
necessary to liquefy the ice. The heat necessary for liquefaction
must be drawn from the ice itself, and the cold water must escape
from the pressure to be re-frozen. Now in the foregoing experiments
the cold water, instead of being allowed to freeze, _issues into
the warm water_, still the floating fragments regelate in a moment.
The touching surfaces may, moreover, be convex; they may be reduced
practically to _points_, clasped all round by the warm water, which
indeed rapidly dissolve them as they approach each other; still they
freeze immediately when they touch.
444. You may learn from this discussion that in scientific matters,
as in all others, there is room for differences of opinion. The frame
of mind to be cultivated here is a suspension of judgment as long
as the meaning remains in doubt. It may be that Faraday's action
and Thomson's action come both into play. I cannot do better than
finish these remarks by quoting Faraday's own concluding words,
which show how in his mind scientific conviction dwelt apart from
dogmatism:--"No doubt," he says, "nice experiments will enable us
hereafter to criticise such results as these, and separating the true
from the untrue will establish the correct theory of regelation."
§ 64. _The Blue Veins of Glaciers._
445. We now approach the end, one important question only remaining
to be discussed. Hitherto we have kept it back, for a wide
acquaintance with the glaciers was necessary to its solution. We had
also to make ourselves familiar by actual experiment with the power
of ice, softened by thaw, to yield to pressure, and to liquefy under
such pressure.
446. Snow is white. But if you examine its individual particles you
would call them _transparent_, not white. The whiteness arises from
the mixture of the ice particles with small spaces of air. In the
case of all transparent bodies whiteness results from such a mixture.
The clearest glass or crystal when crushed becomes a white powder.
The foam of champagne is white through the intimate admixture of a
transparent liquid with transparent carbonic acid gas. The whitest
paper, moreover, is composed of fibres which are individually
transparent.
447. It is not, however, the air or the gas, but the _optical
severance_ of the particles, giving rise to a multitude of reflexions
of the white solar light at their surfaces, that produces the
whiteness.
448. The whiteness of the surface of a clean glacier (112), and of
the icebergs of the Märgelin See (357), has been already referred to
a similar cause. The surface is broken into innumerable fissures by
the solar heat, the reflexion of solar light from the sides of the
little fissures producing the observed appearance.
449. In like manner if you freeze water in a test-tube by plunging it
into a freezing mixture, the ice produced is white. For the most part
also the ice formed in freezing machines is white. Examine such, ice,
and you will find it filled with small air-bubbles. When the freezing
is extremely slow the crystallising force pushes the air effectually
aside, and the resulting ice is transparent; when the freezing is
rapid, the air is entangled before it can escape, and the ice is
_translucent_. But even in the case of quick freezing Mr. Faraday
obtained transparent ice by skilfully removing the air-bubbles as
fast as they appeared with a feather.
450. In the case of lake ice the freezing is not uniform, but
intermittent. It is sometimes slow, sometimes rapid. When slow the
air dissolved in the water is effectually squeezed out and forms a
layer of bubbles on the under-surface of the ice. An act of sudden
freezing entangles the air, and hence we find lake ice usually
composed of layers alternately clear, and filled with bubbles. Such
layers render it easy to detect the planes of freezing in lake ice.
451. And now for the bearing of these facts. Under the fall of the
Géant, at the base of the Talèfre cascade, and lower down the Mer
de Glace; in the higher regions of the Grindelwald, the Aar, the
Aletsch and the Görner glaciers, the ice does not possess the
transparency which it exhibits near the ends of the glaciers. It
is white, or whitish. Why? Examination shows it to be filled with
small air-bubbles; and these, as we now learn, are the cause of its
whiteness.
452. They are the residue of the air originally entangled in the
snow, and connected, as before stated, with the whiteness of the
snow. During the descent of the glacier, the bubbles are gradually
expelled by the enormous pressures brought to bear upon the ice.
Not only is the expulsion caused by the mechanical yielding of the
soft thawing ice, but the liquefaction of the substance at places of
violent pressure, opening, as it does, fissures for the escape of the
air, must play an important part in the consolidation of the glacier.
453. The expulsion of the bubbles is, however, not uniform; for
neither ice nor any other substance offers an absolutely uniform
resistance to pressure. At the base of every cascade that we have
visited, and on the walls of the crevasses there formed, we have
noticed innumerable blue streaks drawn through the white translucent
ice, and giving the whole mass the appearance of lamination. These
blue veins turned out upon examination to be spaces from which the
air-bubbles had been almost wholly expelled, translucency being thus
converted into transparency.
454. This is the _veined_ or _ribboned structure_ of glaciers,
regarding the origin of which diverse opinions are now entertained.
455. It is now our duty to take up the problem, and to solve it if we
can. On the névés of the Col du Géant, and other glaciers, we have
found great cracks, and faults, and _Bergschrunds_, exposing deep
sections of the névé; and on these sections we have found marked the
edges of half-consolidated strata evidently produced by successive
falls of snow. The névé is stratified because its supply of material
from the atmosphere is intermittent, and when we first observed
the blue veins we were disposed to regard them as due to this
stratification.
456. But observation and reflexion soon dispelled this notion. Indeed
it could hardly stand in the presence of the single fact that at the
bases of the ice-falls the veins are always _vertical_, or nearly so.
We saw no way of explaining how the horizontal strata of the névé
could be so tilted up at the base of the fall as to be set on edge.
Nor is the aspect of the veins that of stratification.
457. On the central portions of the cascades, moreover, there are
no signs of the veins. At the bases they first appear, reaching in
each case their maximum development a little below the base. As you
and I stood upon the heights above the Zäsenberg and scrutinised the
cascade of the Strahleck branch of the Grindelwald glacier, we could
not doubt that the base of the fall was the birthplace of the veins.
We called this portion of the glacier a "Structure Mill," intimating
that here, and not on the névé, the veined structure was manufactured.
[Illustration: SECTION OF ICEFALL, AND GLACIER BELOW IT, SHOWING
ORIGIN OF VEINED STRUCTURE.]
458. This, however, is, at bottom, the language of strong _opinion_
merely, not that of _demonstration_; and in science opinion ought to
content us only so long as positive proof is unattainable. The love
of repose must not prevent us from seeking this proof. There is no
sterner conscience than the scientific conscience, and it demands,
in every possible case, the substitution for private conviction of
demonstration which shall be conclusive to all.
459. Let us, for example, be shown a case in which the stratification
of the névé is prolonged into the glacier; let us see the planes of
bedding and the planes of lamination existing side by side, and still
indubitably distinct. Such an observation would effectually exclude
stratification from the problem of the veined structure, and through
the removal of this tempting source of error, we should be rendered
more free to pursue the truth.
460. We sought for this conclusive test upon the Mer de Glace,
but did not find it. We sought it on the Grindelwald, and the Aar
glaciers,[J] with an equal want of success. On the Aletsch glacier,
for the first time, we observed the apparent coexistence of bedding
and structure, the one _cutting_ the other upon the walls of the same
crevasse. Still the case was not sufficiently pronounced to produce
entire conviction, and we visited the Görner glacier with the view of
following up our quest.
[J] M. Agassiz, however, reports a case of the kind upon the glacier
of the Aar.
[Illustration: STRUCTURE AND BEDDING ON ALETSCH GLACIER.]
461. Here day after day added to the conviction that the bedding and
the structure were two different things. Still day after day passed
without revealing to us the final proof. Surely we have not let our
own ease stand in the way of its attainment, and if we retire baffled
we shall do so with the consciousness of having done our best.
Yonder, however, at the base of the Matterhorn, is the Furgge glacier
that we have not yet explored. Upon it our final attempt must be made.
462. We get upon the glacier near its end, and ascend it. We are soon
fronted by a barrier composed of three successive walls of névé, the
one rising above the other, and each retreating behind the other. The
bottom of each wall is separated from the top of the succeeding one
by a ledge, on which threatening masses of broken névé now rest. We
stand amid blocks and rubbish which have been evidently discharged
from these ledges, on which other masses, ready apparently to tumble,
are now poised.
463. On the vertical walls of this barrier we see, marked with the
utmost plainness, the horizontal lines of stratification, while
something exceedingly like the veined structure appears to cross the
lines of bedding at nearly a right angle. The vertical surface is,
however weathered, and the lines of structure, if they be such, are
indistinct. The problem now is to remove the surface, and expose the
ice underneath. It is one of the many cases that have come before
us, where the value of an observation is to be balanced against the
danger which it involves.
464. We do nothing rashly; but scanning the ledges and selecting a
point of attack, we conclude that the danger is not too great to
be incurred. We advance to the wall, remove the surface, and are
rewarded by the discovery underneath it of the true blue veins. They,
moreover, are vertical, while the bedding is horizontal. Bruce, as
you know, was defeated in many a battle, but he persisted and won at
last. Here, upon the Furgge glacier, you also have fought and won
your little Bannockburn.
[Illustration: STRUCTURE AND BEDDING ON FURGGE GLACIER.]
465. But let us not use the language of victory too soon. The
stratification theory has been removed out of the field of
explanation, but nothing has as yet been offered in its place.
§ 65. _Relation of Structure to Pressure._
466. This veined structure was first described by the distinguished
Swiss naturalist, Guyot, now a resident in the United States. From
the Grimsel Pass I have already pointed out to you the Gries glacier
overspreading the mountains at the opposite side of the valley of the
Rhone. It was on this glacier that M. Guyot made his observation.
467. "I saw," he said, "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 glacier
here was composed of two kinds of ice, one that of the furrows,
snowy and more easily melted; the other 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 ridges.
468. "After having followed them for several hundred yards, I reached
a crevasse 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 eyes 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 hard plates of which I have
spoken, the whole forming a regularly laminated mass, which resembled
certain calcareous slates."
469. I have not failed to point out to you upon all the glaciers
that we have visited the little superficial furrows here described;
and you have, moreover, noticed that in the furrows mainly is lodged
the finer dirt which is scattered over the glacier. They suggest
the passage of a rake over the ice. And whenever these furrows were
interrupted by a crevasse, the veined structure invariably revealed
itself upon the walls of the fissure. The surface grooving is indeed
an infallible indication of the interior lamination of the ice.
470. We have tracked the structure through the various parts of the
glaciers at which its appearance was most distinct; and we have
paid particular attention to the condition of the ice at these
places. The very fact of its cutting the crevasses at right angles
is significant. We know the mechanical origin of the crevasses; that
they are cracks formed at right angles to lines of tension. But since
the crevasses are also perpendicular to the planes of structure,
these planes must be parallel to the lines of tension.
471. On the glaciers, however, tension rarely occurs alone. At the
sides of the glacier, for example, where marginal crevasses are
formed, the tension is always accompanied by pressure; the one
force acting at right angles to the other. Here, therefore, the
veined structure, which is parallel to the lines of tension, _is
perpendicular to the lines of pressure_.
472. That this is so will be evident to you in a moment. Let the
adjacent figure represent the channel of the glacier moving in the
direction of the arrow. Suppose three circles to be marked upon the
ice, one at the centre and the two others at the sides. In a glacier
of uniform inclination all these circles would move downward, the
central one only remaining a circle. By the retardation of the sides
the marginal circles would be drawn out to ovals. The two circles
would be _elongated_ in one direction, and _compressed_ in another.
Across the long diameter, which is the direction of strain, we have
the marginal crevasses; across the short diameter _m n_, which is the
direction of pressure, we have the _marginal veined structure_.
[Illustration]
473. This association of pressure and structure is invariable. At
the bases of the cascades, where the inclination of the bed of the
glacier suddenly changes, the pressure in many cases suffices not
only to close the crevasses but to violently squeeze the ice. At
such places the structure always appears, sweeping quite across
the glacier. When two branch glaciers unite, their mutual thrust
intensifies the pre-existing marginal structure of the branches,
and developes new planes of lamination. Under the medial moraines,
therefore, we have usually a good development of the structure. It
is finely displayed, for example, under the great medial moraine of
the glacier of the Aar.
474. Upon this glacier, indeed, the blue veins were observed
independently three years after M. Guyot had first described them. I
say independently, because M. Guyot's description, though written in
1838, remained imprinted, and was unknown in 1841 to the observers on
the Aar. These were M. Agassiz and Professor Forbes. To the question
of structure Professor Forbes subsequently devoted much attention,
and it was mainly his observations and reasonings that gave it the
important position now assigned to it in the phenomena of glaciers.
475. Thus without quitting the glaciers themselves, we establish
the connexion between pressure and structure. Is there anything in
our previous scientific experience with which these facts may be
connected? The new knowledge of nature must always strike its roots
into the old, and spring from it as an organic growth.
§ 66. _Slate Cleavage and Glacier Lamination_.
476. M. Guyot threw out an exceedingly sagacious hint, when he
compared the veined structure to the cleavage of slate rocks. We must
learn something of this cleavage, for it really furnishes the key to
the problem which now occupies us. Let us go then to the quarries
of Bangor or Cumberland, and observe the quarrymen in their sheds
splitting the rocks. With a sharp point struck skilfully into the
edge of the slate, they cause it to divide into thin plates, fit for
roofing or ciphering, as the case may be. The surfaces along which
the rock cleaves are called its _planes of cleavage_.
477. All through the quarry you notice the direction of these planes
to be perfectly constant. How is this laminated structure to be
accounted for?
478. You might be disposed to consider that cleavage is a case of
stratification or bedding; for it is true that in various parts of
England there are rocks which can be cloven into thin flags along the
planes of bedding. But when we examine these slate rocks we verify
the observation, first I believe made by the eminent and venerable
Professor Sedgwick, that the planes of bedding usually run across the
planes of cleavage.
479. We have here, as you observe, a case exactly similar to that of
glacier lamination, which we were at first disposed to regard as due
to stratification. We afterwards, however, found planes of lamination
crossing the layers of the névé, exactly as the planes of cleavage
cross the beds of slate rocks.
480. But the analogy extends further. Slate cleavage continued to
be a puzzle to geologists till the late Mr. Daniel Sharpe made the
discovery that shells and other fossils and bodies found in slate
rocks are invariably flattened out in the planes of cleavage.
481. Turn into any well-arranged museum--for example, into the School
of Mines in Jermyn Street, and observe the evidence there collected.
Look particularly to the fossil trilobites taken from the slate rock.
They are in some cases squeezed to one third of their primitive
thickness. Numerous other specimens show in the most striking manner
the flattening out of shells.
482. To the evidence adduced by Mr. Sharpe, Mr. Sorby added other
powerful evidence, founded upon the microscopic examination of slate
rock. Taking both into account, the conclusion is irresistible that
such rocks have suffered enormous pressure at right angles to the
planes of cleavage, exactly as the glacier has demonstrably suffered
great pressure at right angles to its planes of lamination.
483. The association of pressure and cleavage is thus demonstrated;
but the question arises, do they stand to each other in the relation
of cause and effect? The only way of replying to this question is to
combine artificially the conditions of nature, and see whether we
cannot produce her results.
484. The substance of slate rocks was once a plastic mud, in which
fossils were embedded. Let us imitate the action of pressure upon
such mud by employing, instead of it, softened white wax. Placing a
ball of the wax between two glass plates, wetted to prevent it from
sticking, we apply pressure and flatten out the wax.
485. The flattened mass is at first too soft to cleave sharply; but
you can see, by tearing, that it is laminated. Let us chill it with
ice. We find afterwards that no slate rock ever exhibited so fine a
cleavage. The laminæ, it need hardly be said, are perpendicular to
the pressure.
486. One cause of this lamination is that the wax is an aggregate
of granules the surfaces of which are places of weak cohesion; and
that by the pressure these granules are squeezed flat, thus producing
planes of weakness at right angles to the pressure.
487. But the main cause of the cleavage I take to be the lateral
sliding of the particles of wax over each other. Old attachments
are thereby severed, which the new ones fail to make good. Thus the
tangential sliding produces lamination, as the rails near a station
are caused to exfoliate by the gliding of the wheel.
488. Instead of wax we may take the slate itself, grind it to fine
powder, add water, and thus reproduce the pristine mud. By the proper
compression of such mud, in one direction, the cleavage is restored.
489. Call now to mind the evidences we have had of the power of
thawing ice to yield to pressure. Recollect the shortening of the
Glacier du Géant, and the squeezing of the Glacier de Léchaud, at
Trélaporte. Such a substance, slowly acted upon by pressure, will
yield laterally. Its particles will slide over each other, the
severed attachments being immediately made good by regelation. It
will not yield uniformly, but along special planes. It will also
liquefy, not uniformly, but along special surfaces. Both the sliding
and the liquefaction will take place principally at right angles to
the pressure, and glacier lamination is the result.
490. As long as it is sound the laminated glacier ice resists
cleavage. Regelation, as I have said, makes the severed attachments
good. But when such ice is exposed to the weather the structure is
revealed, and the ice can then be cloven into tablets a square foot,
or even a square yard in area.
§ 67. _Conclusion_.
491. Here, my friend, our labours close. It has been a true pleasure
to me to have you at my side so long. In the sweat of our brows we
have often reached the heights where our work lay, but you have been
steadfast and industrious throughout, using in all possible cases
your own muscles instead of relying upon mine. Here and there I have
stretched an arm and helped you to a ledge, but the work of climbing
has been almost exclusively your own. It is thus that I should like
to teach you all things; showing you the way to profitable exertion,
but leaving the exertion to you more anxious to bring out your
manliness in the presence of difficulty than to make your way smooth
by toning difficulties down.
492. Steadfast, prudent, without terror, though not at all times
without awe, I have found you on rock and ice, and you have shown
the still rarer quality of steadfastness in intellectual effort. As
here set forth, our task seems plain enough, but you and I know how
often we have had to wrangle resolutely with the facts to bring out
their meaning. The work, however, is now done, and you are master of
a fragment of that sure and certain knowledge which is founded on the
faithful study of nature. Is it not worth the price paid for it? Or
rather, was not the paying of the price the healthful, if sometimes
hard, exercise of mind and body, upon alp and glacier--a portion of
our delight?
493. Here then we part. And should we not meet again, the memory of
these days will still unite us. Give me your hand. Good bye.
INDEX.
A