In simpler
English we should say--The mud which has hardened into the slate was
laid down horizontally; and therefore each slate is one of the little
horizontal beds of it, perhaps just what was laid down in a single
tide. We should have a right to do so, because that would be true of
most sedimentary rocks. But it would not be true of slate. The
plane of bedding in slate has nothing to do with the plane of
cleavage. Or, more plainly, the mud of which the slate is made may
have been deposited at the sea-bottom at any angle to the plane of
cleavage. We may sometimes see the lines of the true bedding--the
lines which were actually horizontal when the mud was laid down--in
bits of slate, and find them sometimes perpendicular to, sometimes
inclined to, and sometimes again coinciding with the plane of
cleavage, which they have evidently acquired long after.
Nay, more. These parallel planes of cleavage, at each of which the
slate splits freely, will run through a whole mountain at the same
angle, though the beds through which they run may be tilted at
different angles, and twisted into curves.
Now what has made this change in the rook? We do not exactly know.
One thing is clear, that the particles of the now solid rock have
actually moved on themselves.
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