Structural geology . s present, gneissic structure has been dis-cussed under the heading of rock cleavage. In many cases, how-ever, cleavage in gneisses is not good. The essential mineralogicaldifference between gneisses and schists is the possession by thegneisses of a relatively small amount of the platy and columnarminerals so necessary for a good rock cleavage, and correspond-ingly more feldspar and quartz. The origin of perhaps the majority of gneisses is not yet known.In a few instances the structure has been identified as an originalmagmatic flow structure, the protoclastic structure. I
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Structural geology . s present, gneissic structure has been dis-cussed under the heading of rock cleavage. In many cases, how-ever, cleavage in gneisses is not good. The essential mineralogicaldifference between gneisses and schists is the possession by thegneisses of a relatively small amount of the platy and columnarminerals so necessary for a good rock cleavage, and correspond-ingly more feldspar and quartz. The origin of perhaps the majority of gneisses is not yet known.In a few instances the structure has been identified as an originalmagmatic flow structure, the protoclastic structure. In othercases it is the result of secondary rock flowage, either of igneous orsedimentary rocks, the crystalloblastic structure. Some criteriawhich have been useful in discriminating the igneous and sedi-mentary gneisses resulting from rock flowage are the broaderfield relations, the chemical composition, the possession of igneousor crystalloblastic textures, and the content and form of the heavy ss STRUCTURAL GEOLOGY. Iid.U. Photomicrographs showing the progressive granulation of the Mnrinanorthosite under the influence of pressure. After Adams. GNEISSIC STRUCTURE SO These criteria may often be decisive whenapplied collectively, but seldom when used separately. They arediscussed on a subsequent page (97). Gneisses have been known to develop by rock flowage fromOcks which under other conditions have yielded schists. What, r