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386 FRAGMENTS or SCIENCE.

the one case it is the molecules arranging themselves ac-
cording to organic laws which produce a cleavable struct-
ure, in the other case the easy separation in one direction
is due to the mechanical arrangement of the coarse sensible
masses of the stalks of hay.

This sand-stone rock was once a powder, more or less
coarse, held in mechanical suspension by water. The pow-
der was composed of two distinct parts, fine grains of sand
and small plates of mica. Imagine a wide strand covered by
a tide, or an estuary with water which holds such powder in
suspension: how will it sink ? The rounded grains of sand
will reach the bottom first, because they encounter least re-
sistance, the mica afterward, and when the tide recedes we
have the little plates shining like Spangles upon the surface
of the sand. Each successive tide brings its charge of
mixed powder, deposits its duplex layer day after day, and
finally masses of immense thickness are piled up, ‘which by
preserving the alternations of sand and mica tell the tale
of their formation. Take the sand and mica, and mix them
together in water, and allow them to subside; they will ar-
range themselves in the manner indicated, and by repeating
the process you can actually build up a mass which shall be
the exact counterpart of that presented by Nature. N ow this
structure cleaves with readiness along the planes in which
the particles of mica are strewn. Specimens of such a rock
sent to me from Halifax, and other masses from the quarries
of Over Darwen in Lancashire, are here before you. With
a hammer and chisel I can cleave them into flags ;- indeed,
these flags are employed for roofing purposes in the districts
from which the specimens have come, and receive the name
of “ slate-stone.” But you will discern without a word
from me, that this cleavage is not a crystalline cleavage
any more than that of a hay-rick. It is molar, not molec-
ular.

This, so far as I am aware of, has never been imagined,

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