_STRUCTURE OF GLACIERS._
If a transparent colourless solid be reduced to powder, the powder is
white. Thus rock crystal, rock salt, and glass in powder are all white.
A glass jar, partially filled with a solution of carbonate of soda,
with a little gum added to give it tenacity, presents, on the addition
of a little tartaric acid, the appearance of a tall white column of
foam. In all these cases, the whiteness and the opacity are due to
the intimate and irregular admixture of a solid or a liquid with air;
in like manner the whiteness of snow is due to the mixture of air and
transparent particles of ice.
The snow falls upon mountain eminences, and, above the snow-line, each
year leaves a residue; the substance thus collects in layers, forming
masses of great depth. The lower portions are squeezed by the pressure
of those above them, and a gradual approach to ice is the consequence.
The air being gradually expelled, the transparency of the substance
augments in proportion.
But even after the snow has been squeezed to hard ice in the upper
glacier region, it always contains a large amount of the air originally
entrapped in the snow. The air is distributed through the solid in
the form of bubbles, which give the ice a milky appearance. At the
lower extremity of a glacier the ice, as everybody knows, is blue and
transparent. The transition from one state to the other is not, in all
cases, a gradual change which takes place uniformly throughout the
entire mass. The white ice, on the contrary, of the middle glacier
region is usually striped by veins of a more transparent character,
the air which gives to the ice its whiteness having been, by some
means or other, wholly or partially ejected from the veins. These
veins sometimes give the ice of many glaciers a beautiful laminated
appearance; vast portions, indeed, of various glaciers consist of this
laminated ice.
The theory of the veins which perhaps first presents itself to the
mind, and which is still entertained by many intelligent Alpine
explorers, is that the veining of the middle glaciers is simply a
continuation of the _bedding_ of the _névé_; that not only do the
annual snow-falls produce beds of great thickness, but every successive
fall tends to produce a layer of less thickness, which layers, or the
surfaces separating them, ultimately appear as the blue veins. This
theory demands respectful consideration: on the exposed sections of
the _névé_ the lines of stratification are very manifest, exhibiting
in many cases appearances strongly resembling that of the veined
structure. Indeed, it was with a view to examine this subject more
closely that I withheld my observations on the structure of the Mer
de Glace in 1857, and betook myself once more to the mountains during
the summer of 1858. My desire at that time was to settle once for all
the rival claims of the only two theories which then deserved serious
attention--namely, those of pressure and of stratification.
In pursuance of this idea, I first visited the Lower glacier of
Grindelwald, one of the most accessible, and at the same time most
instructive, in the entire range of the Alps. Ascending the branch of
this glacier which descends from the Schreckhorn, the Strahleck, and
the Finsteraarhorn, I came to the base of an ice-fall which forbade
further advance. Quitting the glacier here, I ascended the side of the
flanking mountain, so as to reach a point from which the fall, and
the glacier below it, are distinctly visible; and from this position
I observed the gradual development and perfecting of the structure at
the base of the fall. On the middle of the fall itself no trace of the
structure was manifest; but where the glacier changed its inclination
at the bottom, being bent upwards so as throw its surface into a state
of intense longitudinal compression, the blue veins first made their
appearance. The base of the fall was a true _structure mill_, where
the transverse veins were manufactured, being afterwards sent forward,
giving a character to portions of the glacier which had no share in
their formation.
I afterwards examined the fall from the opposite side of the valley,
and corroborated the observations. It is difficult, in words, to convey
the force of the evidence which this glacier presents to the observer
who _sees_ it; it seems in fact like a grand laboratory experiment made
by Nature herself with especial reference to the point in question. The
squeezing of the mass, its yielding to the force brought to bear upon
it, its wrinkling and scaling off, and the appearance of the veins at
the exact point where the pressure begins to manifest itself, leave no
doubt on the mind that pressure and structure stand to each other in
the relation of cause and effect, and that the stratification could
have nothing to do with the phenomenon.
I subsequently crossed the Strahleck, descended the glaciers of the
Aar, crossed the Grimsel, and examined the glacier of the Rhone. This
glacier has also its grand ice-fall. In company with Prof. Ramsay, I
climbed in 1858 the precipices flanking the fall at the Grimsel side.
What has been stated regarding the Grindelwald ice-fall is true of
that of the Rhone; the base of the cascade is _the manufactory of the
structure_; and, as all the ice has to pass through this mill, the
entire mass of the glacier from the base of the fall downwards is
beautifully laminated.
Descending the valley of the Rhone to Viesch, I went thence to the
Æggischhorn, and remained for eight days in the vicinity of the
Great Aletsch glacier--the noblest ice-stream of the Alps. A highly
intelligent explorer had adduced certain phenomena of this glacier
as an evidence against the pressure theory of the veined structure;
and I did not think myself justified in quitting the place until I
had perfectly satisfied myself that the Aletsch not only presented no
phenomena at variance with the pressure theory, but exhibited some
which seemed fatal to the theory of the stratification.
I subsequently proceeded to Zermatt, and spent ten days on the
Riffelberg, exploring the entire system of glaciers between Monte Rosa
and the Mont Cervin. These glaciers exhibit, perhaps in a more striking
manner than any others in the Alps, the yielding of glacier ice when
subjected to intense pressure. The great western glacier of Monte
Rosa, the Schwartze glacier, the Trifti glacier, and the glaciers of
St. Theodule, are first spread out as wide and extensive _névés_ over
the breasts of the mountains. They move down, and are finally forced
into the valley containing the trunk, or Görner glacier. Here they are
squeezed to narrow strips, which gradually dwindle in width until they
form driblets not more than a few yards across. From the Görner Grat,
or from the summit of the Riffelhorn, these parallel strips of glacier,
each separated from its neighbour by a medial moraine, present a most
striking and instructive appearance.
The structure of these glaciers was carefully examined, and in all
cases as I travelled from regions where the pressure was feeble to
others where it was intense, the ice changed from a state almost, if
not entirely, structureless, to one in which the veining was exhibited
in great perfection. Each glacier, for example, where it met the
opposing mass in the trunk valley, and was pressed against the latter
by the thrust from behind, exhibited a beautifully developed structure.
Proofs have been already adduced that the Glacier du Géant is in a
state of longitudinal compression; it has also been shown that the
seams of white ice which intersect this glacier are due to the filling
up of the channels of glacier streams by snow, and the subsequent
compression of the substance. Here, then, we have a vast ice-press
which furnishes us with a test of the pressure theory. Both in 1857
and 1858 I found many of these seams of white ice intersected by
blue veins of the finest and most distinct character, their general
direction being at right angles to the direction of pressure.
But the notions of M. Agassiz as to the turning up of the strata so
as to expose their edges at the surface, and the acute remarks and
arguments of Mr. John Ball on the same subject, might still cast a
doubt upon the pressure theory, by suggesting a possible, though
extremely improbable, explanation of the structure in accordance with
the theory of stratification.
Hence my strong desire to discover some crucial phenomenon which should
set this question for ever at rest, and leave no room for doubt, even
on the minds of those who never saw a glacier. On Wednesday, August 18,
I was fortunate enough to make this discovery upon the Furgge glacier.
This ice-field spreads out as an almost level plain at the base of
Mont Cervin. The strata pile themselves one above the other without
disturbance, and hence with great regularity. The ice at length reaches
a brow, over which it is precipitated, forming in its descent four
great terraces, and shutting up the lower valley as a _cul de sac_.
When I reached this place huge blocks of ice stood, like rocking
stones, upon the topmost ledge, and numbers, which had fallen, had been
caught by the other ledges, and occupied very threatening positions:
the base of the fall was cumbered with crushed ice, and large boulders
of the substance had been cast a considerable way down the glacier.
On the faces of the terraces horizontal lines of stratification were
shown in the most perfect manner. Here and there the exertion of a
powerful lateral squeeze was manifest, causing the beds to crumple,
and producing numerous faults. Examining the fall from a distance
through an opera-glass, I thought I could discover lines of veining
running _through_ the strata, at a high angle, exactly as the planes
of cleavage often run at a high angle to the bedding of slate rocks.
The surface of the ice was, however, weathered; and I was unwilling
to accept an observation upon such a cardinal point with a shade of
doubt attached to it. Leaving my field-glass with my guide, who was to
give me warning should the blocks overhead give way, I advanced to the
wall of ice, and at several places cut away with my axe the weathered
superficial portions. Underneath I found the true veined structure,
_running nearly at right angles to the planes of stratification_.
I afterwards climbed the glacier to the right, and, as I ascended,
still better illustrations of the coexistence of the structure and the
strata than those observed upon the terraces exhibited themselves. The
ice was greatly dislocated, and on the faces of the crevasses the
beds were distinctly shown, _with the veins crossing them_. The idea
that the veins could be due to the turning up of the strata is plainly
irreconcileable with these observations.
The same year I visited the Mer de Glace and its tributaries, and found
the pressure key applicable to their phenomena also. The transverse
structure of the Glacier du Géant is formed at the base of the séracs;
that of the Talèfre branch of the Mer de Glace at the base of the
Talèfre ice-fall, where the change of inclination and the thrust from
behind produce the requisite longitudinal compression. I have already
had occasion to remark upon the peculiar dipping of the structure,
and the scaling-off of the protuberances, which are effects of the
same cause. These phenomena are exhibited at the base of all the
ice-cascades.
The principal kinds of structure may be divided into three; as follows:
1st, _Marginal structure_, developed by pressure due to the swifter
motion of the centre of the glacier.
2nd, _Longitudinal structure_, due to mutual pressure of two tributary
glaciers; the structure here is parallel to the medial moraine which
divides the tributaries.
3rd, _Transverse structure_, produced by pressure due to the change of
inclination, and to the longitudinal thrust endured by the glacier at
the base of an ice-fall.
The lamination of a glacier is a peculiarly interesting case of
cleavage. It is produced in the same manner as the lamination of slate
rock, which is known, through the distortion of its fossils, to have
suffered great pressure at right angles to the planes of cleavage.