File:The Quarterly journal of the Geological Society of London (12645461664).jpg

1848..
SHARPE ON SLATY CLEAVAGE.
113
without either bedding or cleavage. Many of these metamorphic slates
appear to have been originally of a brecciated structure, and contain
a confused mixture of crystals with what appear to be fragments of
Fig. 1.
Fig. 2.
foreign origin. In slaty rocks of this description all the enclosed
portions which can be supposed to have had a mechanical origin are
flattened between the planes of cleavage, but nothing of the sort can
be observed of the crystals, which are scattered through the mass
without any reference to the cleavage-planes either in their form or
position.
In good roofing-slates, which are distinguished by the fineness and
uniformity of their grain, it is difficult to observe the forms of the
parts of which they are composed : yet even in these, with the assist-
ance of a lens, we can see that the constituent particles are flattest
between the cleavage-planes, and longest along the dip of the cleavage.
Thus, under whatever circumstances we are able to observe the forms
of the component parts of slate, or of the foreign bodies contained in
it, (whether these are pebbles, fragments of other rocks, or organic
remains,) we find evidence that the mass has been compressed by a
force which has acted in a direction perpendicular to the planes of
cleavage.
The distinctive character of slate appears to he that it is composed