ew him, has so suddenly been taken away from us), Mr. Henry
Clifton Sorby, and others, have furnished us with a body of facts
associated with slaty cleavage, and having a most important bearing
upon the question.

Fossil shells are found in these slate-rocks. I have here several
specimens of such shells in the actual rock, and occupying various
positions in regard to the cleavage planes. They are squeezed,
distorted, and crushed; in all cases the distortion leads to the
inference that the rock which contains these shells has been subjected
to enormous pressure in a direction at right angles to the planes of
cleavage. The shells are all flattened and spread out in these
planes. Compare this fossil trilobite of normal proportions with
these others which have suffered distortion. Some have lain across,
some along, and some oblique to the cleavage of the slate in which
they are found; but in all cases the distortion is such, as required
for its production a compressing force acting, at right angles to the
planes of cleavage. As the trilobites lay in the mud, the jaws of a
gigantic vice appear to have closed upon them and squeezed them into
the shapes you see.

We sometimes find a thin layer of coarse gritty material, between two
layers of finer rock, through which and across the gritty layer pass
the planes of lamination. The coarse layer is found bent by the
pressure into sinuosities like a contorted ribbon. Mr. Sorby has
described a striking case of this kind. This crumpling can be
experimentally imitated; the amount of compression might, moreover, be
roughly estimated by supposing the contorted bed to be stretched out,
its length measured and compared with the shorter distance into which
it has been squeezed. We find in this way that the yielding of the
mass has been considerable.

Let me now direct your attention to another proof of pressure; you see
the varying colours which indicate the bedding on this mass of slate.
The dark portion is gritty, being composed of comparatively coarse
particles, which, owing to their size, shape and gravity, sink first
and constitute the bottom of each layer. Gradually, from bottom to
top the coarseness diminishes, and near the upper surface we have a
layer of exceedingly fine grain. It is the fine mud thus consolidated
from which are derived the German razor-stones, so much prized for the
sharpening of surgical instruments.

When a bed is thin, the fine-grain slate is