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 particle