File:The Quarterly journal of the Geological Society of London (13205826403).jpg

1846..
SHARPE ON SLATY CLEAVAGE.
95
cleavage is southerly ; to the S. of the Stokenham line it is northerly*.
A line of vertical cleavage which must be considered identical with
thatof Start Point appears to run to the N.of the Lizard Point ; at least
we are told that on the N. of the serpentine of that point, the laminae
of the hornblende slate dip to the N, and N.W., which being the
reverse direction to the rest of the district, shows that we have here
passed the vertical line. The bearing of these two places is about W.
by S. to E. by N., which is nearly the line of strike of the cleavage
for the district.
In the N. of Devonshire the cleavage is vertical in the middle of
the quarry of black limestone containing Goniatites at Bickington, a
mile and a half S. of Barnstaple. The cleavage here strikes nearly
E. and W. ; at the N. end of the quarry it dips 70° to the S., and at
the S. end 80° towards the N. Between six and seven miles N. of
this spot, the cleavage is vertical at Swinham Down with a strike to
the E.N.E., and between these two lines the planes usually dip to
the N.N.W., but with occasional reversal of dip and irregularity.
Between Swinham Down and the Bristol Channel the cleavage
always dips S., S.S.W. or S.SE., the angle diminishing as we recede
from the vertical line, so that at Hillsborough near Ilfracombe it has
fallen to 60° and at Linton to 35°. The Bickington line of vertical
cleavage appears the boundary of the Devonshire area of elevation, and
the Swinham line that of another parallel area, of which the northern
limit must be sought on the opposite coast of Pembrokeshire and
Glamorganshire ; the band between Swinham Down and Bickington
being neutral ground lying between the two areas.
We thus establish two lines of vertical cleavage, 60 miles apart,
the one at Stoke Fleming, the other at Bickington, to serve as bound-
aries to the area to be examined. In the centre of this area there
is a broad band of country over which the cleavage planes undulate
in low flat waves, so that considered as a whole they are nearly hori-
zontal. The axes of these flat curves bear between E. and E.N.E.,
and the cleavage dips alternately to about N.N.W. or S.S.E. at angles
which rarely reach 10°. The curves of the cleavage frequently cor-
respond with an anticlinal arch of the bedding, both having the same
axis ; but the inclination of the beds is always greater than that of
the cleavage. The section fig. 19 would represent any one of these
Fig. 19.
The continuous lines represent the bedding, and the dotted lines the cleavage.
waves as they are seen near Launceston, or on the coast near Tin-
tagel. In other cases the two axes have a slightly difl'erent direc-

• Report on Cornwall, &c., p. 77.