File:The Quarterly journal of the Geological Society of London (12512907943).jpg

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PKOCEEDINGS OP THE GEOLOGICAL SOCIETY. .April 10,
rise to the fissure shall have been previously in a state not very far
removed from one of static instability.
Thus, for example, if fig. 2 represent a transverse section of a
mountain-line on both flanks of which incoherent formations (i. e.
Fig. 2. — Ideal Section of Mountain.
clays, gravels, sands, &c.) repose, and a shock transverse to the. anti-
clinal axis be transmitted through the whole in the general direction
a to b, the effect of the wave of shock in its transit through the
entire mass will be, at the arrival side S, to cause the incoherent beds
2^ lying on the inclined flank to slip downward during the first
semipliase of motion of the wave-particle.
Actual slippage must occur to a greater or less extent whenever
the energy of the impressed movement upon the mass oip is equal to
or greater than the resistance due to friction of the mass against its
own inclined bed, or of its own materials against each other in the
plane of the angle of their repose, usually denominated ^ by writers
on mechanics. During the same first semiphase the incoherent
masses r reposing on the flank N are pressed more firmly against
their inclined beds ; but the instant after the wave-particle has
attained its maximum velocity, and so the second semiphase (or
reverse movement of the oscillation) commences, the ordinary pressure
of these masses perpendicular to their beds (due to gravity) is more
or less reduced, the masses themselves, to which forward motion in
the direction a to 6 had been previously given, tending now to be
left behind. And as their equilibrium in situ depends upon their co-
efiicients of friction, either internal or upon their beds, and the effect of
these is greater as the pressure on the bed is greater, so the tendency
to slip is now induced on all the masses at the side JST through the
momentary reduction of their pressure upon their beds.
Thus the conditions for slip are first produced on that side of the
chain from which the wave of shock travels, and afterwards on that
side towards which its transit is directed. In the second semiphase
the tendency to slip in the beds p is diminished, as the movement
of the wave-particle (now from h to a) tends to force them up their
inclined beds.
The fissures, if produced, may be only inceptive, and the slip be
limited to the direction transverse to the wave-motion nearly ; or the
slip, once begun, may assume all the magnitude of great landslips.
In either case, and at both sides of the chain, the general direction