155
Unfortunately the full investigation of the distribution of
temperature after any given time is beset with difficulties; the
conditions being extremely complex. If the radioactive heating
was strictly adiabatic—that is, if all the heat was conserved and
none entered from without—the time required for the attainment of
the equilibrium radioactive temperature would be just about six
million years. The conditions are not, indeed, adiabatic; but, on
the other hand, the rocks upraised by lateral pressure were by no
means at 0° C. to start with. They must be assumed to have
possessed such temperatures as the prior radiothermal effects,
and the conducted heat from the Earth's interior, may have
established.
It would from this appear probable that if a duration of ten
million years was involved, the equilibrium radioactive
temperatures must nearly have been attained. The effects of heat
conducted from the underlying earthcrust have to be added,
leading to a further rise in temperature of not less than 500° or
600° . In such considerations the observed indications of high
temperatures in materials now laid bare by denudation, probably
find their explanation (P1. XIX).
The first fact that we infer from the former existence of such a
temperature distribution is the improbability, indeed the
impossibility, that anything resembling a rigid obstacle, or
deep-seated "horst," can have existed beneath the present
surface-level, and opposed the northerly movement of the
deep-lying synclines. For
156
such a horst can only have been constituted of some siliceous
rock-material such as we find everywhere rising through the
worn-down sediments of the Alps; and the idea that this could
retain rigidity under the prevailing temperature conditions, must
be dismissed. There is no need to labour this question; the horst
cannot have existed. To what, then, is the retardation of the
lower parts of the folds, their overthrow, above, to the north,
and their _déferlement_, to be ascribed?
A little consideration shows that the very conditions of high
temperature and viscosity, which render untenable the hypothesis
of a rigid obstacle, suffice to afford a full explanation of the
retardation of the roots of the folds. For directed translatory
movements cannot be transmitted through a fluid, pressure in
which is necessarily hydrostatic, and must be exerted equally in
every direction. And this applies, not only to a fluid, but to a
body which will yield viscously to an impressed force. There will
be a gradation, according as viscosity gives place to rigidity,
between the states in which the applied force resolves itself
into a purely hydrostatic pressure, and in which it is
transmitted through the material as a directed thrust. The nature
of the force, in the most general case, of course, has to be
considered; whether it is suddenly applied and of brief duration,
or steady and long-continued. The latter conditions alone apply
to the present case.
It follows from this that, although a tangential force
157
or pressure be engendered by a crustal movement occurring to the
south, and the resultant effects be transmitted northwards, these
stresses can only mechanically affect the rigid parts of the
crust into which they are carried. That is to say, they may
result in folding and crushing, or horizontally transporting, the
upper layers of the Earth's crust; but in the deeper-lying
viscous materials they must be resolved into hydrostatic pressure
which may act to upheave the overlying covering, but must refuse
to transmit the horizontal translatory movements affecting the
rigid materials above.