HOW THE MOUNTAINS WERE UPHEAVED.
The notion that the ground is naturally steadfast is an
error,--an error which arises from the incapacity of our senses
to appreciate any but the most palpable, and at the same time
most exceptional, of its movements. The idea of _terra firma_
belongs with the ancient belief that the earth was the centre
of the universe. It is, indeed, by their mobility that the
continents survive the increasing assaults of the ocean waves,
and the continuous down-wearing which the rivers and glaciers
bring about.--PROFESSOR SHALER.
We have found out the quarries which supplied the rocky framework of
mountains, and have learned how the work of transporting these vast
quantities of stone was accomplished by the agency of ever-flowing
glaciers, rivers, and streams.
We must now consider the second stage of the work, and inquire how
the mountains were raised up. Referring back to our illustration
of the cathedral (see pages 143-147), it will be remembered that
this work was included under the head of _Elevation_. But perhaps
some one might ask: "How do you know that the mountains have been
elevated or upheaved? Is it not enough to suppose that they owe their
height entirely to the fact that they are composed of harder rock,
and so have been more successful in resisting the universal decay and
destruction?" Now, such an objection contains a good deal of truth,
for mountains _are_ formed of hard rocks; but at the same time we
know that the agents of denudation are more active among them than on
the plains below, so that, in the higher mountain regions at least,
the work of demolition may actually proceed faster than it does on
low ground.
Mountains are higher than the rest of the world, not merely because
they are built of more lasting material, but also because they have
been uplifted for thousands of feet above the level of the sea; and
the evidence of their upheaval is so plain as to be entirely beyond
doubt.
Let us inquire into the nature of this evidence. We have seen that
the rocks of which mountains are composed were for the most part
formed at the bottom of the sea. When the geologist finds, as he
frequently does, buried in mountain rocks the fossil remains of
creatures that must have lived in the sea (and often very similar to
those living there now), he is compelled to think of the gigantic
upheavals that must have taken place before those remains could
arrive at their present elevated position.
Numerous examples might be given; but we will only mention three. In
the Alps marine fossils have been detected at a height of 10,000 feet
above sea-level, in the Himalayas at a height of 16,500 feet, and in
the Rocky Mountains at a height of 11,000 feet.
Again we must take it for granted that all the stratified or
sedimentary rocks (see pages 148-149) with some trivial exceptions,
such as beds of shingle and conglomerates, have been formed in
horizontal layers. This is one of the simple axioms of geology to
which every one must assent.
Now, if we find in various parts of the continents, and especially
among the mountains, such strata sloping or "dipping" in
various directions, sometimes only slightly, but sometimes very
steeply,--nay, even standing up on end,--the conclusion that they
have been upheaved and pushed or squeezed into these various
positions by some subsequent process is irresistible. But this is
not all; for in every mountain region we find that the rocks have
been crumpled, twisted, and folded in a most marvellous manner. Solid
sheets of limestone may be seen, as it were, to writhe from the
base to the summit of a mountain; yet they present everywhere their
truncated ends to the air, and from their incompleteness it is easy
to see what a vast amount of material has been worn away, leaving,
as it were, mere fragments behind. The whole geological aspect of
the Alps (for example) is suggestive of intense commotion; and they
remain a marvellous monument of stupendous earth-throes, followed by
prolonged and gigantic denudation (see diagrams, chap. ix., p. 307).
There are certain features found in all mountain-chains which must
be carefully borne in mind, especially when we are considering the
explanations that have been suggested with regard to their upheaval.
These may be briefly stated as follows:--
1. Mountain-chains tend to run in straight or gently curving
lines.
2. Their breadth is small compared to their length, and their
height smaller still.
3. They rise sharply and are clearly marked off from the
country on either side.
4. They form the backbones of continents.
5. The rocks of which they are composed have been greatly
disturbed, folded, and contorted.
6. There is often a band of crystalline rocks (granite, gneiss,
etc.) running along the centre of a high range.
7. They are connected with lines of volcanoes.
8. They are frequently affected by earthquakes.
Having arrived at the conclusion that the mountains show evident
signs of upheaval, let us proceed to inquire whether any movements,
either upward or downward, are taking place now on the earth, or can
be proved to have done so within comparatively recent times. On this
question there is ample evidence at our disposal.
More than one hundred and thirty years ago, Celsius, the Swedish
astronomer, was aware, from the unanimous testimony of the
inhabitants of the sea-coasts, that the Gulf of Bothnia was
constantly diminishing both in depth and extent. He resorted to
measurements in order to prove (as he thought) that the waters of
the Baltic were changing their level. This was a mistaken idea; and
we now understand that the level of the sea does not change, except
under the influence of the daily rise and fall of the tide, which is
easily allowed for. However, that was the idea then; and it survived
for some time. But if the sea-level were continually sinking, the
water, which, owing to the influence of gravitation, must always
remain horizontal, would equally retreat all round the Scandinavian
peninsula and on all our seashores. But this is not the case. Again,
it would be impossible on this theory to explain the curious fact
that in some parts of the world the sea is gaining on the land, while
in other places it is as surely retreating; for we cannot believe
that in one part the sea-level is rising, while in another (not far
off in some cases) it is sinking. No body of water could behave in
this irregular fashion; and the sea could not possibly be rising and
falling at the same time.
Hence we may take it for granted that any change that we may notice
in the relative level of land and sea _must_ be due to upward or
downward movements in the land.
But to return to Celsius. Old men pointed out to him various points
on the coast, over which during their childhood the sea was wont to
flow, and besides, showed him the water-lines which the waves had
once traced out farther inland. And besides this, the names of places
which implied a position on the shore, former harbours or ports now
abandoned and situated inland, the remains of boats found far from
the sea, and lastly, the written records and popular songs, left no
doubt that the sea had retreated; and it seemed both to themselves
and to the astronomer that the waters were sinking. In the year 1730
Celsius, after comparing all the evidence he had collected, announced
that the Baltic had sunk three feet, four inches, every hundred
years. In the course of the following year, in company with Linnæus,
the naturalist, he made a mark at the base of a rock in the island
of Leoffgrund, not far from Jelfe, and thirteen years afterwards was
able to prove, as he thought, that the waters were still subsiding
at the same rate, or a little faster. In reality, he had proved, not
that the sea was sinking, but that the land was rising.
Similar observations show that nearly the whole of Scandinavia is
slowly rising out of the sea. At the northern end of the Gulf of
Bothnia the land is emerging at the rate of five feet, three inches,
in a century; but by the side of the Aland Isles it only rises three
and one quarter feet in the same time. South of this archipelago it
rises still more slowly; and farther down, the line of shore does not
alter as compared with the level of the sea.
But it is a curious fact that the extreme southern end of this
peninsula is subsiding, as proved by the forests that have been
submerged. Several streets of some towns there have already
disappeared, and the coast has lost on the average a belt of land
thirty-two yards in breadth.
The upward movement of the Scandinavian peninsula must have been
going on for a long time, if we assume that it was always at the same
rate as at present; for we find beds of seashells of living species
at heights of six or seven hundred feet above the level of the sea.
Great dead branches of a certain pink coral, found in the sea at a
depth of over one hundred and fifty to three hundred fathoms, are
now seen in water only ten or fifteen fathoms deep. It must have
been killed as it was brought up into the upper and warmer layers of
water. This is striking testimony.
The pine woods too, which clothe the hills, are continually being
upheaved towards the lower limit of snow, and are gradually withering
away in the cooler atmosphere; and wide belts of forest are composed
of nothing but dead trees, although some of them have stood for
centuries.
Geologists have proved that the Baltic Sea formerly communicated
by a wide channel with the North Sea, the deepest depressions of
which are now occupied by lakes in the southern part of Sweden; for
considerable heaps of oyster-shells are now found in several places
on the heights commanding these great lakes. Then we have in Denmark
the celebrated "kitchen-middens," heaps of rubbish also largely
composed of oyster-shells which the inhabitants, in the "Stone
Age," collected from the bottoms of the neighbouring bays. At the
present day the waters of the Baltic, into which rivers bring large
quantities of fresh water, do not contain enough salt for oysters to
grow there; but the oyster-shells prove that the Baltic Sea and these
inland lakes were once as salt as the North Sea is now. This can only
be explained by supposing that the Baltic was not so shut in then as
it is in these days. The bed of the old wide channel has risen, and
what once was sea is now land.
Again, it is very probable that the great lakes and innumerable
sheets of water which fill all the granite basins of Finland have
taken the place of an arm of the sea which once united the waters of
the Baltic to those of the great Polar Ocean. And so there must have
been upheaval here as well.
The old sea-beaches, now above the level of the highest tides, that
are found in many parts of the Scandinavian, Scottish, and other
coasts, furnish plain evidence of upheaval.
At the present day, between the lines of high tide and low tide, the
sea is constantly engaged in producing sand and shingle, spreading
them out upon the beach, mingling them with the remains of shells
and other marine animals, and sometimes piling them up, sometimes
sweeping them away. In this way a beach often resembles a terrace.
When the land is upheaved rapidly enough to carry up this line
of beach-deposits before they are washed away by the waves, they
form a flat terrace, or what is known as a "raised beach." The old
high-water mark is then inland; its sea-worn caves become in time
coated with ferns and mosses; the old beach forms an admirable
platform on which meadows, fields, villages, and towns spring up; and
the sea goes on forming a new beach below and beyond the margin of
the old one.
The Scottish coast-line, on both sides, is fringed with raised
beaches, sometimes four or five occurring above each other, at
heights of from twenty-five to seventy-five feet above the present
high-water mark. Each of these lines of terrace marks a former lower
level at which the land stood with regard to the sea; and the spaces
between them represent the amount of each successive rise of the
land. Each terrace was formed during a pause, or interval, in the
upward movement, during which the waves had time to make a terrace,
whereas, while the land kept on rising, they had no time to do so.
Thus we learn that the upheaval of the country was interrupted by
considerable pauses.
Sometimes old ports and harbours furnish evidence of upheaval. Thus,
the former Roman port of Alaterva (Cramond) in Scotland, the quays of
which are still visible, is now situated at some distance from the
sea, and the ground on which it stands has risen at least twenty-four
feet. In other places the scattered débris shows that the coast has
risen twenty-six feet. And by a remarkable coincidence, the ancient
wall of Antoninus, which in the time of the Romans stretched from sea
to sea, and served as a barrier against the Picts, comes to an end at
a point twenty-six feet above the level of high tides. In the estuary
of the Clyde there are deposits of mud, containing rude canoes and
other relics of human workmanship, several feet above the present
high-water mark.
Raised beaches are found on many parts of the coast of Great Britain.
Excellent examples occur on the coasts of Devon and Cornwall. On the
sides of the mountainous fiords of Norway similar terraces are found
up to more than six hundred feet above the sea; and as some of these
rise to a greater height at a distance of fifty miles inland, it
seems that there was a greater upward movement towards the interior
of Norway than on the coasts.
There is a celebrated raised beach on the side of a mountain in
North Wales, known as Moel Tryfaen, where the writer gathered a
number of marine shells at a height of 1,357 feet.
But Scandinavia and Great Britain are not the only parts of Europe
where an upward movement has taken place, for the islands of Nova
Zembla and Spitzbergen show evidence of the same kind; and the coast
of Siberia, for six hundred miles to the east of the river Lena, has
also been upraised. On the banks of the Dwina and the Vega, 250 miles
to the south of the White Sea, Murchison found beds of sand and mud
with shells similar to those which inhabit the neighbouring seas, so
well preserved that they had not lost their colours.
Again, the ground of the Siberian _toundras_ is to a large extent
covered with a thin coating of sand and fine clay, exactly similar
to that which is now deposited on the shores of the Frozen Ocean.
In this clay, the remains of the mammoth, or woolly elephant, now
extinct, are preserved in great numbers.
Parts of Northern Greenland have also risen; while at the southern
end of this frozen land a downward movement is still taking place.
The best-known example of these slow movements within historic times
is the so-called Temple of Serapis in the Bay of Baie, near Naples.
The ruins of this building, which was probably a Roman bath, consist
of a square floor paved with marble, showing that it possessed a
magnificent central court. This court, when perfect, was covered with
a roof supported by forty-six fine columns, some of marble, others
of granite. There is still a hot spring behind, from which water was
conducted through a marble channel. All the columns but three were
nearly buried in the soil which covered the whole court, when the
ruins were first discovered. Now, each of the three marble columns
that are still standing shows clear evidence of having been depressed
below the sea-level, for they all exhibit a circular row of little
holes bored by a certain marine shell-fish, known as _Lithodomus
dactylus_, at a height of twelve feet from the floor; each row is
about eight feet broad. The shells may still be seen inside the
little pear-shaped holes which the shell-fish bored for themselves;
and the same shell-fish still live in the waters of the Mediterranean
and bore holes in the limestone rocks.
It is therefore quite clear that these columns must have been under
water to a depth of twenty feet or so, and also that they must have
remained under water for some considerable time, during which the
shell-fish made these borings. Then an upheaval took place whereby
the whole building was elevated to its present level. But underneath
the present floor, at a depth of five feet, were discovered the
remains of an older floor. This probably belonged to an earlier
building which had in like manner been depressed below sea-level. We
thus learn that the land in this spot had been sinking for a long
time, and that at some subsequent time it rose. The fallen columns
suggest the idea that they were thrown down by earthquakes. At the
present time the land here is again sinking at the rate of one inch
in three or four years.
But the first example of upheaval within comparatively recent times,
and one which is instructive as throwing some light on the subject of
the present chapter,--namely, the upheaval of mountain-chains,--is to
be found along the western mountainous coast of South America. Here
we have the magnificent ranges of the Andes running along the whole
length of this continent. The illustrious Charles Darwin, during
his famous trip in the "Beagle," discovered numerous raised beaches
along this coast, and at once perceived their importance to the
geologist. The terraces are not quite horizontal, but rise towards
the south. On the frontier of Bolivia, they are seen at heights of
from sixty-five to eighty feet above sea-level; but nearer the higher
mass of the Chilian Andes they are found at one thousand feet, and
near Valparaiso, in Chili, at thirteen hundred feet above the sea.
Darwin also discovered that some of the upheavals thus indicated took
place during the human period; for he found in one of the terraces
opposite Callao, in Peru, at a height of eighty feet, shells with
bones of birds, ears of wheat, plaited reeds, and cotton thread,
showing that men had lived on the terrace. These relics of human
industry are exactly similar to those that are found in the _huacas_,
or burial-places, of the ancient Peruvians. There can be no doubt
that the island of San Lorenzo, and probably the whole of the coast
in its neighbourhood, have risen eighty feet or more since the Red
Man inhabited the country.
Callao probably forms the northern limit of the long strip of coast
that has been upheaved, and the island of Chiloe the southern limit;
but even thus the region of elevation has a length from north to
south of about 2,480 miles.
We noticed in the case of Scandinavia that the upward movement is
greater in the interior of the mountain-range than at or near the
coast; and it is interesting to find that the same difference has
been observed in the case of the Andes. The upheaving force, whatever
its nature, acts with more energy under the Chilian Andes than under
the rocks of the adjacent coast.
In New Zealand we have also evidences of upheaval; and if we trace
out on the map a long line from the Friendly Isles and Fiji, through
the Eastern Archipelago, and then on through the Philippine Islands,
and finally to Japan and the Kurile Islands, we shall find scattered
regions of elevation all along this great line, which is probably a
mountain-chain, partly submerged, and along which numerous active
volcanoes are situated.
Putting together all the evidence that has been gathered on this
subject, of which only a very small part is here given, we are
warranted in concluding that taking the world generally, regions
where active volcanoes exist are generally regions where upheaval is
taking place. There is also a very interesting connection between
mountain-chains and lines of volcanic action. From this it seems to
follow, if lines of volcanic action are also lines of upheaval, that
mountain-chains are undergoing upheaval at the present time. This
is a conclusion in favour of which a good deal may be said. It is
certainly true in the cases of the Scandinavian range, and also of a
very large part of the Andes, to which we have already referred. The
Highlands of Scotland and Scandinavia form the northern end of an old
line of volcanic action running down the Atlantic Ocean through the
Azores, Madeira, Cape Verde Islands, Ascension, St. Helena, right
down to Tristan d'Acunha.
In many other parts of the world we have evidences from submerged
forests, the positions of certain landmarks with regard to the sea,
and in some cases submerged towns, that movements of a downward
nature are taking place.
It is important to distinguish from these evidences the changes that
take place where the waves of the sea are rapidly washing away the
coast-line. Putting aside these cases, however, it has been clearly
proved that in many regions a slow sinking of the land is going on.
The eastern side of South America has not been so thoroughly observed
as its western side; but there is still good reason to believe that
a large part of this coast is sinking. So it appears that a see-saw
movement is affecting South America, and that while one side is going
up, the other is going down; and it is interesting to observe other
examples of the same thing,--such as are afforded by Greenland and
Norway.
[Illustration: THE SKAEGGDALFORS, NORWAY.
FROM A PHOTOGRAPH BY J. VALENTINE.]
Again, while part of Labrador is rising, parts of the eastern coast
of North America, as far down as Florida, are slowly sinking. Thus
along the New England coast between New York and Maine, and again
along the Gulf of St. Lawrence, we find numerous submerged forests
with quantities of trees standing upright with their roots in old
forest-beds, but with the tops of their stumps some feet below the
level of high tide. In the case of New Jersey the subsidence is
probably taking place at the rate of two feet in a hundred years.
Before passing on to consider upward movements of a more rapid
nature, such as are frequently caused by earthquakes, we may pause
for a few moments to consider certain very slight, but nevertheless
very interesting little movements, such as _slight pulsations_ and
tremors, which have been observed to take place in the earth's crust
(as it is called), and which of late years have been carefully
studied.
Professor Milne, a great authority on earthquakes, has noticed slight
swayings of the earth, which though occupying a short time--from a
few seconds to a few hours--are still too slow to produce a shock of
any kind. These he calls "earth pulsations." They have been observed
by means of delicate spirit-levels, the bubbles of which move with
very slight changes of level at either end of the instrument. At
present only a few experiments of this kind have been made; but they
tell us that the surface of the earth (which is apparently so firm
and immovable) is subject to slight but frequent oscillations. Some
think that they depend upon changes in the weight of the atmosphere.
If this is so, the balance between the forces at work below the
earth's surface and those that operate on its surface must be
very easily disturbed. Still we cannot see that this is a serious
objection; on the contrary, there is much reason to think that any
slight extra weight on the surface, such as might be caused by an
increase of the pressure of the atmosphere, and still more by the
accumulation of vast sedimentary deposits on the floor of the ocean,
may be quite sufficient to cause a movement to take place. Moreover,
Mr. G. H. Darwin has shown that the earth's crust daily heaves up and
down under the attraction of the moon in the same kind of way that
the ocean does; so that we must give up all idea of the solid earth
being fixed and immovable, and must look upon it as a flexible body,
like a ball of india-rubber (see chap. ix., pp. 314-315).
Slight movements of rather a different kind have been noticed, to
which the name of "earth-tremors" has been given. These are very
slight jarrings or quiverings of the earth, too slight to be observed
by our unaided senses, but rendered visible by means of very delicate
pendulums and other contrivances. Now wherever such observations
have been made it has been discovered that the earth is constantly
quivering as if it were a lump of jelly. In Italy, where this subject
has been very carefully studied, the tremors that are continually
going on are found to vary considerably in strength; for instance,
when the weather is very disturbed and unsettled, the movements of
the pendulum are often much greater. Again, before an earthquake the
instrument shows that the tremors are more frequent and violent.
Another way of observing these curious little movements is by burying
microphones in the ground. The microphone is a little instrument
invented of late years which is capable of enormously magnifying the
very slightest sounds, such as our ears will not detect. By its means
one can hear, as some one said, "the tramp of a fly's foot," if he
will be so obliging as to walk over it. It has thus been proved in
Italy that the earth sends forth a confused medley of sounds caused
by little crackings and snappings in the rocks below our feet.
In this way it will be possible to predict a serious earthquake,
because it will give warning some days before, by the increase of the
little tremors and sounds; and it is to be hoped that by this simple
means human lives may be saved.
Now, these disturbances are of precisely the same nature as
earthquakes,--in fact, we may call them microscopic earthquakes. To
the geologist they are of great interest, as they seem to afford
some little insight into the difficult question of the upheaval of
mountains, and to show us something of the constant _working_ of
those wonderful forces below the surface of the earth by means of
which continents are raised up out of the sea, and mountain-chains
are elevated thousands of feet. It is probable that both are due to
the working of the same forces, and are accomplished by the same
machinery.
We now pass on to consider those more violent movements of the solid
land known as earthquakes. This kind of disturbance is such as might
be produced by a sudden shock or blow given below the ground, from
which waves travel in all directions. First comes a rumbling noise
like the roar of distant artillery; then come the earthquake waves
one after another, causing the ground to rise and fall as a ship does
on the waves of the sea; the ground is frequently rent asunder, so
that chasms are formed, into which in some cases men and animals
have been hurled alive. In the case of a very violent earthquake
the waves travel long distances. Thus the great earthquake by which
Lisbon was destroyed in the year 1755 disturbed the waters of Loch
Lomond in Scotland. In this fearful catastrophe sixty thousand human
beings perished. If the disturbance takes place near the sea, great
sea waves are formed, which cause fearful destruction to life and
property. This happened in the case of the Lisbon earthquake; and
in the year 1868, when Ecuador and Peru were visited by a fearful
earthquake, a great sea wave swept over the port of Arica, and in a
few minutes every vessel in the harbour was either driven ashore or
wrecked, and a man-of-war was swept inland for a quarter of a mile.
Earthquakes bring about many changes on the surface of the earth. For
example, on mountain-slopes forests are shattered, and large masses
of soil and débris are shaken loose from the rock on which they
rested, and hurled into the valleys; streams are thus choked up, and
sometimes lakes formed, either by the damming up of a river or by
the subsidence of the ground.
It is frequently found after an earthquake that the level of
the ground has been permanently altered; and this effect of
earthquakes is important in connection with the subject we are now
considering,--namely, how mountains are upheaved. Sometimes, it is
true, the movement is a downward one; but more generally it takes
place in an upward direction. As an example of this, we may mention
the Chilian earthquake of 1835, which was very violent, and destroyed
several towns on that coast, from Copiapo to Chile. It was afterwards
found that the land in the Bay of Conception had been raised four or
five feet. At the island of Santa Maria, to the southwest of this
bay, the land was raised eight feet, and in one part ten feet; for
beds of dead mussels were seen at that height above high water, and
a considerable rocky flat that formerly was covered by the sea now
became dry land. It was also proved by means of soundings that the
sea round the island was shallower by about nine feet.
Now the question arises, "How are earthquakes caused?" Various
suggestions have been made; but it is pretty clear that all
earthquakes are not produced in the same way. For instance, volcanic
eruptions are frequently attended by earthquakes. Violent shocks of
this nature generally precede and accompany a great eruption, as is
frequently the case before an eruption of Mount Vesuvius.
Steam plays a very important part in all volcanic eruptions; and
these earthquakes are probably caused by great quantities of pent-up
steam at a high pressure struggling to escape. It is also possible
that when molten rock is forcibly injected into the crevices and
joints of overlying rocks earthquake shocks may be produced by
the concussion. The old Roman poet and philosopher, Lucretius,
endeavoured to solve this problem, and concluded that "the shakings
of the surface of the globe are occasioned by the falling in of
enormous caverns which time has succeeded in destroying." But
though the explanation might possibly apply to a few cases of small
earthquakes, it is not a satisfactory one, for it is not at all
likely that many large cavities exist below the earth's surface,
because the great weight of the overlying rock would inevitably crush
them in.
We have already pointed out that earthquakes frequently happen in
mountainous regions; and this fact alone suggests that perhaps the
same causes which upheave mountains may have something to do with
earthquakes. But there are other reasons for believing that the same
force which causes earthquakes also upheaves mountain-chains. The
reader will remember the case of the Chilian earthquake that raised
part of the Andes a few feet in height.
Now, it is quite clear that the rocks of which mountains are composed
have suffered a great deal of disturbance. We have only to look at
the crumbled and contorted strata to see that they have been forced
into all kinds of positions, sometimes standing bolt upright (see
diagrams, chap. ix., p. 307). And as we cannot believe, for many
reasons, that these movements were of a very sudden or violent kind,
we must consider that they took place slowly on the whole; but
besides being folded and twisted, the rocks of mountains frequently
exhibit clear signs of having been split and cracked. The fractures
are of all sizes, from an inch or more up to hundreds or even
thousands of feet. They tell us plainly that the rocks were once
slowly bent, and that after a certain amount of bending had taken
place, the strain put upon them became greater than they could bear,
and consequently they snapped and split along certain lines. This is
just what might be expected. For instance, ice on a pond will bend a
good deal, but only up to a certain amount; after that, it cracks in
long lines with a remarkably sharp and smooth fracture. But suppose
the pressure came from below instead of from above, as when a number
of people are skating on a pond. Should we not see the ice forced
up in some places, so that some sheets stood up above the others
after sliding past their broken edges? This is just what the rocks
in different places have frequently done. After a fracture has taken
place the rock on one side has slid up over the other, and the two
surfaces made by the fracture--like two long walls--are no longer
seen at the same level. One has been pushed up, while the other has
gone down (see diagram of the ranges of the Great Basin, chap. viii.,
p. 273).
Now, it is almost impossible to conceive of these tremendous
fractures taking place in the rocks below our feet without causing
sudden jars or shocks. Here, then, we seem to have a clue to the
problem. Even if the movements took place only a few inches or a few
feet at a time, that does not spoil our theory, but rather favours
it; for in that case the upheaval of a mountain-chain will have
taken a very long time (which is almost certain), and may have been
accomplished bit by bit. Hundreds and thousands of earthquake shocks,
some slight, and others severe, may have attended the upheaval of a
mountain-range.
This explanation is accepted by many authorities. It does not exactly
imply that mountains were upheaved by earthquakes; but it means that
the same forces that elevate continents, heaving them up out of the
sea into ridges and very low arches, have been at work to crumple and
fold their rocks in some places into stupendous folds, such as we now
find form part of the general structure of mountains; and that in so
doing they caused fearful strains, too great for the rocks to bear,
so that they split over and over again, and in so doing produced jars
and shocks that must have been very similar to, if not identical
with, earthquake shocks as we know them at the present day.
Such an explanation is in striking harmony with what we have
already learned about the operations of Nature. It was from the
long-continued operation of rain and rivers that the materials now
forming mountains were transported to the seas in which they were
slowly formed. It was also by the ordinary operations of frost,
heat and cold, snow and ice, streams, rain, and rivers that the
mountains received their present shapes (see chapters v. and vii.).
And now we learn that the gigantic work of upheaval took place in a
tolerably quiet and uniform manner,--with perhaps only an occasional
catastrophe of a more violent kind, but still according to the same
law of uniformity which is the very basis of modern geology, and by
means of which so much can be explained.
We could give other proofs of the gradual elevation of mountains
if they were wanted. But at least enough has been said to give
the reader a glimpse into the methods employed by geologists in
endeavouring to explain how mountains were upheaved; and to show that
it is only by a careful study of all that is taking place now on the
earth that we can ever hope to solve the difficult questions that
present themselves to all who study those stony records on which the
earth has written for our enlightenment the chapters of her ancient
history.
In conclusion, it may be asked what is the nature of the force that
accomplishes all this titanic work of upheaval. Although the question
has been much discussed, and some very ingenious suggestions brought
forward, we cannot say that any of them are entirely satisfactory.
But we know that the earth is a cooling body which loses so much heat
every year; and it may be that the shrinking that takes place as it
cools, by leaving the crust of the earth in some places unsupported,
causes it to settle down, to adapt itself to a smaller surface below,
and in so doing it would inevitably throw itself into a series of
folds, or wrinkles, like those on the skin of a dried apple. Many
think that mountain-ranges may be explained in this way.