xample of
this kind of cleavage.

Turn we now to the consideration of some other phenomena to which the
term cleavage may be applied. Beech, deal, and other woods cleave
with facility along the fibre, and this cleavage is most perfect when
the edge of the axe is laid across the rings which mark the growth of
the tree. If you look at this bundle of hay severed from a rick, you
will see a sort of cleavage in it also; the stalks lie in horizontal
planes, and only a small force is required to separate them laterally.
But we cannot regard the cleavage of the tree as the same in character
as that of the hayrick. In the one case it is the molecules arranging
themselves according to organic laws which produce a cleavable
structure, in the other case the easy separation in one direction is
due to the mechanical arrangement of the coarse sensible stalks of
hay.

This sandstone rock was once a powder held in mechanical suspension by
water. The powder 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 resistance, the mica afterwards,
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, mix them together in water, and
allow them to subside; they will arrange 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.
Now 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 'slatestone.' But you will discern without a word
from me, that this cleavage is not a crystalline cleavage any more
than that of a hayrick is. I