When yielding has begun in any geosyncline, and the materials are
faulted and overthrust, there results a considerably increased
thickness. As an instance, consider the piling up of sediments
over the existing materials of the Alps, which resulted from the
compressive force acting from south to north in the progress of
Alpine upheaval. Schmidt of Basel has estimated that from 15 to
20 kilometres of rock covered the materials of the Simplon as now
exposed, at the time when the orogenic forces were actively at
work folding and shearing the beds, and injecting into their
folds the plastic gneisses coming from beneath.[1] The lateral
compression of the area of deposition of the Laramide, already
referred to, resulted in a great thickening of the deposits. Many
other cases might be cited; the effect is always in some degree
necessarily produced.
[1] Schmidt, Ec. Geol. _Helvelix_, vol. ix., No. 4, p. 590
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If time be given for the heat to accumulate in the lower depths
of the crushed-up sediments, here is an additional source of
increased temperature. The piled-up masses of the Simplon might
have occasioned a rise due to radioactive heating of one or two
hundred degrees, or even more; and if this be added to the
interior heat, a total of from 800° to 1000° might have prevailed
in the rocks now exposed at the surface of the mountain. Even a
lesser temperature, accompanied by the intense pressure
conditions, might well occasion the appearances of thermal
metamorphism described by Weinschenk, and for which, otherwise,
there is difficulty in accounting.[1]
This increase upon the primarily developed temperature conditions
takes place concurrently with the progress of compression; and
while it cannot be taken into account in estimating the
conditions of initial yielding of the crust, it adds an element
of instability, inasmuch as any progressive thickening by lateral
compression results in an accelerated rise of the goetherms. It
is probable that time sufficient for these effects to develop, if
not to their final, yet to a considerable extent, is often
available. The viscous movements of siliceous materials, and the
out-pouring of igneous rocks which often attend mountain
elevation, would find an explanation in such temperatures.
[1] Weinschenk, _Congrès Géol. Internat._, 1900, i., p. 332.
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There is no more striking feature of the part here played by
radioactivity than the fact that the rhythmic occurrence of
depression and upheaval succeeding each other after great
intervals of time, and often shifting their position but little
from the first scene of sedimentation, becomes accounted for. The
source of thermal energy, as we have already remarked, is in fact
transported with the sediments—that energy which determines the
place of yielding and upheaval, and ordains that the mountain
ranges shall stand around the continental borders. Sedimentation
from this point of view is a convection of energy.
When the consolidated sediments are by these and by succeeding
movements forced upwards into mountains, they are exposed to
denudative effects greatly exceeding those which affect the
plains. Witness the removal during late Tertiary times of the
vast thickness of rock enveloping the Alps. Such great masses are
hurried away by ice, rivers, and rain. The ocean receives them;
and with infinite patience the world awaits the slow accumulation
of the radioactive energy beginning afresh upon its work. The
time for such events appears to us immense, for millions of years
are required for the sediments to grow in thickness, and the
geotherms to move upwards; but vast as it is, it is but a moment
in the life of the parent radioactive substances, whose atoms,
hardly diminished in numbers, pursue their changes while the
mountains come and go, and the
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