gneisses are mainly rocks of igneous origin. While recognizing the undoubted evidence of secondary dynamic action in many regions, and the absence at present of criteria by which original and secondary structures may be discriminated, the authors are strongly of the opinion that much of the banding of gneisses, as distinguished from mere foliation, may be an original structure due to the conditions in which the igneous magma was erupted and consolidated. The necessity of establishing the

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... nce of such differentiated lamination in basic rocks, and of recognizing its occurrence to any considerable extent in granitic masses, is self-evident after the bearing of such facts on the nature and origin of the ancient gneisses has been so clearly set forth.