the roots of the folds. For directed translatory
movements cannot be transmitted through a fluid, pressure in
which is necessarily hydrostatic, and must be exerted equally in
every direction. And this applies, not only to a fluid, but to a
body which will yield viscously to an impressed force. There will
be a gradation, according as viscosity gives place to rigidity,
between the states in which the applied force resolves itself
into a purely hydrostatic pressure, and in which it is
transmitted through the material as a directed thrust. The nature
of the force, in the most general case, of course, has to be
considered; whether it is suddenly applied and of brief duration,
or steady and long-continued. The latter conditions alone apply
to the present case.

It follows from this that, although a tangential force

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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
_deferlement_ 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