File:The Quarterly journal of the Geological Society of London (13205961024).jpg

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PROCEEDINGS OF THE GEOLOGICAL SOCIEEY. .DeC. 2,
judging from their present forms, they appear shortened at least
twenty-five per cent, in a direction perpendicular to the strike of
the cleavage across the bed.
Fig. 3.
Similar instances are common where flat shells are found in slaty
beds : the large Posidonise in the black shale above the limestones
at the base of the culm-series of Devonshire are frequently distorted
in the same manner ; at the limestone quarry of Bickington, one
mile and a half south of Barnstaple, thousands may be seen in the
shale thrown off the top of the quarry, every one of which is con-
tracted in a direction perpendicular to the line of strike of the cleavage
across the bed, and which are all covered with folds parallel to that
line. The cleavage intersects the bedding at an angle of about 50°.
The case of the Trilobites in the Llandeilo flags, mentioned by Pro-
fessor Phillips in the passage already quoted, is analogous to these.
In all these cases the shells appear to have suflTered from lateral
pressure, which has altered their forms and produced the parallel folds
on their surface.
Fig. 4.
The same lateral pressure has produced a different effect on gibbous
shells, which are usually flattened in a direction perpendicular to the
cleavage and drawn out or squeezed out considerably in the diiection
of the dip of the cleavage planes. The result is seen in the Spirifer
disjunctus, represented in figs. 4- and 5, from one of the slaty beds at
South Petherwin, met by the cleavage at an angle of about 60 .
Figure 4.Shows the shell as it lies on the bed, its length reduced
about one-half, from A to B, by compression perpendicular to the