Abstract

Phenomena and Plan of Discussion.

Evidences of Movement:—All observers are aware that few rock masses are continuous for any considerable distance. It is seldom that more than a few yards of a rock exposure can be examined without revealing joints, fissures or slickensides. Still more frequently rock masses show slaty or schistose cleavage,* impressed upon them by dynamical causes. In a very great proportion of such cases a little attention also discloses the fact that the partings are locally arranged on a definite system. In slaty cleavage the cleavage planes are substantially parallel and very close together; in flags of the slaty class the intervals between cleavage planes are greater; in schists the partings range through small angles, and in these last rocks there are frequently two sets of partings, each cleavage making a small angle with others of the same set, but a large angle with those of . . .

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