SLATE AND SHALE.
Slate is one of the great commercial products of the world. As far back
as the year 1877 the output of slate was not less than 1,000,000 tons
per annum. The chief use to which slate is put is for covering
buildings, and for this purpose it is better than any other known
material. It is also used in the construction of billiard tables and for
writing-slates; these latter uses are very insignificant as compared to
its use in architecture. Slate, like building-stone and limestone, is
quarried from the earth's crust and is found in the strata close above
the Metamorphic rocks, near the beginning of what is called the Primary,
or Paleozoic period. As compared with the coal formations it is very,
very old.
There are different substances called slate that are not slate in the
scientific use of that word. In general all stone formations are called
slates that split up into thin layers. But the true slate is a special
material which is formed by special processes of nature. The difference
between slate and shale, for instance, is not one of ingredients, but of
the process by which the ingredients are put together. All of the
sedimentary rocks are formed by a deposit of sediment from the water on
the bottom of the ocean. At one period the floods have brought down a
certain kind of material in greater profusion than at others, and this
is deposited in thin layers, and as it hardens there will be seams in it
and the stratifications will be differently colored, the color depending
upon the deposit at any particular time.
A bed of shale, like a bed of coal, has lines of cleavage in it, and if
it is examined under a microscope it will be found that the sedimentary
particles, like the twigs and leaves in the coal veins, lie with their
longest dimensions in line with the plane of cleavage. Shale in color
looks like slate, and an analysis of the material of which it is formed
shows that shale and slate are both made from the same. There is,
however, a structural difference between the two which is very peculiar
and very interesting. The slate is ordinarily a denser material and the
lines of cleavage are often at right angles with those that we find in
ordinary shale.
A slab of shale will be of a uniform color on any one line of cleavage.
The color may change at the next line, and generally does, to a slight
extent. It is easy to see, then, if we could change the lines of
cleavage in the shale, so as to run at right angles with their present
lines, the face of a slab would show bands of different colors or
shadings, such as we often see in slate. If you take a piece of clay
that has been thoroughly mixed, and subject it to a very great pressure,
and then examine the piece that has been submitted to pressure under a
microscope and compare it with a piece of the clay after it has been
thoroughly mixed, but has not been submitted to pressure, you will find
that the two are very different in structure. The pressed clay will show
that the particles of which it is made up have all turned, so that their
longest dimensions are in a line at right angles with the direction of
pressure. Here is an interesting fact that we must remember. And it is
in this that we find the reason for the structural difference between
shale and slate. The lines of cleavage in shale are not formed
necessarily by pressure, but because in the disposition of the material
of which it was formed the particles naturally laid themselves down so
that their longest dimensions were on a horizontal line.
Ages after, when other rock and other formations had been laid down on
top of the bed of deposited mud, the upheavals of the earth have so
changed the lines of pressure upon this material and the pressure is so
great that a rearrangement of the particles 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.