File:The Quarterly journal of the Geological Society of London (13205801933).jpg

1846..'
SHARPE ON SLATY CLEAVAGE.
83
sides being drawn out and the other contracted till it takes the form
c a d of fig. 9. In most cases these two operations appear to have
acted uniformly together ; the contraction of the one side being
in proportion to the expansion of the other.
The origin of the oblique pressure on these fossils is easily found:
the expansion of the masses of rock in the direction of the dip of the
cleavage must cause an oblique pressure on the surface of every bed
which is cut obliquely by the cleavage; the fossil shells lying between
an expanding mass and a resisting weight of matter have given
way before the pressure in the manner described. But as the ex-
pansion of the rock in the one direction may have been caused by
its compression in the contrary direction, it follows that all the
effects yet described may have originated in the compression of the
mass of the rock in a direction perpendicular to the cleavage planes.
The next cases chosen for illustration are those where the angle
of incidence of the planes of bedding and cleavage is below 5°.
These need not detain us long as they are exactly the same as the
last in principle, and only differ in the distortion being more exag-
gerated. The specimens are from Tintagel, and belong to Spirifer
giganteus of Sowerby. The shells are placed in the same position
relatively to the cleavage as before: that of fig. 10 is analogous to
that of fig, 6; the wing jpj„^ Iq^
DAB lengthened and so
much flattened that the ribs - —
are nearly lost; the other
wing CAB contracted and
with the ribs well-marked.
The shell fig. 1 1 is more dis- ^
torted ; and as the two valves
are united, the change of
form can be determined
accurately: fig. 12 shows ^
a longitudinal section of '.
this shell. The left wings
DAB are nearly equally
elongated in both valves in
the direction of the dip of
the cleavage Y Y, their ex-
tent, as seen in the section,
being D E and D F ; the
iwo right wings are so much
contracted that in the sec-
tion they only reach from C to E and F. Many other specimens
show a similar result in different degrees ; but the most extraordinary
that I have seen is the S. giganteus^ as represented in fig. 13, from
a bed at Tintagel intersected by the cleavage at an angle of about
1°. In this cast the elongated half of the hinge-line DA is three
times the length of the other half C A : the hinge area and foramen
are of an unnatural size, while a great part of the body of the shell
is lost. There is here the most distinct proof that a flexible cast of
the shell has been pressed forward towards the upper line Z Z.