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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.
Between the regions in which these two opposing conditions
prevail there will be no hard and fast line; but with the
downward increase of fluidity there will be a gradual failure of
the mechanical conditions and an increase of the hydrostatic.
Thus while the uppermost layers of the crust may be transported
to the full amount of the crustal displacement acting from the
south (speaking still of the Alps) deeper down there will be a
lesser horizontal movement, and still deeper there is no
influence to urge the viscous rock-materials in a northerly
direction. The consequences of these conditions must be the
recumbence of the folds formed under the crust-stress, and their
_déferlement_ towards the north. To see this, we must follow the
several stages of development.
The earliest movements, we may suppose, result in flexures of the
Jura-Mountain type--that is, in a
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succession of undulations more or less symmetrical. As the
orogenic force continues and develops, these undulations give
place to folds, the limbs of 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 _déferlement_ or overlap must necessarily arise.