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other useful purposes. This again is succeeded by a slate of a
softer kind, in which crystals of chiastolite are plentifully
imbedded; these crystals gradually disappear, and the rock
becomes a more homogeneous clay-slate, which, contrary to general
observation, has its outgoing at a higher elevation than either
the granite or the gneiss.
These rocks are of a blackish colour, and divide by natural
partings into slates of various thickness, which are sometimes
curiously bent and waved: when these partings are very numerous,
though indistinct at first, they open by exposure to the weather,
and in time it becomes shivered into thin flakes, which lessens
its value as a roofing slate. In some places the thin laminae
alternate with others of a few inches in thickness; which are
harder, and of a lighter colour, containing more siliceous
matter; they have been by some taken for greywacké slate,
though apparently belonging to a different formation.
Rocks of this description have generally been represented as
stratified, and the strata parallel to the slaty cleavage; but
this proposition should not be received without some hesitation.
If it be supposed that these varieties of rock between which
there is no natural parting have been deposited upon the granite
in the order in which they have been mentioned; then, the strata
may be said to be mantle-shaped round the granitic nucleus; only
interrupted in its continuity by the anomalous rocks of Carrock;
but if it be assumed that the stratifica-
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