which are approximately vertical,
and the synclinal parts of which become ever more and more
depressed into the deeper, and necessarily hotter, underlying
materials; the anticlines being probably correspondingly
elevated. These events are slowly developed, and the temperature
beneath is steadily rising in consequence of the conducted
interior heat, and the steady accumulation of radioactive energy
in the sedimentary rocks and in the buried radioactive layer of
the Earth. The work expended on the crushed and sheared rock also
contributes to the developing temperature. Thus the geotherms
must move upwards, and the viscous conditions extend from below;
continually diminishing the downward range of the translatory
movements progressing in the higher parts. While above the folded
sediments are being carried northward, beneath they are becoming
anchored in the growing viscosity of the medium. The anticlines
will bend over, and the most southerly of the folds will
gradually become pushed or bent over those lying to the north.
Finally, the whole upper part of the sheaf will become
horizontally recumbent; and as the uppermost folds will be those
experiencing the greatest effects of the continued displacement,
the _deferlement_ or overlap must necessarily arise.

We may follow these stages of mountain evolution

159

in a diagram (Fig. 9) in which we eliminate intermediate
conditions, and regard the early and final stages of development
only. In the upper sketch we suppose the lateral compression much
developed and the upward movement of the geotherms in progress.
The dotted line may be assumed to be a geotherm having a
temperature of viscosity. If the conditions here shown persist

{Fig. 9}

indefinitely, there is no doubt that the only further
developments possible are the continued crushing of the sediments
and the bodily displacement of the whole mass to the north. The
second figure is intended to show in what manner these results
are evaded. The geotherm of viscosity has risen. All above it is
affected mechanically by the continuing stress, and borne
northwards in varying

160

degree depending upon the rigidity. The folds have been
overthrown and drawn out; those which lay originally most to the
south have become the uppermost; and, experiencing the maximum
amount of displacement, overlap those lying beneath. There has
also been a certain amount of upthrow owing to the hydrostatic
pressure. This last-mentioned