MOUNTAIN ARCHITECTURE.
The splendour falls on castle walls
And snowy summits old in story;
The long light shakes across the lakes,
And the wild cataract leaps in glory.
Blow, bugle, blow, set the wild echoes flying;
Blow, bugle; answer, echoes, dying, dying, dying.
TENNYSON.
The dying splendours of the sun slowly sinking and entering the
"gates of the West" may well serve as a fitting emblem of the
mountains in their beautiful old age, awaiting in silent and calm
dignity the time when they also must be brought low, and sink in the
waters of the ocean, as the sun appears daily to do. Yes, they too
have their day. They too had their rising, when mighty forces brought
them up out of their watery bed. Many of them have passed their
hey-day of youth, and their midday; while others, far advanced in old
age, are nearing the end of their course.
But as the sun rises once more over eastern seas to begin another
day, so will the substance of the mountains be again heaved up after
a long, long rest under the sea, and here and there will rise up from
the plains to form the lofty mountain-ranges of a distant future.
Everywhere we read the same story, the same circle of changes. The
Alpine peak that proudly rears its head to the clouds must surely
be brought low, and finally come back to the same ocean from which
those clouds arose. It is in this way that the balance between land
and water is preserved. In passing through such a great circle of
changes, the mountains assume various forms and shapes which are
determined by:--
1. Their different ages and states of decay.
2. The different kinds of rocks of which they are composed, and
especially by their "joints," or natural divisions.
3. The different positions into which these rocky layers have
been squeezed, pushed, and crumpled by those stupendous forces
of upheaval of which we spoke in chapter vi.
Let us therefore glance at some of these external forms, and then
look at the internal structure of mountains.
In so doing we shall find that we have yet a good deal more to learn
about mountains and how they were made; and also we shall then be in
a better position to realise not only how very much denudation they
have suffered, but also how greatly they have been disturbed since
their rocks were first made.
Every one who knows mountains must have observed how some are smooth
and rounded, others sharp and jagged, with peaks and pinnacles
standing out clearly against the sky; some square and massive, with
steep walls forming precipices; others again spread out widely at
their base, but the sloping sides end in a sharp point at the top,
giving to the mountain the appearance of a cone. Their diversities of
shape are so endless that we cannot attempt to describe them all.
First, with regard to the general features of mountains. Looked
at broadly, a mountain-range is not a mere line of hills or
mountains rising straight up from a plain on each side, such as
school-boys often draw in their maps; very far from it. Take the
Rocky Mountains, for instance. "It has been truly said of the Rocky
Mountains that the word 'range' does not express it at all. It is a
whole country populous with mountains. It is as if an ocean of molten
granite had been caught by instant petrifaction when its billows were
rolling heaven high."[28]
[28] "The Crest of the Continent," by Ernest Ingersoll, Chicago,
1885.
It has often been observed by mountain climbers that when they get
to the top of a high mountain, and take a bird's-eye view of the
country, all the mountain-tops seem to reach to about the same
height, so that a line joining them would be almost level. For this
reason, perhaps, writers so often compare them to the waves of an
ocean. This feature is very conspicuous in the case of the Scotch
Highlands.
Sir A. Geikie has well described what he saw from the top of Ben
Nevis:--
"Much has been said and written about the wild, tumbled sea of
the Highland Hills. But as he sits on his high perch, does it
not strike the observer that there is after all a wonderful
orderliness, and even monotony, in the waves of that wide
sea? And when he has followed their undulations from north to
south, all round the horizon, does it not seem to him that
these mountain-tops and ridges tend somehow to rise to a
general level; that, in short, there is not only on the great
scale a marked similarity of contour about them, but a still
more definite uniformity of average height? To many who have
contented themselves with the bottom of the glen, and have
looked with awe at the array of peaks and crags overhead, this
statement will doubtless appear incredible. But let any one get
fairly up to the summits and look along them, and he will not
fail to see that the statement is nevertheless true. From the
top of Ben Nevis this feature is impressively seen. Along the
sky-line, the wide sweep of summits undulates up to a common
level, varied here by a cone and there by the line of some
strath or glen, but yet wonderfully persistent round the whole
panorama. If, as sometimes happens in these airy regions, a
bank of cloud with a level under-surface should descend upon
the mountains, it will be seen to touch summit after summit,
the long line of the cloud defining, like a great parallel
ruler, the long level line of the ridges below. I have seen
this feature brought out with picturesque vividness over the
mountains of Knoydart and Glen Garry. Wreaths of filmy mist had
been hovering in the upper air during the forenoon. Towards
evening, under the influence of a cool breeze from the north,
they gathered together into one long band that stretched for
several miles straight as the sky-line of the distant sea,
touching merely the higher summits and giving a horizon by
which the general uniformity of level among the hills could be
signally tested. Once or twice in a season one may be fortunate
enough to get on the mountains above such a stratum of mist,
which then seems to fill up the irregularities of the general
platform of hill-tops, and to stretch out as a white phantom
sea, from which the highest eminences rise up as little islets
into the clear air of the morning.... Still more striking
is the example furnished by the great central mass of the
Grampians, comprising the Cairngorm Mountains and the great
corries and precipices round the head of the Dee. This tract
of rugged ground, when looked at from a distance, is found to
present the character of a high, undulating plateau."[29]
[29] Scenery of Scotland page 130, new edition.
This long level line of the Highland mountain-tops may be seen very
well from the lower country outside; for example, from the isles of
Skye and Eigg, where one may see the panorama between the heights of
Applecross and the Point of Ardnamurchan showing very clearly the
traces of the old table-land.
How are we to explain this curious fact, so opposed to our first
impressions of a mountain region? It is quite clear that the
old plateau thus marked out cannot be caused by the arrangement
or position of the rocks of which the Highlands are composed. If
these rocks were found to be lying pretty evenly in flat layers,
or strata, undisturbed by great earth-movements, we could readily
understand that they would form a plateau. But the reverse is the
case: the rocks are everywhere thrown into folds, and frequently
greatly displaced by "faults;" yet these important geological
features have little or no connection with the external aspect of the
country. It is therefore useless to look to internal structure for
an explanation. We must look outside, and consider what has been for
ages and ages taking place here.
As already pointed out, an enormous amount of solid rock has been
removed from this region--thousands and thousands of feet. It was
long ago planed down by the action of water, so that a table-land
once existed of which the tops of the present mountains are isolated
fragments. No other conclusion is possible. To the geologist every
hill and valley throughout the whole length and breadth of the
Highlands bears striking testimony to this enormous erosion. The
explanation we are seeking may therefore be summed up in one word,
"denudation." The valleys that now intersect the table-land have been
carved out of it. If we could in imagination put back again onto
the present surface what has been removed, we should have a mental
picture of the Highlands as a wide, undulating table-land; and this
rolling plain would suggest the bottom of the sea. The long flat
surfaces of the Highland ridges, cut across the edges of inclined
or even upright strata, are the fragments of a former base-line of
erosion; that is, they represent the general submarine level to which
the Highlands were reduced after exposure to the action of "rain and
rivers," and finally of the sea. As the sea gradually spread over it,
it planed down everything that had not been previously worn away, and
so reduced the whole surface to one general level like the sea-bed of
the present day. But it is not necessary to suppose that the whole
region was under water at the same time, and it is probable that
there were separate inland seas or lakes. In these the rocks of the
Old Red Sandstone were formed; and they in their turn have suffered
so much denudation that only patches and long strips of them are left
on the borders of the Highlands.
Before we speak of individual mountains and their shapes, it is
important to bear in mind another fact about mountain-chains;
namely, that they are very low in proportion to their breadth and
length. The great heights reached by some mountains produce such a
powerful impression on our senses that we hardly realise how very
insignificant they really are. It is only by drawing them on a true
scale that we can realise this. The surface of the earth is so vast
that even the highest mountains are in proportion but as the little
roughnesses on the skin of an orange. Fig. 2 (see chap, vii., p. 236)
represents a section through the Highlands, drawn on the same scale
for height as for length.
What has been said about the Highland plateau applies equally well
to many other mountain-ranges. Mr. Ruskin observed something rather
similar in the Alps. He says,--
"The longer I stayed in the Alps, and the more closely I
examined them, the more I was struck by the one broad fact of
there being a vast Alpine plateau, or mass of elevated land,
upon which nearly all the highest peaks stood like children set
upon a table, removed, in most cases, far back from the edge
of the plateau, as if for fear of their falling; ... and for
the most part the great peaks are not allowed to come to the
edge of it, but remain like the keeps of castles, withdrawn,
surrounded league beyond league by comparatively level fields
of mountains, over which the lapping sheets of glaciers writhe
and flow, foaming about the feet of the dark central crests
like the surf of an enormous sea-breaker hurled over a rounded
rock and islanding some fragment of it in the midst. And the
result of this arrangement is a kind of division of the whole
of Switzerland into an upper and a lower mountain world,--the
lower world consisting of rich valleys, bordered by steep but
easily accessible, wooded banks of mountain, more or less
divided by ravines, through which glimpses are caught of the
higher Alps; the upper world, reached after the first steep
banks of three thousand or four thousand feet in height have
been surmounted, consisting of comparatively level but most
desolate tracts of moor and rock, half covered by glacier, and
stretching to the feet of the true pinnacles of the chain."
He then points out the wisdom of this arrangement, and shows how it
protects the inhabitants from falling blocks and avalanches; and
moreover, the masses of snow, if cast down at once into the warmer
air, would melt too fast and cause furious inundations.
All the various kinds of rocks are differently affected by the
atmospheric influences of decay, and so present different external
appearances and shapes, so that after a little experience the
geologist can recognize the presence of certain rocks by the kind
of scenery they produce; and this knowledge is often of great use
in helping him to unravel the geological structure of a difficult
region. Thus granite, crystalline schists, slates, sandstones, and
limestones, all "weather" in their own ways, and moreover split up
differently, because their joints and other natural lines of division
run in different ways.
Thus granite is jointed very regularly, some of the joints running
straight down and others running horizontally, so that the rain and
atmosphere seize on these lines and widen them very considerably; and
thus the granite is weathered out either in tall upright columns,
like those seen at Land's End, or else into great square-shaped
blocks with their corners rounded off, presenting the appearance
of a number of knapsacks lying one over the other. In this way
we can account for the well-known "Tors" of Devonshire, and the
"Rocking Stones." Granite weathers rapidly along its joints, and
its surfaces crumble away more rapidly than might be expected,
considering how hard a rock it is; but the felspar which is its chief
mineral constituent is readily decomposed by rain water, which acts
chemically upon it. The deposits of China clay in Devonshire are
the result of the decomposition and washing away of the granite of
Dartmoor.
Granite mountains are generally rounded and "bossy," breaking now
and then into cliffs, the faces of which are riven by huge joints,
and present a very different appearance from those composed of
crystalline schists with their sharp crests and peaks. Ben Nevis and
the Cairngorms are partly composed of granite.
Gneiss is a rock composed of the same minerals as granite; namely,
mica, quartz, and felspar. And yet mountains composed of this rock
have quite a different aspect, and sometimes, as in the Alps, produce
very sharp and jagged pinnacles. The reason of this is that gneiss
splits in a different way from granite, because its minerals are
arranged in layers, and so it is more like a crystalline schist.
Mica-schist is another rock very abundant in mountain regions. This
rock is composed of quartz and mica arranged in wavy layers. The
mica, which is very conspicuous, lies in thin plates, sometimes
so dovetailed into each other as to form long continuous layers
separating it from those of the quartz; and it readily splits along
the layers of mica. This mineral is easily recognised by its bright,
shiny surface. There are, however, two varieties,--one of a light
colour and the other black.
Mica-schist and gneiss are often found in the same region, and are
the materials of which most of the highest peaks in Europe are
composed. We find them abounding in the district of Mont Blanc; and
all the monarch's attendant _aiguilles_, with the splintered ridges
enclosing the great snowfields in the heart of the chain, consist
mostly of these two rocks. The Matterhorn, Weisshorn, Monte Viso, the
Grand Paradis, the Aiguille Verte and Aiguille du Dru are examples of
the wonderful forms produced by the breaking up and decay of these
two rocks.
The different varieties of slate split in a very marked way. Slates
are often associated with the schists, and exert their influence in
modifying the scenery.
Limestone ranges, though less striking in the outlines of their
crests than those composed of slates and crystalline schists, and not
reaching to such heights, are nevertheless not at all inferior in the
grandeur of their cliffs, which frequently extend for miles along the
side of a valley in vast terraces, whose precipitous walls are often
absolutely inaccessible. The beauty of limestone mountains is often
enhanced by the rich pastures and forests which clothe their lower
slopes. The dolomitic limestone of the Italian Tyrol, being gashed by
enormous vertical joints and at the same time having been formed in
rather thin layers which break up into small blocks, produces some
very striking scenery. But wild as these mountainous ridges may be,
their forms can never be confounded with those of the crystalline
schists; for however sharp their pinnacles may appear at first sight,
careful examination will always show that their outline is that
of ruined masonry, suggesting crumbling battlements and tottering
turrets, and not the curving, flame-like crests and splintered peaks
of the crystalline schists.[30]
[30] Bonney.
It has already been explained that all sedimentary rocks have been
formed under water in layers or strata, and it must be obvious
that the stratification of such rocks has an important influence
on scenery; and very much depends on whether the strata have been
left undisturbed, with perhaps just a slight slope, or whether they
have been folded and crumpled; for the position of the strata, or
"bedding," as it is called,--whether flat, inclined, vertical, or
contorted,--largely determines the nature of the surface. Undoubtedly
the most characteristic scenery formed by stratified rocks is to be
seen in those places where the "bedding" is horizontal, or nearly so,
and the strata are massive. A mountain constructed of such materials
appears as a colossal pyramid, the level lines of stratification
looking like great courses of masonry. The joints that cut across the
strata allow it to be cleft into great blocks and deep chasms; so
that, as in the case of the dolomitic limestone above mentioned, we
find a resemblance to ruined buildings.
We cannot find a better example of this in our own country than the
mountains of sandstone and conglomerate (of the Cambrian age) that
here and there lie on the great platform of old gneiss in the west of
Sutherland and Ross. Sir A. Geikie says,--
"The bleak, bare gneiss, with its monotonous undulations,
tarns, and bogs, is surmounted by groups of cones, which for
individuality of form and independence of position better
deserve to be called mountains than most of the eminences to
which that name is given in Scotland. These huge pyramids,
rising to heights of between two thousand and four thousand
feet, consist of dark red strata, so little inclined that
their edges can be traced by the eye in long, level bars on
the steeper hillsides and precipices, like lines of masonry.
Here and there the hand of time has rent them into deep rifts,
from which long 'screes' (slopes of loose stones) descend into
the plains below, as stones are detached from the shivered
walls of an ancient battlement. Down their sides, which have in
places the steepness of a bastion, vegetation finds but scanty
room along the projecting ledges of the sandstone beds, where
the heath and grass and wildflowers cluster over the rock in
straggling lines and tufts of green; and yet, though nearly as
bare as the gneiss below them, these lofty mountains are far
from presenting the same aspect of barrenness. The prevailing
colour of their component strata gives them a warm red hue,
which even at noon contrasts strongly with the grey of the
platform of older rock.... These huge isolated cones are among
the most striking memorials of denudation anywhere to be seen
in the British Isles. Quinag, Canisp, Suilven, Coulmore, and
the hills of Coygoch, Dundonald, Loch Maree, and Torridon are
merely detached patches of a formation not less than seven
thousand or eight thousand feet thick, which once spread
over the northwest of Scotland. The spaces between them were
once occupied by the same dull red sandstone; the horizontal
stratification of one hill, indeed, is plainly continuous with
that of the others, though deep and wide valleys, or miles
of low moorland, may now lie between. While the valleys have
been worn down through the sandstone, these strange pyramidal
mountains that form so singular a feature in the landscapes of
the northwest highlands have been left standing, like lonely
sea-stacks, as monuments of long ages of waste."[31]
[31] Scenery of Scotland, page 201, new edition.
Again, the vast table-lands of the Colorado region illustrate on
a truly magnificent scale, to which there is no parallel in the
Old World, the effects of atmospheric erosion on undisturbed and
nearly level strata. Here we find valleys and river gorges deeper
and longer than any others in the world; great winding lines of
escarpment, like ranges of sea cliffs; terraced slopes rising at
various levels; huge buttresses and solitary monuments, standing like
islands out of the plains; and lastly, great mountain masses carved
out into the most striking and picturesque shapes, yet with their
lines of "bedding" clearly marked out.
On the other hand, where, as is almost always the case in
mountain-ranges, the stratified rocks have been folded, crumpled,
twisted, and fractured by great "faults," we find a very different
result. In these cases the rocks have generally been very much
altered by the action of heat. For here we find crystalline schists,
gneiss, granite, and other rocks in the formation of which heat has
played an important part; and very often the igneous rocks have
forced their way through those of sedimentary origin and altered them
into what are called metamorphic rocks (see chapter v., page 156).
Thus they have lost much of their original character and structure.
The repeated uplifts and subsidences of the earth's crust, by which
the continents of the world have been raised up out of the sea
to form dry land, have, broadly speaking, thrown the rocky strata
into a series of wave-like undulations. In some extensive regions
these undulations are so broad and low that the curvature is quite
imperceptible, and the strata appear to lie in horizontal layers, or
to slope very slightly in a certain direction. This is, in a general
way, the position of the strata of which plains and plateaux are
composed.
But in the longer and comparatively narrow mountain regions that
traverse each of the great continents, forming, as it were, backbones
to them, the undulations are very much more frequent, narrower, and
higher. Sometimes the rocks have been thrown into huge open waves, or
the folds are closely crowded together, so that the strata stand on
their ends, or are even completely overturned, and thus their proper
order of succession is reversed, and the older ones actually lie on
the top of the newer ones.
As we approach a great mountain-chain we observe many minor ridges
and smaller chains running roughly parallel with it, and, as it
were, foreshadowing the great folds met with in the centre of the
chain and among its highest peaks. These small folds become sharper
and closer the nearer we get to the main chain, and evidently were
formed by the same movements that uplifted the higher ranges beyond;
but the force was not so great. Thus we find the great Alpine chain
flanked to the north by the smaller ranges of the Jura Mountains; and
on the south, side of the Himalayas we find similar smaller ranges of
hills.
Ruskin thus describes his impression of the Jura ranges, which he
very aptly compares with a swell on the sea far away from a storm,
the storm being represented by the wild sea of Alpine mountains:--
"Among the hours of his life to which the writer looks back
with peculiar gratitude, as having been marked with more
than ordinary fulness of joy or clearness of teaching, is
one passed, now some years ago, near time of sunset, among
the masses of pine forest which skirt the course of the Ain,
above the village of Champagnole, in the Jura. It is a spot
which has all the solemnity, with none of the savageness, of
the Alps; where there is a sense of a great power beginning
to be manifested in the earth, and of a deep and majestic
concord in the rise of the long low lines of piny hills,--the
first utterance of those mighty mountain symphonies, soon to
be more loudly lifted and wildly broken along the battlements
of the Alps. But their strength is as yet restrained; and the
far-reaching ridges of pastoral mountain succeed each other,
like the long and sighing swell which moves over quiet waters
from some far-off stormy sea.
"And there is a deep tenderness pervading that vast monotony.
The destructive forces and the stern expression of the central
ranges are alike withdrawn. No frost-ploughed, dust-encumbered
paths of ancient glacier fret the soft Jura pastures; no
splintered heaps of ruin break the fair ranks of her forests;
no pale, defiled, or furious rivers rend their rude and
changeful ways among her rocks. Patiently, eddy by eddy, the
clear green streams wind along their well-known beds; and under
the dark quietness of the undisturbed pines there spring up,
year by year, such company of joyful flowers as I know not the
like among all the blessings of the earth."
Long faults, or fractures, where the strata have been first bent
and then broken, and afterwards have been forced up or have slid
down hundreds or even thousands of feet, are very numerous in
mountain-ranges; and by suddenly bringing quite a different set of
rocks to the surface, these faults cause considerable difficulty to
the geologist, as he goes over the ground and endeavours to trace the
positions of the different rocks.
In these vast folds it sometimes happens that portions of older (and
lower) strata are caught up and so embedded among those of newer
rocks. It will therefore be readily perceived that to unravel the
geological structure of a great mountain-chain is no easy task.
We need not then be surprised if in some cases the arrangement of
the rocks of mountains is not thoroughly understood. The wonder
is, when we think of the numerous difficulties which the geologist
encounters,--the arduous ascents, the precipices, glaciers,
snowfields obscuring the rocks from his view, the overlying soil
of the lower parts, and the steep crests and dangerous ridges that
separate the snowfields,--that so much has already been discovered in
this difficult branch of geology.
However, the general arrangement of the rocks of which many
mountain-chains are composed has been satisfactorily made out in not
a few cases. Let us look into some of these and see what has been
discovered.
You will remember the structure of the Weald, described in chap.
vii., pp. 235-238, and how we showed that a great low arch of chalk
strata has been entirely removed over that area, so that at the
present time only its ends are seen forming the escarpments of the
North and South Downs. This area, then, is now a great open valley,
or rather a gently undulating plain enclosed by low chalk hills. Now,
an arch of this kind is called an "anticline," and it might have been
expected that it would have remained more or less unbroken to the
present day. Why, then, has it suffered destruction?
In the first place, chalk is a soft rock, and one that rain water can
dissolve; but more than that, its arch-like structure was against it,
and its chance of preservation was decidedly small. In architecture
the arch is the most firm and stable structure that can be made; but
not so with strata, and this is the reason. Such an arch was not made
of separate blocks, closely fitting and firmly cemented together;
on the contrary, the arch was stretched and heaved up from below.
It therefore must have been more or less cracked up; for rocks are
apt to split when bent, although when deeply buried under a great
thickness of overlying rocks, they will bend very considerably
without snapping. But this was not the case here. And so the forces
of denudation set to work upon an already somewhat broken mass of
rock. Try to picture to yourself this old low arch of chalk as it was
when it first appeared as dry land. Probably some of it had already
been planed away by the waves of the sea, and what was left was by
no means well calculated to withstand the action of the agents of
denudation. If you look back to the figure, you will see the dotted
lines showing the former outline of this anticline, or arch, and you
perceive at once that the strata must have been sloping outwards away
from the middle. Now, this one fact greatly influenced its fate, for
an anticline cannot be regarded as a strong or stable arrangement
of strata. It is easy to see why; suppose a little portion were cut
away on one side at its base by some stream. It is clear that a kind
of overhanging cliff would be left, and blocks of chalk would sooner
or later come rolling down into the valley of the little stream.
When these had fallen, they would leave an inclined plane down which
others would follow; and this would continue to take place until the
top of the arch was reached. The same reasoning applies to the other
side. It is very seldom that arches, or anticlines, can last for a
long time. The outward slope of the strata and their broken condition
are against them.
But when the rocks dip _inwards_, to form a kind of trough or basin,
it is just the opposite. Such basins are known as "synclines;" and
a structure of this kind can be shown to be much more stable and
permanent than an anticline. The strata, instead of being stretched
out and cracked open, have been squeezed together.
It is very important to bear this in mind, and to remember how
differently anticlines and synclines are affected; for this simple
rule is illustrated over and over again in mountain-ranges:--
Anticlines, being unstable, are worn away until they become
valleys.
Synclines, being stable, are left and frequently form mountains.
Now look at the section through the Appalachian chain (see Fig.
1), and you will see that each hill is a syncline, and the valleys
between them are anticlines. This happens so frequently that almost
every range of mountains furnishes examples; but as every rule has
its exceptions, so this one has, and we may find an example in the
case of the Jura Mountains outside the Alps.
It will be seen from the section that the ridges are formed by
anticlines, and the valleys by synclines. But on looking a little
more closely, we see that the tops of the former have suffered a
considerable amount of erosion (as indicated by the dotted lines).
Now, the reason why they have not been completely worn down into
valleys is that these rocks were once covered by others overlying
them, so that this outer covering of rocks had first to be removed
before they could be attacked by rain and rivers. These wave-like
ridges of the Jura are being slowly worn down; and the time must
come when they will be carved out into valleys, while the synclines
between them will stand out as hills. It is simply a question of
time. But many mountain-chains have a far more complicated structure
than that of the Appalachians, and consist of violently crumpled and
folded strata (see section of Mont Blanc, Fig. 3).
[Illustration: SECTIONS OF MOUNTAIN-RANGES, SHOWING THEIR
STRUCTURE AND THE AMOUNT OF ROCK WORN AWAY.]
It might naturally be asked how such sections are made, considering
that we cannot cut through mountains in order to find out their
structure; but Nature cuts them up for us, gashing their sides with
ravines and valleys carved out by streams and rivers, and in steep
cliffs and precipices we find great natural sections that serve our
purpose almost equally well. Sometimes, however, we get considerable
help from quarries and railway-cuttings.
Take, for example, one of the synclinal folds in the Appalachian
chain. Its structure is ascertained somewhat as follows. Suppose you
began to ascend the hill, armed with a good map, a pocket-compass, a
clinometer,--a little instrument for measuring the angles at which
strata dip or slope,--and with a bag on your back for specimens
of rocks and fossils. At the base of the hill you might notice at
starting a certain layer of rock--say a limestone--exposed by the
side of the stream. It will be so many feet thick, and will contain
such-and-such fossils, by means of which you can identify it; and
it will dip into the interior of the hill at a certain angle, as
measured by the clinometer. As you rise higher, this rock may be
succeeded by sandstone of a certain thickness, and likewise dipping
into the hill; and so with the other rocks that follow, until you
reach the summit.
By the time you have reached the top of the hill, you know the nature
of all the rocks up that side, and the way they dip; and all your
observations are carefully recorded in a notebook. Then you begin
to descend on the other side, and in so doing you find the same set
of rocks coming out at the surface all in the same order; only this
order is now reversed, because you are following them downwards
instead of upwards. Of course they are hidden in many places by soil
and loose stones; but that does not matter, because at other places
they are exposed to view, especially along ravines, carved out of
the mountain-side. Also rocks "weather" so differently that they can
often be distinguished even at a distance.
In this kind of way you can find out the structure of a mountain,
and draw a section of it when you get home, by following out and
completing the curves of the strata as indicated at or near the
surface; and you find they fit in nicely together.
Fig. 3 (see page 307) represents what is believed to be the general
arrangement of the rocks of Mont Blanc. The section is greatly
simplified, because many minor folds and all the faults, or
dislocations, are omitted. Now, in this case we have an example of
what is known as the "fan-structure." It will be seen at once that
the folds have been considerably squeezed together; and the big fold
in the centre indicated by dotted lines has been so much compressed
in the lower part--that is, in what is now Mont Blanc--that its sides
were brought near to each other until they actually sloped inwards
instead of outwards.
You may easily imitate this structure by taking a sheet of paper,
laying it on the table, and then, putting one hand on each side of
it, cause it to rise up in a central fold by pressing your hands
towards each other. Notice carefully what happens. First, you get a
low arch, or anticline, like that of the Weald. Then as you press it
more, the upward fold becomes sharper and narrower; then continue
pressing it, and you will find the fold bulging out at the top, but
narrowing in below until you get this fan-structure.
This is just what has happened in the case of the Alps. A tremendous
lateral pressure applied to the rocks heaved them up and down
into great and small folds, and in some places, as in Mont Blanc,
fan-structure was produced. Imagine the top of the fan removed, and
you get what looks like a syncline, but is really the lower part of a
very much compressed anticline.
Now, it is believed that all mountain-ranges have been enormously
squeezed by lateral pressure; and the little experiment with the
sheet of paper furnishes a good illustration of what has happened. A
table-cloth lying on a smooth table will serve equally well. You can
easily push it into a series of folds; notice how they come nearer
as you continue pushing. You see also that in this way you get long
narrow ridges with valleys between. These represent the original
anticlines and synclines of mountain-ranges, which in course of time
are carved out, as explained above, until the synclines become hills
and the anticlines valleys.
Every mountain-chain must originally have had long ridges like these,
which in some cases determined the original directions of the streams
and valleys; and it is easy to see now why mountain-chains are long
and narrow, why their strata have been so greatly folded, and why
we get in every mountain-chain long ranges of hills roughly parallel
with each other (see chapter vi., pages 177-178).
The reason why granite, gneiss, and crystalline schists are
frequently found in the central and highest peaks of mountain-ranges
is that we have the oldest and lowest rocks exposed to the surface,
on account of the enormous amount of denudation that has taken place.
There may be great masses of granite underlying all mountain-chains;
but it is only exposed to view when a very great deal of overlying
rock has been removed.
It was thought at one time that granite was the oldest of all rocks,
and that mountain-chains had been upheaved by masses of granite
pushing them up from below; but we know now that both these ideas
are mistaken. Some granites are certainly old geologically, but
others are of later date; and it is certain that granite was not the
upheaving agent, but more likely it followed the overlying rocks as
they were heaved up by lateral pressure, because the upward bending
of the rocks would tend to relieve the enormous pressure down below,
and so the granite would rise up.
[Illustration: MONT BLANC. SNOWFIELDS, GLACIERS. AND STREAMS.]
We now pass on to a very different example, where mountains are the
result of huge fractures and displacements; namely, the numerous
and nearly parallel ranges of the Great Basin, of Western Arizona,
and Northern Mexico. The region between the Sierra Nevada and the
Wahsatch Mountains, extending from Idaho to Mexico, is composed
of very gently folded rocks deeply buried in places by extensive
outflows of lava.
Now, in this case the earth-movements caused great cracks, or splits,
doubtless attended by fearful earthquakes. We find here a series of
nearly parallel fractures, hundreds of miles long, and fifteen to
thirty miles apart. These traverse the entire region, dividing the
rocks into long narrow blocks. There is evidence to show that the
whole region was once much more elevated than it is now, and has
subsided thousands of feet. During the subsidence along these lines
of fracture, or faults, the blocks were tilted sideways; and the
uptilted blocks, carved by denudation, form the isolated ranges of
this very interesting region (see illustration, chap. viii., p. 273,
Fig. 1). The faults are indicated by arrows pointing downwards; and
the dotted lines indicate the erosion of the uptilted blocks.
But this must be regarded as a very exceptional case, for we do not
know of any other mountain-range formed quite in the same way. Why
the strata, although only slightly bent, should have snapped so
violently in this case, while in other mountain-ranges they have
suffered much more bending without so much fracture and displacement,
we cannot tell, but can only suggest that possibly it was because
they were not buried up under an enormous thickness of overlying
rocks, which would exert an enormous downward pressure, and so tend
to prevent fracturing.
There are many other deeply interesting questions with regard to the
upheaval of mountains which at present cannot be answered.
We have already learned to alter our preconceived ideas about the
stability and immovable nature of the earth's crust, and have seen
that it is in reality most unstable, and is undergoing continual
movements, both great and small. But here we have an equally
startling discovery, which quite upsets all our former ideas of the
hard and unyielding nature of the rocks composing the earth's crust;
for we find that not only can they be bent into innumerable folds
and little puckerings, but that in some cases they have been drawn
out and squeezed as if they were so much soft putty. The imagination
almost fails to grasp such facts as these.
Of late years geologists in Switzerland and in Great Britain have
discovered that in some parts of mountains rocks have been enormously
distorted and crushed, so that they have assumed very different
states from those in which they were made, and curious mineral
changes have taken place under the influence of this crushing.
In the very complicated region of the Northwest Highlands of
Sutherland and Ross, the structure of which has only lately been
explained, some wonderful discoveries of this nature have been
made. Certain of the crystalline schists found there have been
formed by the crushing down and rearrangement of older rocks that
once presented a very different appearance. In this district,
where the rocks have been squeezed by enormous lateral pressure,
the dislocations sometimes have assumed the form of inclined or
undulating planes, the rocks above which have been actually pushed
over those below, and in some cases the horizontal displacement
amounts to many miles.
Not only have the rocks been ruptured, and older, deep-seated masses
been torn up and driven bodily over younger strata (that once were
_above_ them), but there has been at the same time such an amount
of internal shearing as to crush the rocks into a finely divided
material, and to give rise to a streaky arrangement of the broken
particles, closely resembling the flow-structure of a lava. In the
crushed material new minerals have been sometimes so developed as to
produce a true schist.[32]
[32] Geikie.