1848..
SHARPE ON SLATY CLEAVAGE.
119
organic remains. The lines y y show the direction of the cleavage-
planes. The plane of bedding a a has been broken into a very irre-
gular series of steps by some force which has either moved each
sheet of slate forward in ad-
vance of its neighbour, or has
caused an uneven expansion
of the parts of the rock. The
fossils have suffered from this
movement of the surface.
The whole bed is covered by
small irregular ridges parallel
to the cleavage, which make
it impossible to measure the
angles accurately.
In figure 7, « « represents
the section of the surface of
a bed of black calcareous
slate in the Bickington lime-
stone quarry, North Devon,
which is bent over in a steep
arch, and dips rapidly to the
north ; it is intersected by
cleavage y y, dipping nearly
70° to the south. In this
instance the surface of the
bed « a is bent into parallel
waves without its continuity
being broken : but the out-
lines of the beds below h h
and c c may be traced in the
solid rock unchanged; so that
the disturbance of the surface seems only to have taken place where
the looser packing of the rocks allowed room for some play of their
parts.
Figure 8 represents another case of the same kind, observed in the
Brathay flag-stone quarry near Ambleside ; a a being the surfr.ee of
a bed, and yy the planes of cleavage.
In cases like the last two, the whole surface of the beds a a is