The pre-Cambrian banded gneiss of a part of the Piedmont Plateau near Philadelphia is interpreted as a hybrid rock, resulting from the thorough injection of felsic material into a plagioclase amphibolite. The composition of the injecting solutions probably ranged from mostly water to that of an aqueous potash-silica-alumina magma. In the western part of the area vertical, winding mylonite zones cut these rocks. These zones outline lenticular masses of less sheared rocks and in some places attain a thickness of a quarter of a mile. East of Wissahickon Creek schistose shear zones run parallel to the nearly vertical foliation of the gneiss. A post-Ordovician high-angle fault forms the northern border of the gneiss west of Wissahickon Creek, crosses it and forms the southern border of the gneiss east of Wissahickon Creek. A branch of this fault bounds the western part of the gneiss on the south. The paper presents the results of a structural and petrographic study of a belt of the gneiss 37 miles long, extending from Trenton to Bryn Mawr.