one-hundredth the area of the land. For let us consider the strain
upon a single line drawn over the summit of the protuberance from a
point on its rim to a point opposite. Regarding the protuberance as a
spherical swelling, the length of the arc corresponding to a chord of
100 miles and a versed sine of 3 miles is 100.24 miles; consequently
the surface to reach its new position must stretch 0.24 of a mile, or
be broken. A fissure or a number of cracks with this total width
would relieve the strain; that is to say, the sum of the widths of all
the cracks over the length of 100 miles would be 420 yards. If,
instead of comparing the width of the fissures with the length of the
lines of tension, we compared their areas with the area of the
unfissured land, we should of course find the proportion much less.
These considerations will help the imagination to realise what a small
ratio the area of the open fissures must bear to the unfissured crust.
They enable us to say, for example, that to assume the area of the
fissures to be one-tenth of the area of the land would be quite
absurd, while that the area of the fissures could be one-half or more
than one-half that of the land would be in a proportionate degree
unthinkable. If we suppose the elevation to be due to the shrinking
or subsidence of the land all round our assumed circle, we arrive
equally at the conclusion that the area of the open fissures would be
altogether insignificant as compared with that of the unfissured
crust.
To those who have seen them from a commanding elevation, it is
needless to say that the Alps themselves bear no sort of resemblance
to the picture which this theory presents to us. Instead of deep
cracks with approximately vertical walls, we have ridges running into
peaks, and gradually sloping to form valleys. Instead of a fissured
crust, we have a state of things closely resembling the surface of the
ocean when agitated by a storm. The valleys, instead of being much
narrower than the ridges, occupy the greater space. A plaster cast of
the Alps turned upside down, so as to invert the elevations and
depressions, would exhibit blunter and broader mountains, with
narrower valleys between them, than the present ones. The valleys
that exist cannot, I think, with any correctness of language be called
fissures. It may be urged that they originated in fissures: but even
this is unproved, and, were it proved, the fissures would still play
the subordinate part of giving direction to the agents which are to be
regarded as the real sculptors of the Alps.
The fracture theory, then, if it regards the elevation of the Alps as
due to the operation of a force acting throughout the entire region,
is, in my opinion, utterly incompetent to account for the conformation
of the country. If, on the other hand, we are compelled to resort to
local disturbances, the manipulation of the earth's crust necessary to
obtain the valleys and the mountains will, I imagine, bring the
difficulties of the theory into very strong relief. Indeed an
examination of the region from many of the more accessible
eminences--from the Galenstock, the Grauhaupt, the Pitz Languard, the
Monte Confinale--or, better still, from Mont Blanc, Monte Rosa, the
Jungfrau, the Finsteraarhorn, the Weisshorn, or the Matterhorn, where
local peculiarities are toned down, and the operations of the powers
which really made this region what it is are alone brought into
prominence--must, I imagine, convince every physical geologist of the
inability of any fracture theory to account for the present
conformation of the Alps.
A correct model of the mountains, with an unexaggerated vertical
scale, produces the same effect upon the mind as the prospect from one
of the highest peaks. We are apt to be influenced by local phenomena
which, though insignificant in view of the general question of Alpine
conformation, are, with reference to our customary standards, vast and
impressive. In a true model those local peculiarities disappear; for
on the scale of a model they are too small to be visible; while the
essential facts and forms are presented to the undistracted attention.
A minute analysis of the phenomena strengthens the conviction which
the general aspect of the Alps fixes in the mind. We find, for
example, numerous valleys which the most ardent plutonist would not
think of ascribing to any other agency than erosion. That such is
their genesis and history is as certain as that erosion produced the
Chines in the Isle of Wight. From these indubitable cases of
erosion--commencing, if necessary, with the small ravines which run
down the flanks of the ridges, with their little working navigators at
their bottoms--we can proceed, by almost insensible gradations, to the
largest valleys of the Alps; and it would perplex the plutonist to fix
upon the point at which fracture begins to play a material part.
In ascending one of the larger valleys, we enter it where it is wide
and where the eminences are gentle on either side. The flanking
mountains become higher and more abrupt as we ascend, and at length we
reach a place where the depth of the valley is a maximum. Continuing
our walk upwards, we find ourselves flanked by gentler slopes, and
finally emerge from the valley and reach the summit of an open col, or
depression in the chain of mountains. This is the common character of
the large valleys. Crossing the col, we descend along the opposite
slope of the chain, and through the same series of appearances in the
reverse order. If the valleys on both sides of the col were produced
by fissures, what prevents the fissure from prolonging itself across
the col? The case here cited is representative; and I am not
acquainted with a single instance in the Alps where the chain has been
cracked in the manner indicated. The cols are simply depressions; in
many of which the unfissured rock can be traced from side to side.
The typical instance just sketched follows as a natural consequence
from the theory of erosion. Before either ice or water can exert
great power as an erosive agent, it must collect in sufficient mass.
On the higher slopes and plateaus--in the region of cols--the power is
not fully developed; but lower down tributaries unite, erosion is
carried on with increased vigour, and the excavation gradually reaches
a maximum. Lower still the elevations diminish and the slopes become
more gentle; the cutting power gradually relaxes, until finally the
eroding agent quits the mountains altogether, and the grand effects
which it produced in the earlier portions of its course entirely
disappear.
I have hitherto confined myself to the consideration of the broad
question of the erosion theory as compared with the fracture theory;
and all that I have been able to observe and think with reference to
the subject leads me to adopt the former. Under the term erosion I
include the action of water, of ice, and of the atmosphere, including
frost and rain. Water and ice, however, are the principal agents, and
which of these two has produced the greatest effect it is perhaps
impossible to say. Two years ago I wrote a brief note 'On the
Conformation of the Alps,' [Footnote: Phil. Mag. vol. xxiv. p. 169]
in which I ascribed the paramount influence to glaciers. The facts on
which that opinion was founded are, I think, unassailable; but whether
the conclusion then announced fairly follows from the facts is, I
confess, an open question.
The arguments which have been thus far urged against the conclusion
are not convincing. Indeed, the idea of glacier erosion appears so
daring to some minds that its boldness alone is deemed its sufficient
refutation. It is, however, to be remembered that a precisely similar
position was taken up by many excellent workers when the question of
ancient glacier extension was first mooted. The idea was considered
too hardy to be entertained; and the evidences of glacial action were
sought to be explained by reference to almost any process rather than
the true one. Let those who so wisely took the side of 'boldness' in
that discussion beware lest they place themselves, with reference to
the question of glacier erosion, in the position formerly occupied by
their opponents.
Looking at the little glaciers of the present day--mere pigmies as
compared to the giants of the glacial epoch--we find that from every
one of them issues a river more or less voluminous, charged with the
matter which the ice has rubbed from the rocks. Where the rocks are
soft, the amount of this finely pulverised matter suspended in the
water is very great. The water, for example, of the river which flows
from Santa Catarina to Bormio is thick with it. The Rhine is charged
with this matter, and by it has so silted up the Lake of Constance as
to abolish it for a large fraction of its length. The Rhone is
charged with it, and tens of thousands of acres of cultivable land are
formed by the silt above the Lake of Geneva.
In the case of every glacier we have two agents at work--the ice
exerting a crushing force on every point of its bed which bears its
weight, and either rasping this point into powder or tearing it bodily
from the rock to which it belongs; while the water which everywhere
circulates upon the bed of the glacier continually washes the detritus
away and leaves the rock clean for further abrasion. Confining the
action of glaciers to the simple rubbing away of the rocks, and
allowing them sufficient time to act, it is not a matter of opinion,
but a physical certainty, that they will scoop out valleys. But the
glacier does more than abrade. Rocks are not homogeneous; they are
intersected by joints and places of weakness, which divide them into
virtually detached masses. A glacier is undoubtedly competent to root
such masses bodily away. Indeed the mere _à priori_ consideration of
the subject proves the competence of a glacier to deepen its bed.
Taking the case of a glacier 1,000 feet deep (and some of the older
ones were probably three times this depth), and allowing 40 feet of
ice to an atmosphere, we find that on every square inch of its bed
such a glacier presses with a weight of 375 lbs, and on every square
yard of its bed with a weight of 486,000 lbs. With a vertical pressure
of this amount the glacier is urged down its valley by the pressure
from behind. We can hardly, I think, deny to such a tool a power of
excavation.
The retardation of a glacier by its bed has been referred to as
proving its impotence as an erosive agent; but this very retardation
is in some measure an expression of the magnitude of the erosive
energy. Either the bed must give way, or the ice must slide over
itself. We get indeed some idea of the crushing pressure which the
moving glacier exercises against its bed-from the fact that the
resistance, and the effort to overcome it, are such as to make the
upper layers of a glacier move bodily over the lower ones--a portion
only of the total motion being due to the progress of the entire mass
of the glacier down its valley.
The sudden bend in the valley of the Rhone at Martigny has also been
regarded as conclusive evidence against the theory of erosion. 'Why,'
it has been asked, I did not the glacier of the Rhone go straight
forward instead of making this awkward bend?' But if the valley be a
crack, why did the crack make this bend? The crack, I submit, had at
least as much reason to prolong itself in a straight line as the
glacier had. A statement of Sir John Herschel with reference to
another matter is perfectly applicable here: 'A crack once produced
has a tendency to run--for this plain reason, that at its momentary
limit, at the point at which it has just arrived, the divellent force
on the molecules there situated is counteracted only by half of the
cohesive force which acted when there was no crack, viz. the cohesion
of the uncracked portion alone' ('Proc. Roy. Soc.' vol. xii.
p. 678). To account, then, for the bend, the adherent of the fracture
theory must assume the existence of some accident which turned the
crack at right angles to itself; and he surely will permit the
adherent of the erosion theory to make a similar assumption.
The influence of small accidents on the direction of rivers is
beautifully illustrated in glacier streams, which are made to cut
either straight or sinuous channels by causes apparently of the most
trivial character. In his interesting paper 'On the Lakes of
Switzerland,' M. Studer also refers to the bend of the Rhine at
Sargans in proof that the river must there follow a pre-existing
fissure. I made a special expedition to the place in 1864; and though
it was plain that M. Studer had good grounds for the selection of this
spot, I was unable to arrive at his conclusion as to the necessity of
a fissure.
Again, in the interesting volume recently published by the Swiss
Alpine Club, M. Desor informs us that the Swiss naturalists who met
last year at Samaden visited the end of the Morteratsch glacier, and
there convinced themselves that a glacier had no tendency whatever to
imbed itself in the soil. I scarcely think that the question of
glacier erosion, as applied either to lakes or valleys, is to be
disposed of so easily. Let me record here my experience of the
Morteratsch glacier.
I took with me in 1864 a theodolite to Pontresina, and while there had
to congratulate myself on the aid of my friend Mr. Hirst, who in 1857
did such good service upon the Mer de Glace and its tributaries. We
set out three lines across the Morteratsch glacier, one of which
crossed the ice-stream near the well-known hut of the painter Georgei,
while the two others were staked out, the one above the hut and the
other below it. Calling the highest line A, the line which crossed
the glacier at the hut B, and the lowest line C, the following are the
mean hourly motions of the three lines, deduced from observations
which extended over several days. On each line eleven stakes were
fixed, which are designated by the figures 1, 2, 3, &c. in the
Tables.
Morteratsch Glacier, Line A.
No. of Stake. Hourly Motion.
1 0.35 inch.
2 0.49 inch.
3 0.53 inch.
4 0.54 inch.
5 0.56 inch.
6 0.54 inch.
7 0.52 inch.
8 0.49 inch.
9 0.40 inch.
10 0.29 inch.
11 0.20 inch.
As in all other measurements of this kind, the retarding influence of
the sides of the glacier is manifest: the centre moves with the
greatest velocity.
Morteratsch Glacier, Line B.
No. of Stake. Hourly Motion.
1 0.05 inch.
2 0.14 inch.
3 0.24 inch.
4 0.32 inch.
5 0-41 inch.
6 0.44 inch.
7 0.44 inch.
8 0.45 inch.
9 0.43 inch.
10 0.44 inch.
11 0.44 inch.
The first stake of this line was quite close to the edge of the
glacier, and the ice was thin at the place, hence its slow motion.
Crevasses prevented us from carrying the line sufficiently far across
to render the retardation of the further side of the glacier fully
evident.
Morteratsch Glacier, Line C.
No. of Stake Hourly Motion.
1 0.05 inch.
2 0.09 inch.
3 0.18 inch.
4 0.20 inch.
5 0.25 inch.
6 0.27 inch.
7 0.27 inch.
8 0.30 inch.
9 0.21 inch.
10 0.20 inch.
11 0.16 inch.
Comparing the three lines together, it will be observed that the
velocity diminishes as we descend the glacier. In 100 hours the
maximum motion of three lines respectively is as follows:
Maximum Motion in 100 hours.
Line A 56 inches
Line B 45 inches.
Line C 30 inches.
This deportment explains an appearance which must strike every
observer who looks upon the Morteratsch from the Piz Languard, or from
the new Bernina Road. A medial moraine runs along the glacier,
commencing as a narrow streak, but towards the end the moraine
extending in width, until finally it quite covers the terminal portion
of the glacier. The cause of this is revealed by the foregoing
measurements, which prove that a stone on the moraine where it is
crossed by the line A approaches a second stone on the moraine where
it is crossed by the line C with a velocity of twenty-six inches per
one hundred hours. The moraine is in a state of longitudinal
compression. Its materials are more and more squeezed together, and
they must consequently move laterally and render the moraine at the
terminal portion of the glacier wider than above.
The motion of the Morteratsch glacier, then, diminishes as we descend.
The maximum motion of the third line is thirty inches in one hundred
hours, or seven inches a day--a very slow motion; and had we run a
line nearer to the end of the glacier, the motion would have been
slower still. At the end itself it is nearly insensible. [Footnote:
The snout of the Aletsch Glacier has a diurnal motion of less than two
inches, while a mile or so above the snout the velocity is eighteen
inches. The spreading out of the moraine is here very striking.] Now
I submit that this is not the Place to seek for the scooping power of
a glacier. The opinion appears to be prevalent that it is the snout
of a glacier that must act the part of ploughshare; and it is
certainly an erroneous opinion. The scooping power will exert itself
most where the weight and the motion are greatest. A glacier's snout
often rests upon matter which has been scooped from the glacier's bed
higher up. I therefore do not think that the inspection of what the
end of a glacier does or does not accomplish can decide this question.
The snout of a glacier is potent to remove anything against which it
can fairly abut; and this power, notwithstanding the slowness of the
motion, manifests itself at the end of the Morteratsch glacier. A
hillock, bearing pine-trees, was in front of the glacier when Mr.
Hirst and myself inspected its end; and this hillock is being bodily
removed by the thrust of the ice. Several of the trees are
overturned; and in a few years, if the glacier continues its reputed
advance, the mound will certainly be ploughed away.
The question of Alpine conformation stands, I think, thus: We have,
in the first place, great valleys, such as those of the Rhine and the
Rhone, which we might conveniently call valleys of the first order.
The mountains which flank these main valleys are also cut by lateral
valleys running into the main ones, and which may be called valleys of
the second order. When these latter are examined, smaller valleys are
found running into them, which may be called valleys of the third
order. Smaller ravines and depressions, again, join the latter, which
may be called valleys of the fourth order, and so on until we reach
streaks and cuttings so minute as not to merit the name of valleys at
all. At the bottom of every valley we have a stream, diminishing in
magnitude as the order of the valley ascends, carving the earth and
carrying its materials to lower levels. We find that the larger
valleys have been filled for untold ages by glaciers of enormous
dimensions, always moving, grinding down and tearing away the rocks
over which they passed. We have, moreover, on the plains at the feet
of the mountains, and in enormous quantities, the very matter derived
from the sculpture of the mountains themselves.
The plains of Italy and Switzerland are cumbered by the _débris_ of the
Alps. The lower, wider, and more level valleys are also filled to
unknown depths with the materials derived from the higher ones. In
the vast quantities of moraine-matter which cumber many even of the
higher valleys we have also suggestions as to the magnitude of the
erosion which has taken place. This moraine-matter, moreover, can
only in small part have been derived from the falling of rocks upon
the ancient glacier; it is in great part derived from the grinding and
the ploughing-out of the glacier itself. This accounts for the
magnitude of many of the ancient moraines, which date from a period
when almost all the mountains were covered with ice and snow, and
when, consequently, the quantity of moraine-matter derived from the
naked crests cannot have been considerable.
The erosion theory ascribes the formation of Alpine valleys to the
agencies here briefly referred to. It invokes nothing but true
causes. Its artificers are still there, though, it may be, in
diminished strength; and if they are granted sufficient time, it is
demonstrable that they are competent to produce the effects ascribed
to them. And what does the fracture theory offer in comparison? From
no possible application of this theory, pure and simple, can we obtain
the slopes and forms of the mountains. Erosion must in the long run
be invoked, and its power therefore conceded. The fracture theory
infers from the disturbances of the Alps the existence of fissures;
and this is a probable inference. But that they were of a magnitude
sufficient to produce the conformation of the Alps, and that they
followed, as the Alpine valleys do, the lines of natural drainage of
the country, are assumptions which do not appear to me to be justified
either by reason or by observation.
There is a grandeur in the secular integration of small effects
implied by the theory of erosion almost superior to that involved in
the idea of a cataclysm. Think of the ages which must have been
consumed in the execution of this colossal sculpture. The question
may, of course, be pushed further. Think of the ages which the molten
earth required for its consolidation. But these vaster epochs lack
sublimity through our inability to grasp them. They bewilder us, but
they fail to make a solemn impression. The genesis of the mountains
comes more within the scope of the intellect, and the majesty of the
operation is enhanced by our partial ability to conceive it. In the
falling of a rock from a mountain-head, in the shoot of an avalanche,
in the plunge of a cataract, we often see more impressive
illustrations of the power of gravity than in the motions of the
stars. When the intellect has to intervene, and calculation is
necessary to the building up of the conception, the expansion of the
feelings ceases to be proportional to the magnitude of the phenomena.
*****
I will here record a few other measurements executed on the Rosegg
glacier: the line was staked out across the trunk formed by the
junction of the Rosegg proper with the Tschierva glacier, a short
distance below the rocky promontory called Agaliogs.
Rosegg Glacier.
No. of Stake. Hourly Motion.
1 0.01 inch.
2 0.05
3 0.07
4 0.10
5 0.11
6 0.13
7 0.14
8 0.18
9 0.24
10 0.23
11 0.24
This is an extremely slowly moving glacier; the maximum motion hardly
amounts to seven inches a day. Crevasses prevented us from continuing
the line quite across the glacier.
********************
X. RECENT EXPERIMENTS ON FOG-SIGNALS.
[Footnote: A discourse delivered in the Royal Institution, March 22,
1878.]
The care of its sailors is one of the first duties of a maritime
people, and one of the sailor's greatest dangers is his proximity to
the coast at night. Hence, the idea of warning him of such proximity
by beacon-fires placed sometimes on natural eminences and sometimes on
towers built expressly for the purpose. Close to Dover Castle, for
example, stands an ancient Pharos of this description.
As our marine increased greater skill was invoked, and lamps
reinforced by parabolic reflectors poured their light upon the sea.
Several of these lamps were sometimes grouped together so as to
intensify the light, which at a little distance appeared as if it
emanated from a single source. This 'catoptric' form of apparatus is
still to some extent employed in our lighthouse-service, but for a
long time past it has been more and more displaced by the great lenses
devised by the illustrious Frenchman, Fresnel.
In a first-class 'dioptric' apparatus the light emanates from a lamp
with several concentric wicks, the flame of which, being kindled by a
very active draught, attains to great intensity. In fixed lights the
lenses refract the rays issuing from the lamp so as to cause them to
form a luminous sheet which grazes the sea-horizon. In revolving
lights the lenses gather up the rays into distinct beams, resembling
the spokes of a wheel, which sweep over the sea and strike the eye of
the mariner in succession.