Nature . reading ofthe same problems after attacking them by the way ofmicro-sections and field observations, the author makesthe term rock cleavage very comprehensive; he re-cognises among cleavable rocks two broad divisions, whichhe calls respectively protoclase, or original cleavage rock,and metaclase, or secondary cleavage rock. The formerclass includes such structures as bedding in sediments andflow structure in lavas; the latter class is considered underthe heads fracture cleavage and flow cleavage is conditioned by the existence of in-cipient or cemented and welded par
Nature . reading ofthe same problems after attacking them by the way ofmicro-sections and field observations, the author makesthe term rock cleavage very comprehensive; he re-cognises among cleavable rocks two broad divisions, whichhe calls respectively protoclase, or original cleavage rock,and metaclase, or secondary cleavage rock. The formerclass includes such structures as bedding in sediments andflow structure in lavas; the latter class is considered underthe heads fracture cleavage and flow cleavage is conditioned by the existence of in-cipient or cemented and welded parallel fractures, and isindependent of the parallel arrangement of the mineralconstituents. Flow cleavage is conditioned solely by aparallel arrangement of the minerals. The one is a pheno-menon of the zone of fracture, the other of the zone ofHowage in the lithosphere. Fracture cleavage is madeto include, wholly or in part, those structures that havebeen variously described as close-joint-cleavage, false. Chloritoid cry cleavage, strain-slip-cleavage, slip cleavage, ausweichungscleavage, rift and fissility in part (the term is retained forclosely spaced parallel partings). Flow cleavage includes,wholly or in part, the ultimate cleavage of Sorby, cleavage of most authors, slaty i leavage, schistosity,and parallel structures in certain gneisses. Flow cleavageis a molecular phenomenon, and the dominating factor inits production is re-crystallisation. Much space is devotedto the study of the behaviour of the more important rock-forming minerals in relation to the direction of the cleavagein rocks, and many thin slices have been examined todetermine how far there existed a parallelism between the ByC. Indu< 1 (0 Bulletin 23 (2) Bulletin (3) Bulletin 2S2, 1905, Preliminary ReporResources of Central Oregon. By I C Rui (4) Bulletin 235. 1904, A Geological RecorRange. By G. O. Smith and F. C. Calkins. (s) Bulletin 242, rQo4, Geology of the HudHoosic and the Kinderhook. By
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