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 laminae, 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 exactly, as explained in the last chapter, showing its parent
form, shape and characteristics with microscopic perfection, but more
and more in miniature as its size is reduced.

This may clearly be seen by taking a very small quantity of such a
substance as chlorate of potash. If a crystal of this is examined under
a magnifying glass till its crystalline form and structure are familiar,
and it is then placed in a test-tube and gently heated, cleavage will at
once be evident. With a little crackling, the chlorate splits itself
into many crystals along its chief lines of cleavage (called the
cleavage planes), every one of which crystals showing under the
microscope the identical form and characteristics of the larger crystal
from which it came.

The cleavage of minerals must, therefore, be considered as a part of
their crystalline structure, since this is caused by cleavage, so that
both cleavage and crystalline structure should be considered together.
Thus we see that given an unchangeable crystal with cleavage planes
evident, it is p