nd the same conclusion, namely, that slate-rocks have been
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 yielding of the mass above and below
the bed. Such a measurement, 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 something 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 another angle. When the pressure began to act, the
intermediate bed, which is not entirely unyielding, suffered
longitudinal pressure; as it bent, the pressure became gradually more
transverse, and the direction of its cleavage is exactly 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 intermediate 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 phenomena of cleavage
and pressure--that they accompany each other; but the question still
remains, Is the pressure sufficient to account for the cleavage? 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 confusedly mixed up in it? It will be, he
argues, and he argues rightly, to place the plates with their flat
surfaces more or less perpendicular to the direction in which the
pressure is exerted. He takes scales of the oxide of iron, mixes them
with a fine powder, and on squeezing the mass finds that the tendency
of the scales is to set themselves at right angles to the line of
pressure. Along the planes of