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SLATES. 393

confusedly mixed up in it ? It will be, he argues, and he
argues rightly, to place the plates with their flat surfaces
more or less perpendicular to the direction in which the
pressure is exerted. He takes scales of the oxide of iron,
mixes them with a fine powder, and on squeezing the mass
finds that the tendency of the scales is to set themselves at
right angles to the line of pressure. ,Along the planes of
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, nor any thing analogous to them.
Here is the self-same substance submitted to pressure. I
would invite the attention of the eminent geologists now
before me to the structure of this wax. N 0 slate ever ex-
hibited so clean a cleavage; it splits into laminae of sur-
passing 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 pur-
posely 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 any thing, I should
say they rather impair its fineness and clearness than pro-
mote it.

The finer the slate is the more perfect will be the re-
semblance 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

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