erature may be--gradually
reducing the thickness of that part

130

of the upper crust which is bearing the simultaneously increasing
compressive stresses. Below this geotherm long-continued stress
resolves itself into hydrostatic pressure; above it (there is, of
course, no sharp line of demarcation) the crust accumulates
elastic energy. The final yielding and flexure occur when the
resistant cross-section has been sufficiently diminished. It is
probable that there is also some outward hydrostaitic thrust over
the area of rising temperature, which assists in determining the
upward throw of the folds.

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

131

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 deg. to 1000 deg. 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 in