The mica schists and mica gneisses of the Wissahickon formation in the Philadelphia region are described in terms of metamorphic facies. Representatives of the amphibolite facies (sillimanite-almandine and staurolite-kyanite subfacies) predominate, but show incipient alteration to minerals characteristic of the epidote-albite amphibolite facies. The status of rocks containing the assemblages kyanite-orthoclase, sillimanite-muscovite, and kyanite-almandine is also discussed.
Metamorphism of the highest grade is found in the southwestern end of the schist belt; but evidence is presented to show that the most intense metamorphism took place, not at the time of highest temperatures, but during a succeeding period of declining temperatures, when mineral changes were facilitated by copious hydrothermal solutions and strong regional deformation. The decipherable history is therefore largely one of retrograde metamorphism; many of the higher grade rocks have been converted by granitization to microcline gneiss (“granodiorite”), especially in the southern part of the schist belt, while farther to the north and west, incipient chloritization has been favored by late crushing. The period of metamorphism is tentatively dated as Paleozoic.