Recent Reports of Geological Surveys 1

Abstract

Cleavage. THE subject of rock cleavage is one of perennial interest; only a short time ago were Dr. Becker's views noticed in these columns, views founded upon experiment and analysis. Now, Dr. Leith (1) lays before us his reading of the same problems after attacking them by the way of micro-sections and field observations. The author makes the term “rock cleavage” very comprehensive; he recognises among cleavable rocks two broad divisions, which he calls respectively protoclase, or original cleavage rock, and metaclase, or secondary cleavage rock. The former class includes such structures as bedding in sediments and flow structure in lavas; the latter class is considered under the heads “fracture cleavage” and “flow cleavage.” Fracture cleavage is conditioned by the existence of incipient or cemented and welded parallel fractures, and is independent of the parallel arrangement of the mineral constituents. Flow cleavage is conditioned solely by a parallel arrangement of the minerals. The one is a phenomenon of the zone of fracture, the other of the zone of flowage in the lithosphere. Fracture cleavage is made to include, wholly or in part, those structures that have been variously described as close-joint-cleavage, false cleavage, strain-slip-cleavage, slip cleavage, ausweichungs cleavage, rift and fissility in part (the term is retained for closely spaced parallel partings). Flow cleavage includes, wholly or in part, the ultimate cleavage of Sorby, “cleavage” of most authors, slaty cleavage, schistosity, and parallel structures in certain gneisses. Flow cleavage is a molecular phenomenon, and the dominating factor in its production is re-crystallisation. Much space is devoted to the study of the behaviour of the more important rockforming minerals in relation to the direction of the cleavage in rocks, and many thin slices have been examined to determine how far there existed a parallelism between the cleavage of the rock and dimensional and vector properties of given mineral species.