posits; but the limit had apparently been attained,
and at this time the Laramide range, as well as its southerly
continuation into the United States, the Rockies, had their
beginning. Chamberlin and Salisbury[1] estimate that the height
of the mountains developed in the Laramide range at this time was
20,000 feet, and that, owing to the further elevation which has
since taken place, from 32,000 to 35,000 feet would be their
present height if erosion had not reduced them. Thus on either
side of the American continent we have the same forces at work,
throwing up mountain ridges where the sediments had formerly been
shed into the ocean.

These great events are of a rhythmic character; the crust, as it
were, pulsating under the combined influences of sedimentation
and denudation. The first involves downward movements under a
gathering load, and ultimately a reversal of the movement to one
of upheaval; the second factor, which throughout has been in

[1] Chamberlin and Salisbury, _Geology_, 1906, iii., 163.

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 downwa