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SLATES. 387

and it has been agreed among geologists not to. call such
splitting as this cleavage at all, but to restrict the term to
a phenomenon of a totally different character.

Those who have visited the slate-quarries of Cumberland
and North Wales will have witnessed the phenomenon to
which I refer. We have long drawn our supply of roofing-
slates from such quarries ; school-boys ciphered on these
slates, they were, used for tombstones in church-yards, and
for billiard-tables in the metropolis; but not until a com-
paratively late period did men begin to inquire how their
wonderful structure was produced. What is the agency
which enables us to split Honister Crag, or the cliffs of
Snowdon, into laminae from crown to base ? This question
is at the present moment one of the great difficulties of
geologists, and occupies their attention perhaps more than
any other. You may wonder at this. Looking into the
quarry of 'Penrhyn, you may be disposed to offer the ex-
planation I heard given two years ago. “These planes of
cleavage,” said a friend who stood beside me on the quarry’s
edge, “ are the planes : of . stratification which have been
lifted by some convulsion into an almost vertical. position.”
But this was a mistake, and indeed here lies the grand difli-
culty of the problem. The planes of cleavage stand in most
cases at a high angle to the bedding. Thanks to Sir Roder-
ick Murchison, I am able to plaee the proof of this before
you. Here, is a specimen of slate in which both the planes
of cleavage and of bedding are distinctly marked, one of
them making a large angle with the other. This is com-
mon. The cleavage of slates, then, is not a question of
stratification ; what, then, is its cause ?

In an able and elaborate essay published in 1835, Pro-
fessor Sedgwick proposed the theory that cleavage is due
to the action of crystalline or polar forces subsequent to the
consolidation of the rock. “ We may affirm,” he says, “that
no retreat of the parts, no contraction of dimensions in pass-

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