of this class of mountains. These are, of course, formed by the
accumulation of igneous materials around the focus or foci of eruption,
and their mode of origin is so generally understood, and, indeed,
so obvious, that I need do no more than mention them. Of course,
they are all subject to erosion, and many long-extinct volcanoes are
highly denuded. Some very ancient ones, as those of our own country,
have been so demolished that frequently all that remains are the now
plugged-up pipes or flues through which the heated materials found a
passage to the surface--all those materials, consisting of lavas and
ashes, having in many cases entirely disappeared. In former times
volcanic eruptions often took place along the line of an extensive
fissure--the lava, instead of being extruded at one or more points,
welled-up and overflowed along the whole length of the fissure, so as
to flood the surrounding regions. And this happening again and again,
vast plateaux of igneous rock came to be built up, such as those of the
Rocky Mountains, Iceland, the Faröes, Antrim and Mull, Abyssinia and
the Deccan. These are called _plateaux of accumulation_ (see Fig. 1),
and all of them are more or less highly denuded, so that in many cases
the plateaux have quite a mountainous appearance. Of course, plateaux
of accumulation are not always formed of igneous rocks. Any area of
approximately horizontal strata of aqueous origin, rising to a height
of a thousand feet or more above the sea, would come under this class
of plateau--the plateau of the Colorado being a good example. Although
that plateau is of recent origin, yet its surface, as we have seen,
has been profoundly modified by superficial erosion; and this is true
to a greater extent of plateaux which have been much longer exposed
to denudation. It is obvious that even mountains and plateaux of
accumulation often owe many of their present features to the action of
the surface-agents of change.
2. Mountains of Elevation.--We have seen that the strata which enter
most largely into the composition of the earth's crust, so far as that
is open to observation, consist of rocks which must originally have
been disposed in horizontal or approximately horizontal layers. But,
as every one knows, the stratified rocks are not always horizontally
arranged. In Scotland they rarely are so. On the contrary, they are
inclined at all angles from the horizon, and not infrequently they even
stand on end. Moreover, they are often traversed by dislocations, large
and small. No one doubts that these tilted and disturbed rocks are
evidence of wide-spread earth-movements. And it has been long known to
geologists that such movements have happened again and again in this
and many other countries where similar disturbed strata occur. Some of
these movements, resulting in the upheaval of enormous mountain-masses,
have taken place within comparatively recent geological times. Others
again date back to periods inconceivably remote. The Pyrenees, the
Alps, the Caucasus, the Himalaya, which form the back-bone of Eurasia,
are among the youngest mountains of the globe. The Highlands of
Scotland and Scandinavia are immeasurably more ancient; they are, in
point of fact, the oldest high grounds in Europe, nor are there any
mountain-masses elsewhere which can be shown to be older. But while the
Alps and other recent mountains of elevation still retain much of their
original configuration, not a vestige of the primeval configuration of
our own Highlands has been preserved; their present surface-features
have no direct connection with those which must have distinguished them
in late Silurian times. Our existing mountains are not, like those of
the Alps, mountains of elevation.
The structure of a true mountain-chain is frequently very complicated,
but the general phenomena can be readily expressed in a simple diagram.
Let Fig. 5 be a section taken across a mountain-chain, _i.e._ at right
angles to its trend or direction. The dominant point of the chain is
shown at _B_, while _A_ and _C_ represent the low grounds. Now, an
observer at _A_, advancing towards _B_, would note that the strata, at
first horizontal, would gradually become undulating as he proceeded on
his way--the undulations getting always more and more pronounced. He
would observe, moreover, that the undulations, at first symmetrical,
as at _a_, would become less so as he advanced--one limb of an arch
or _anticline_, as it is termed, being inclined at a greater angle
than the other, as at _b_. Approaching still nearer to =B=, the arches
or anticlines would be seen eventually to bend over upon each other,
so as to produce a general dip or inclination of the strata towards
the central axis of the chain. Crossing that axis (_B_), and walking
in the direction of the low grounds (_C_), the observer would again
encounter the same structural arrangement, but of course in reverse
order. Thus, in its simplest expression, a true mountain-chain consists
of strata arranged in a series of parallel undulations--the greater
mountain ridges and intervening hollows corresponding more or less
closely to the larger undulations and folds of the strata. Now, could
these plicated strata be pulled out, could the folds and reduplications
be smoothed away, so as to cause the strata to assume their original
horizontal position, it is obvious that the rocks would occupy a
greater superficial area. We see, then, that such a mountain-chain
must owe its origin to a process of tangential or lateral thrusting
and crushing. The originally horizontal strata have been squeezed
laterally, and have yielded to the force acting upon them by folding
and doubling up. It seems most probable that the larger contortions
and foldings which are visible in all true mountain-chains, owe their
origin to the sinking down of the earth's crust upon the cooling and
contracting nucleus. During such depressions of the crust the strata
are necessarily subjected to enormous lateral compression; they are
forced to occupy less space at the surface, and this they can only do
by folding and doubling-back upon themselves. If the strata are equally
unyielding throughout a wide area, then general undulation may ensue;
but should they yield unequally, then folding and contortion will take
place along one or more lines of weakness. In other words, the pressure
will be relieved by the formation of true mountain-chains. Thus,
paradoxical as it may seem, the loftiest mountains of the globe bear
witness to profound depression or subsidence of the crust. The Andes,
for example, appear to owe their origin to the sinking down of the
earth's crust under the Pacific; and so in like manner the Alps would
seem to have been ridged up by depression of the crust in the area of
the Mediterranean. Mountain-chains, therefore, are true wrinkles in the
crust of the earth; they are lines of weakness along which the strata
have yielded to enormous lateral pressure.
A glance at the geological structure of the Alps and the Jura shows us
that these mountains are a typical example of such a chain; they are
mountains of elevation. In the Jura the mountains form a series of long
parallel ridges separated by intervening hollows; and the form or shape
of the ground coincides in a striking manner with the foldings of the
strata. In these mountains we see a succession of symmetrical flexures,
the beds dipping in opposite directions at the same angle from the axis
of each individual anticline. There each mountain-ridge corresponds
to an _anticline_, and each valley to a _syncline_, or trough-shaped
arrangement of strata. But as we approach the Alps the flexures become
less and less symmetrical, until in the Alps themselves the most
extraordinary convolutions and intricate plications appear, the strata
being often reversed or turned completely upside down.
Though it is true that the slopes of this great mountain-chain
not infrequently correspond more or less closely to the slope or
inclination of the underlying rocks, it must not be supposed that this
correspondence is often complete. Sometimes, indeed, we find that the
mountains, so far from coinciding with anticlines, are in reality
built up of synclinal or basin-shaped strata; while in other cases
deep and broad valleys run along the lines of anticlinal axes (Fig.
6). All this speaks to enormous erosion. A study of the geological
structure of the Alps demonstrates that thousands of feet of rock have
been removed from those mountains since the time of their elevation. A
section drawn across any part of the chain would show that the strata
have been eroded to such an extent, and the whole configuration so
profoundly modified, that it is often difficult, or even impossible,
to tell what may have been the original form of the surface when the
chain was upheaved. And yet the Alps, it must be remembered, are of
comparatively recent age, some of their highly-confused and contorted
rocks consisting of marine strata which are of no greater antiquity
than the incoherent clays and sands of the London Tertiary basin.
Now, when we reflect upon the fact that, in the case of so young a
mountain-chain, the configuration due to undulations of the strata
has been so greatly modified, and even in many places obliterated, it
is not hard to believe that after sufficient time has elapsed--after
the Alps have existed for as long a period, say, as the mountains
of middle Germany--every mountain formed of anticlinal strata shall
have disappeared, and those synclines which now coincide with valleys
shall have developed into hills. The reader who may have paid little
or no attention to geological structure and its influence upon the
form of the ground, will probably think this a strange and extravagant
statement; yet I hope to show presently that it is supported by all
that we know of regions of folded strata which have been for long
periods of time subjected to denudation.
* * * * *
3. Mountains of Circumdenudation.--In countries composed of undulating
and folded strata which have been for long ages exposed to the
action of eroding agents, the ultimate form assumed by the ground
is directly dependent on the character of the rocks, and the mode
of their arrangement. The various rock-masses which occur in such a
neighbourhood as Edinburgh, for example, differ considerably in their
power of resisting denudation. Hence the less readily eroded rocks
have come in time to form hills of less or greater prominence. Such
is the case with the Castle Rock, Corstorphine Hill, the Braids, the
Pentlands, etc. These hills owe their existence, as such, to the fact
that they are composed of more enduring kinds of rock than the softer
sandstones and shales by which they are surrounded, and underneath
which they were formerly buried to great depths. Some hills, again,
which are for the most part built up of rocks having the same character
as the strata that occur in the adjacent low grounds, stand up as
prominences simply because they have been preserved by overlying caps
or coverings of harder rocks--rocks which have offered a stronger
resistance to the action of the denuding agents. The Lomond Hills are
good examples. Those hills consist chiefly of sandstones which have
been preserved from demolition by an overlying sheet of basalt-rock.
But the mode in which rocks are arranged is a not less important factor
in determining the shape which the ground assumes under the action of
the agents of erosion. Thus, as we have already seen, flat-topped,
pyramidal mountains, and more or less steep-sided or trench-like
valleys, are characteristic features in regions of horizontal strata.
When strata dip or incline in one general direction, then we have a
succession of escarpments or dip-slopes, corresponding to the outcrops
of harder or less readily eroded beds, and separated from each other
by long valleys, hollows, or undulating plains, which have the same
trend as the escarpments (Fig. 7). This kind of configuration is
well exemplified over a large part of England. The general dip or
inclination of the Mesozoic or Secondary strata throughout that
country, between the shores of the North Sea and the English Channel,
is easterly and south-easterly--so that the outcrops of the more
durable strata form well-defined escarpments that face the west and
north-west, and can be followed almost continuously from north to
south. Passing from the Malvern Hills in a south-easterly direction,
we traverse two great escarpments--the first coinciding with the
outcrop of the Oolite, and forming the Cotswold Hills; and the second
corresponding to the outcrop of the Chalk, and forming the Chiltern
Hills. The plains and low undulating tracts that separate these
escarpments mark the outcrops of more yielding strata--the low grounds
that intervene between the Cotswolds and the Malvern Hills being
composed of Liassic and Triassic clays and sandstones. In Scotland
similar escarpments occur, but owing to sudden changes of the dip,
and various interruptions of the strata, the Scottish escarpments are
not so continuous as those of the sister-country. Many of the belts
of hilly ground in the Scottish Lowlands, however, exemplify the
phenomena of escarpment and dip-slope. Thus, the Sidlaws in Forfarshire
consist of a series of hard igneous rocks and interbedded sandstones
and flags--the outcrops of which form a succession of escarpments with
intervening hollows. The same appearances recur again and again all
over the Lowlands. Wherever, indeed, any considerable bed of hard rock
occurs in a series of less enduring strata--the outcrop of the harder
rock invariably forms a well-marked feature or escarpment. As examples,
I may refer to Salisbury Crags, Craiglockhart Hill, Dalmahoy Crags, the
Bathgate Hills, King Alexander's Crag, etc. All these are conspicuous
examples of the work of denudation--for it can be demonstrated
that each of these rock-masses was at one time deeply buried under
sandstones and shales, and they now crop out at the surface, and form
prominent features simply because the beds which formerly covered and
surrounded them have been gradually removed.
From what has now been said it will be readily understood that in
regions composed of strata the inclination or dip of which is not
constant but continually changing in direction, the surface-features
must be more or less irregular. If the strata dip east the outcrops
of the harder beds will form escarpments facing the west, and the
direction of the escarpments will obviously change with the direction
of the dip. Undulating strata of variable composition will, in short,
give rise to an undulating surface, but the superficial undulations
will not coincide with those of the strata. On the contrary, in
regions consisting of undulating strata of diverse consistency the
hills generally correspond with synclinal troughs--or, in other words,
trough-shaped strata tend to form hills; while, on the other hand,
arch-shaped or anticlinal strata most usually give rise to hollows
(see Fig. 2). This remarkable fact is one of the first to arrest the
attention of every student of physical geology, and its explanation
is simple enough. An anticlinal arrangement of strata is a weak
structure--it readily succumbs to the attacks of the denuding agents; a
synclinal arrangement on the contrary, is a strong structure, which is
much less readily broken up. Hence it is that in all regions which have
been exposed for prolonged periods to sub-aërial denudation synclinal
strata naturally come to form hills, and anticlinal strata valleys or
low grounds. In the case of a mountain-chain so recently elevated as
that of the Alps, the mountain-ridges, as we have seen, often coincide
roughly with the greater folds of the strata. Such anticlinal mountains
are weakly built, and consequently rock-falls and landslips are of
common occurrence among them--far more common, and on a much larger
scale, than among the immeasurably older mountains of Scandinavia and
Scotland. The valleys of the Pyrenees, the Alps, and the Apennines,
are cumbered with enormous chaotic heaps of fallen rock-masses. From
time to time peaks and whole mountain-sides give way, and slide into
the valleys, burying hamlets and villages, and covering wide tracts of
cultivated land. Hundreds of such disastrous rock-falls have occurred
in the Alps within historical ages, and must continue to take place
until every weakly-formed mountain has been demolished. The hills
and mountains of Scotland have long since passed through this phase
of unstable equilibrium. After countless ages of erosion our higher
grounds have acquired a configuration essentially different from that
of a true mountain-chain. Enormous landslips like that of the Rossberg
are here impossible, for all such weakly-constructed mountains have
disappeared.
A little consideration will serve to show how such modifications and
changes have come about. When strata are crumpled up they naturally
crack across, for they are not elastic. During the great movements
which have originated all mountains of elevation, it is evident
that the strata forming the actual surface of the ground would often
be greatly fissured and shattered along the crests of the sharper
anticlinal ridges. In the synclinal troughs, however, although much
fissuring would take place, yet the strata would be compelled by the
pressure to keep together. Now, when we study the structure of such a
region as the Alps, we find that the tops of the anticlines have almost
invariably been removed, so as to expose the truncated ends of the
strata--the ruptured and shattered rock-masses having in the course
of time been carried away by the agents of erosion. Such mountains
are pre-eminently weak structures. Let us suppose that the mountains
represented in the diagram (Fig. 8) consist of a succession of strata,
some of which are more or less permeable by water, while others are
practically impermeable. It is obvious that water soaking down from the
surface will find its way through the porous strata (_p_), and come out
on the slopes of the mountains along the joints and cracks (_c_) by
which all strata are traversed. Under the influence of such springs and
the action of frost, the rock at the surface will eventually be broken
up, and ever and anon larger and smaller portions will slide downwards
over the surface of the underlying impermeable stratum. The undermining
action of rivers will greatly intensify this disintegrating and
disrupting process. As the river deepens and widens its valley (_v_),
it is apparent that in doing so it must truncate the strata that are
inclined towards it. The beds will then crop out upon the slopes of the
valley (as at _b_, _b_), and so the conditions most favourable for a
landslip will arise. Underground water, percolating through the porous
beds (p), and over the surface of the underlying impermeable beds (_i_,
_i_, _i_), must eventually bring about a collapse. The rocks forming
the surface-slopes of the mountain will from time to time give and
slide into the valley, or the whole thickness of the truncated strata
may break away and rush downwards; and this process must continue so
long as any portion of the anticlinal arch remains above the level of
the adjacent synclinal troughs.
Thus it will be seen that an anticlinal arch is a weak structure--a
mountain so constructed falls a ready prey to the denuding agents; and
hence in regions which have been exposed to denudation for as long a
period as the Scottish or Scandinavian uplands, a mountain formed of
anticlinally arranged strata is of very exceptional occurrence. When
it does appear, it is only because the rocks of which it is composed
happen to be of a more enduring character than those of the adjacent
tracts. The Ochil Hills exemplify this point. These hills consist of a
great series of hard igneous rocks, which are arranged in the form of a
depressed anticlinal arch--the low grounds lying to the north and south
being composed chiefly of sandstones and shales. Here it is owing to
the more enduring character of the igneous rocks that the anticlinal
arch has not been entirely removed. We know, however, that these
igneous rocks were formerly buried under a great thickness of strata,
and that their present appearance at the surface is simply the result
of denudation.
If an anticlinal arch be a weak structure, a synclinal arrangement of
strata is quite the opposite. In the case of the former each bed has
a tendency to slip or slide away from the axis, while in a syncline
it is just the reverse--the strata being inclined towards and not
away from the axis. Underground water, springs, and frost are enabled
to play havoc with anticlinal strata, for the structure is entirely
in their favour. But in synclinal beds the action of these powerful
agents is opposed by the structure of the rocks--and great rock-falls
and landslips cannot take place. Synclinal strata therefore endure,
while anticlinal strata are worn more readily away. Even in a true
mountain-range so young as the Alps, denudation has already demolished
many weakly-built anticlinal mountains, and opened up valleys along
their axes; while, on the other hand, synclinal troughs have been
converted into mountains. And if this be true of the Alps, it is still
more so of much older mountain-regions, in which the original contours
due to convolutions of the strata have entirely disappeared (see Fig.
9).
The mountains of such regions, having been carved out and modelled by
denuding agents, are rightly termed _mountains of circumdenudation_,
for they are just as much the work of erosion as the flat-topped and
pyramidal mountains which have been carved out of horizontal strata.
The Scottish Highlands afford us an admirable example of a mountainous
region of undulating and often highly-flexed strata, in which the
present surface-features are the result of long-continued erosion. As
already remarked, this region is one of the oldest land-surfaces in the
world. In comparison with it, the Pyrenees, the Alps, and the Himalayas
are creations of yesterday. The original surface or configuration
assumed by the rocks composing our Highland area at the time when these
were first crushed and folded into anticlines and synclines had already
been demolished at a period inconceivably more remote than the latest
grand upheaval of the Alps. Even before the commencement of Old Red
Sandstone times, our Archæan, Cambrian, and Silurian rocks had been
planed down for thousands of feet, so that the bottom beds of the Old
Red Sandstone were deposited upon a gently undulating surface, which
cuts across anticlines and synclines alike. In late Silurian and early
post-Silurian times the North-west Highlands probably existed as a true
mountain-chain, consisting of a series of parallel ranges formed by the
folding and reduplication of the strata. The recent observations of my
friends, Professor Lapworth and Messrs. Peach and Horne, in Sutherland,
have brought to light the evidence of gigantic earth-movements, by
which enormous masses of strata have been convoluted and pushed
for miles out of place. We see in that region part of a dissected
mountain-chain. The mountain-masses which are there exposed to view
are the basal or lower portions of enormous sheets of disrupted rock,
the upper parts of which have been removed by denudation. In a word,
the mountains of Sutherland are mountains of circumdenudation--they
have been carved out of elevated masses by the long-continued action
of erosion. To prove this, one has only to draw an accurate section
across the North-west Highlands, when it becomes apparent that the
form or shape of the ground does not correspond or coincide with the
convolutions of the strata, and that a thickness of thousands of
feet of rock has been denuded away since those strata were folded and
fractured. All over the Highlands we meet with similar evidence of
enormous denudation. The great masses of granite which appear at the
surface in many places are eloquent of the result produced by erosion
continued for immeasurable periods of time. Every geologist knows that
granite is a rock which could only have been formed and consolidated at
great depths. When, therefore, such a rock occurs at the surface, it is
evidence beyond all doubt of prodigious erosion. The granite has been
laid bare by the removal of the thick rock-masses underneath which it
cooled and consolidated.