The Wissahickon schist of the region in and near Philadelphia originally was a sediment made up of sandy, shaly, and arkosic layers of varying thickness and composition. These sediments have been completely recrystallized and highly deformed during: regional metamorphism; however, the original compositional banding has been retained. The presence of the metamorphic index minerals, garnet, staurolite, kyanite, and sillimanite, in definite zonal arrangement, indicates that the regional metamorphism increased in intensity from northwest to southeast. The rocks of the area belong to the amphibolite and epidote-amphibolite facies.
Granitization along the southwestern border of the area has altered the schist. Intrusive basic igneous rocks in the southern and eastern part of the area also show some granitization. Apart from the granitized rocks the chemical composition of the schist has not been changed appreciably, and the metamorphism may be considered as isochemical.
The structure of the rocks is believed to be that of an abnormal synclinorium produced by compression. No explanation is offered for the coincidence of the northern limb of the synclinorium with the garnet and staurolite zones, the center with the kyanite zone, and the southern limb with the sillimanite zone. The northern limb of the synclinorium and the zones of regional metamorphism are truncated by faults.