weakness produced by the scales the
mass cleaves.

By tests of a different character from those applied by Mr. Sorby, it
might be shown how true his conclusion is--that the effect of pressure
on elongated particles, or plates, will be such as he describes it.
But while the scales must be regarded as a true cause, I should not
ascribe to them a large share in the production of the cleavage. I
believe that even if the plates of mica were wholly absent, the
cleavage of slate-rocks would be much the same as it is at present.

Here is a mass of pure white wax: it contains no mica particles, no
scales of iron, or anything analogous to them. Here is the selfsame
substance submitted to pressure. I would invite the attention of the
eminent geologists now before me to the structure of this wax. No
slate ever exhibited so clean a cleavage; it splits into laminae of
surpassing tenuity, and proves at a single stroke that pressure is
sufficient to produce cleavage, and that this cleavage is independent
of intermixed plates or scales. I have purposely mixed this wax with
elongated particles, and am unable to say at the present moment that
the cleavage is sensibly affected by their presence--if anything, I
should say they rather impair its fineness and clearness than promote
it.

The finer the slate is the more perfect will be the resemblance of its
cleavage to that of the wax. Compare the surface of the wax with the
surface of this slate from Borrodale in Cumberland. You have
precisely the same features in both: you see flakes clinging to the
surfaces of each, which have been partially torn away in cleaving. Let
any close observer compare these two effects, he will, I am persuaded,
be led to the conclusion that they are the product of a common cause.
[Footnote: I have usually softened the wax by warming it, kneaded it
with the fingers, and pressed it between thick plates of glass
previously wetted. At the ordinary summer temperature the pressed wax
is soft, and tears rather than cleaves; on this account I cool my
compressed specimens in a mixture of pounded ice and salt, and when
thus cooled they split cleanly.]

But you will ask me how, according to my view, does pressure produce
this remarkable result? This may be stated in a very few words.

There is no such thing in nature as a body of perfectly homogeneous
structure. I break this clay which seems so uniform, and find that
the fracture presents to my eyes innumerab