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392 FRAGMEN TS OF SCIENCE.

subjected to enormous pressure in a direction at right angles
to the planes of cleavage.

In reference to Mr. Sorby’s contorted bed, I have said
that by supposing it to be stretched out and its length
measured, it would give us an idea of the amount of yield-
ing of the mass above and below the bed. Such a measure-
ment, however, would not give the exact amount of yielding.
I hold in my hand a specimen of slate with its bedding
marked upon it; the lower portions of each layer being
composed of a comparatively coarse gritty material some-
thing like what you may suppose the contorted bed to be
composed of. Now, in crossing these gritty portions, the
cleavage turns, as if tending to cross the bedding at an-
other angle. When the pressure began to act, the inter-
mediate bed, which is not entirely unyielding, suffered
longitudinal pressure; as it bent, the pressure became grad-
ually more lateral, and the direction of its cleavage is ex-
actly such as you would infer from an action of this kind—it
is neither quite across the bed nor yet in the same direction
as the cleavage of the slate above and below it, but inter-
mediate between both. Supposing the cleavage to be at
right angles to the pressure, this is the direction which it
ought to take across these more unyielding strata.

Thus we have established the concurrence of the phe-
nomena of cleavage and pressure—that they accompany
each other; but the question still remains, Is the pressure
sufficient to account for the cleavage ‘P A single geologist,
as far as I am aware, answers boldly in the affirmative.
This geologist is Sorby, who has attacked the question in
the true spirit of a physical investigator. Call to mind the
cleavage of the flags of Halifax and Over Darwen, which is
caused by the interposition of layers of mica between the
gritty strata. Mr. Sorby finds plates of mica to be also a
constituent of slate-rock. He asks himself, what will be
the effect of pressure upon a mass containing such plates

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