The pre-Cambrian igneous rocks of eastern Pennsylvania and Maryland

The Blue Ridge and Piedmont geomorphic provinces, topographically distinct but geologically a unit, extend southwestward across eastern Pennsylvania and central Maryland, in a belt with an average width in these States of some 50 miles. In these provinces are exposed the crystalline formations of the Atlantic belt. Gneisses (with sporadic interbedded graphitic schist and marble), quartz-schist, crystalline limestone, and scnlsts, constitute the pre-Cambrian sedimentary series which has been folded, overturned, and overthrust with faulting, to the northwest.