121
operation as creator of the sediments, then intervenes as an
assailant of the newly-raised mountains, transporting their
materials again to the ocean, when the rhythmic action is
restored to its first phase, and the age-long sequence of events
must begin all over again.
It has long been inferred that compressive stress in the crust
must be a primary condition of these movements. The wvork
required to effect the upheavals must be derived from some
preexisting source of energy. The phenomenon—intrinsically one of
folding of the crust—suggests the adjustment of the earth-crust
to a lessening radius; the fact that great mountain-building
movements have simultaneously affected the entire earth is
certainly in favour of the view that a generally prevailing cause
is at the basis of the phenomenon.
The compressive stresses must be confined to the upper few miles
of the crust, for, in fact, the downward increase of temperature
and pressure soon confers fluid properties on the medium, and
slow tangential compression results in hydrostatic pressure
rather than directed stresses. Thus the folding visible in the
mountain range, and the lateral compression arising therefrom,
are effects confined to the upper parts of the crust.
The energy which uplifts the mountain is probably a surviving
part of the original gravitational potential energy of the crust
itself. It must be assumed that the crust in following downwards
the shrinking subcrustal magma, develops immense compressive
stresses in
122
its materials, vast geographical areas being involved. When
folding at length takes place along the axis of the elongated
syncline of deposition, the stresses find relief probably for
some hundreds of miles, and the region of folding now becomes
compressed in a transverse direction. As an illustration, the
Laramide range, according to Dawson, represents the reduction of
a surface-belt 50 miles wide to one of 25 miles. The marvellous
translatory movements of crustal folds from south to north
arising in the genesis of the Swiss Alps, which recent research
has brought to light, is another example of these movements of
relief, which continue to take place perhaps for many millions of
years after they are initiated.
The result of this yielding of the crust is a buckling of the
surface which on the whole is directed upwards; but depression
also is an attendant, in many cases at least, on mountain
upheaval. Thus we find that the ocean floor is depressed into a
syncline along the western coast of South America; a trough
always parallel to the ranges of the Andes. The downward
deflection of the crust is of course an outcome of the same
compressive stresses which elevate the mountain.
The fact that the yielding of the crust is always situated where
the sediments have accumulated to the greatest depth, has led to
attempts from time to time of establishing a physical connexion
between the one and the other. The best-known of these theories
is that of Babbage and Herschel. This seeks the connexion in the
rise of the
123