strip
from another. Advantage is taken of these lines of cleavage to procure
long and extremely thin even strips from trees of the willow variety
for such trades as basket-making.
The
same effect is seen in house-coal, which may easily be split the way of
the grain (on the lines of cleavage), but is much more difficult and
requires greater force to break across the grain. Rocks also show
distinct lines of cleavage, and are more readily split one way than
another, the line of cleavage or stratum of break being at any angle
and not necessarily parallel to its bed. A striking example of this is
seen in slate, which may be split in plates, or lamina}, with great
facility, though this property is the result of the pressure to which
the rock has been for ages subjected, which has caused a change in the
molecules, rather than by " cleavage " as the term is strictly
understood, and as existing in minerals. Mica is also another example
of laminated cleavage, for given care, and a thin, fine knife to divide
the plates, this mineral may be " cleaved " to such remarkably thin
sheets as to be unable to sustain the most delicate touch without
shattering.
These
are well-known examples of simple cleavage, in one definite direction,
though in many instances there are several forms and directions of
cleavage, but even in these there is generally one part or line in and
on which cleavage will take place much more readily than on the others,
these planes or lines also showing different properties and angular
characters, which, no matter how much fractured, always remain the
same. It is this " cleavage " which causes a crystal to reproduce itself
