s of which the slate is made
up has taken place, so that their longest dimensions now are in a
direction that crosses the stratifications as originally laid down.

The effect of this is twofold. First, the material is compressed into a
denser, closer form, and then, the lines of cleavage are changed, or to
express it in more common language, the grain has been changed. So that
when it splits up it runs crosswise of the original layers as the water
deposited them, and this produces the different shadings so often seen
in different slate. Shale splits in line with its layers; slate splits
across that line.

Let us go back a moment to our experiment with the lump of clay. If we
examined the mixture before submitted to pressure we should find that
the oblong particles of which it was made up would stand in all
directions, hit or miss, and if we should dry this lump of clay it would
have no special lines of cleavage. But the moment we have submitted it
to a certain amount of pressure we find that lines of cleavage have been
established, and that the particles have been rearranged so that their
longest dimensions are all in one direction, which coincides with the
cleavage lines. If we should now take this same piece of clay and
subject it to a pressure at right angles to that of the first
experiment we should find that the lines of cleavage had also changed
and that the particles had all been rearranged. Apply the principle to
the formation of slate, and we can understand how it happens that what
we call the grain runs crosswise of the deposits that were made at
different times. It is not a chemical, but purely a mechanical
difference. Or, to express it differently--the difference is a
structural one produced by mechanical causes.

The origin of cleavage in slate has been the subject of much speculation
and investigation, but like many other problems it was solved through
the invention and application of the microscope. Thin layers of slate
have been made, the same as with limestone and chalk, so thin that the
light would readily pass through it and that an examination of the
particles could be readily made, showing their arrangement under varied
conditions. Science is indebted to the microscope for the solution of
very many problems that for ages before had puzzled philosophers.

CHAPTER V.

SALT.

It may seem curious to the reader that we should care to discuss a
subject seemingly so simple as common salt. Bu