ANNIVEESAEY ADDEESS OF THE PEESIDEl^T.
73
Tarbet; only they are on a scale about -^\ as large, and therefore
the rock appears to have an imperfect cleavage throughout rather
than a iointed structure. Here, then, we have an imperfect clea-
vage due to the lateral yielding of a rock composed of material not
truly plastic in its ultimate constitution.
Fig. 3. — Slate, Liskeard, with
minute contortions.
Fig. 4. — Slate, LisJceard, with
planes of discontinuity.
Pig. 5. — Slate, Shap, disturbed
Yig, 6. — Slate, Llanheris, with
well-c
The peculiar laminated structure of this Liskeard slate appears to
have favoured the production of this close-jointed structure, and
the joints are still further marked by the subsequent introduction
of black oxide of iron ; but in some cases, as in the pencil-slate of
Shap, the flakes of mica which constitute nearly the whole of the
rock have been thrown out of their original position in a similar
manner to the contorted material of the Liskeard slate, often with-
out the formation of definite contortions or joints, as shown some-
what diagramaticaUy by fig. 5, magnified about 200 linear.
We thus arrive at a structure in which the flakes in each minute
broken portion do lie in the plane of stratification, but this plane
is so contorted and broken up, that, as seen in a section cut at
right angles to both bedding and cleavage, they are nearly equally
inclined at all azimuths. The amount of the lateral yielding of
the mass necessary to give rise to this structure is apparently
small compared with that which has certainly occurred in the case
of many well-cleaved slates ; but it clearly shows how the particles
may soon become so arranged that the further yielding of the rock
would totally efface all evidence of the first stages of the process,
and cause the constituent laminse to become approximately parallel
to one plane, as shown, somewhat diagramaticaUy, by fig. 6, magni-