File:The Quarterly journal of the Geological Society of London (12645106773).jpg

118
PROCEEDINGS OF THE GEOLOGICAL SOCIETY. .NoV. 1,
.?;
obviously mechanical ; the resistance offered by the oblique sheet of
quartz to the pressure forward of the mass, has caused the bending
of the sheets of slate at the ends which abut against the quartz.
A slight change in the direction of the clea-
Fig. 5. vage may frequently be observed in its passage
^. from one bed of rock to another, arising pro-
/ bably from beds of different hardness offering
different degrees of resistance to pressure.
Figure 5 represents the section of a fragment
of slate from Langdale, in which the thin beds
a a and h h are rather softer than the other beds
c c ; the lines y y show the direction of the
planes of cleavage, which are perpendicular in
passing through the beds c c, but dip 80° to the
southward in the beds a a and b b, the beds
dipping to the north at an angle of 60°.
Here the change in the direction of the
cleavage in passing from bed to bed is fully
10°, which is an unusual amount of deviation ;
but a variation of two or three degrees under
such circumstances is often met with. The
K
cause of these irregularities must also be me-
chanical.
The surface of particular beds of slate is
sometimes found broken up into a series of
steps parallel to the strike of the cleavage :
the front of each step is formed by the plane
of cleavage, and the top by a portion of the
bed which has been displaced from its original
position and appears to lie at an angle towards
(f the original plane of bedding.
6 represents the section of a fragment of slate from the

• • Pilton beds " two miles north of Barnstaple : the upper broken