OUR ancestors regarded mountainous regions with feelings of
horror, mingled with commiseration for those whom an unkindly
destiny had condemned to dwell therein. We, on the other hand,
find in the contemplation of the great alps of the Earth such
peaceful and elevated thoughts, and such rest to our souls, that
it is to those very solitudes we turn to heal the wounds of ife.
It is difficult to explain the cause of this very different point
of view. It is probably, in part, to be referred to that cloud of
superstitious horror which, throughout the Middle Ages, peopled
the solitudes with unknown terrors; and, in part, to the
asceticism which led the pious to regard the beauty and joy of
life as snares to the soul's well-being. In those eternal
solitudes where the overwhelming forces of Nature are most in
evidence, an evil principle must dwell or a dragon's dreadful
brood must find a home.
But while in our time the aesthetic aspect of the hills appeals
to all, there remains in the physical history of the mountains
much that is lost to those who have not shared in the scientific
studies of alpine structure and genesis. They lose a past history
which for interest com-
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petes with anything science has to tell of the changes of the
Earth.
Great as are the physical features of the mountains compared with
the works of Man, and great as are the forces involved compared
with those we can originate or control, the loftiest ranges are
small contrasted with the dimensions of the Earth. It is well to
bear this in mind. I give here (Pl. XV.) a measured drawing
showing a sector cut from a sphere of 50 cms. radius; so much of
it as to exhibit the convergence of its radial boundaries which
if prolonged will meet at the centre. On the same scale as the
radius the diagram shows the highest mountains and the deepest
ocean. The average height of the land and the average depth of
the ocean are also exhibited. We see how small a movement of the
crust the loftiest elevation of the Himalaya represents and what
a little depression holds the ocean.
Nevertheless, it is not by any means easy to explain the genesis
of those small elevations and depressions. It would lead us far
from our immediate subject to discuss the various theoretical
views which have been advanced to account for the facts. The idea
that mountain folds, and the lesser rugosities of the Earth's
surface, arose in a wrinkling of the crust under the influence of
cooling and skrinkage of the subcrustal materials, is held by
many eminent geologists, but not without dissent from others.
The most striking observational fact connected with mountain
structure is that, without exception, the ranges
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of the Earth are built essentially of sedimentary rocks: that is
of rocks which have been accumulated at some remote past time
beneath the surface of the ocean. A volcanic core there may
sometimes be--probably an attendant or consequence of the
uplifting--or a core of plutonic igneous rocks which has arisen
under the same compressive forces which have bowed and arched the
strata from their original horizontal position. It is not
uncommon to meet among unobservant people those who regard all
mountain ranges as volcanic in origin. Volcanoes, however, do not
build mountain ranges. They break out as more or less isolated
cones or hills. Compare the map of the Auvergne with that of
Switzerland; the volcanoes of South Italy with the Apennines.
Such great ranges as those which border with triple walls the
west coast of North America are in no sense volcanic: nor are the
Pyrenees, the Caucasus, or the Himalaya. Volcanic materials are
poured out from the summits of the Andes, but the range itself is
built up of folded sediments on the same architecture as the
other great ranges of the Earth.
Before attempting an explanation of the origin of the mountains
we must first become more closely acquainted with the phenomena
attending mountain elevation.
At the present day great accumulations of sediment are taking
place along the margins of the continents where the rivers reach
the ocean. Thus, the Gulf of Mexico receiving the sediment of the
Mississippi and Rio Grande;
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the northeast coast of South America receiving the sediments of
the Amazons; the east coast of Asia receiving the detritus of the
Chinese rivers; are instances of such areas of deposition. Year
by year, century by century, the accumulation progresses, and as
it grows the floor of the sea sinks under the load. Of the
yielding of the crust under the burthen of the sediments we are
assured; for otherwise the many miles of vertically piled strata
which are uplifted to our view in the mountains, never could have
been deposited in the coastal seas of the past. The flexure and
sinking of the crust are undeniable realities.
Such vast subsiding areas are known as geosynclines. From the
accumulated sediments of the geosynclines the mountain ranges of
the past have in every case originated; and the mountains of the
future will assuredly arise and lofty ranges will stand where now
the ocean waters close over the collecting sediments. Every
mountain range upon the Earth enforces the certainty of this
prediction.
The mountain-forming movement takes place after a certain great
depth of sediment is collected. It is most intense where the
thickness of deposit is greatest. We see this when we examine the
structure of our existing mountain ranges. At either side where
the sediments thin out, the disturbance dies away, till we find
the comparatively shallow and undisturbed level sediments which
clothe the continental surface.