This paper is a comparative study of the fabrics of granitized and ungranitized Wissahickon schists near Philadelphia.

Granitization was effected chiefly by the introduction of alkalis and some lime, probably carried in hydrothermal solutions. The mica schists were reconstituted, becoming coarser in grain, with relative increase in feldspars and decrease in micas. A complementary enrichment of less-granitized patches and narrow border zones in iron and magnesia has produced biotite-rich rocks. Hydrothermal activity following the main period of granitization has produced some retrograde metamorphism.

Folding in the ungranitized schist is of the flexural slip type. The development of folds can be traced in both mica and quartz fabrics, and the rotation of S-surf aces in the fabrics can be correlated with the overturned folds observed in the field. The same features can be identified in the granitized rocks, but less distinctly, since the physical properties of the latter made them react differently from the ungranitized schists during regional metamorphism. Granitization was syntectonic.

Late movements, occurring after the main period of granitization, are recorded locally in the sheared or crushed rocks.

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