306 F. BASCOM — PIEDMONT DISTRICT OF PENNSYLVANIA 



nized Paleozoic material to the west of Buck ridge, and this affirmation 

 has been made the ground for inferring an igneous unconformity be- 

 tween the western Paleozoics and eastern gneisses. One mile northwest 

 of Romansville and 9 miles west of West Chester there occur intrusives 

 in the Ordovician mica-schist overlying the Cambro-Ordovician lime- 

 stone, gabbro and peridotite altered to serpentine and precisely similar 

 to intrusives in the Wissahickon mica-gneiss. 



The Huntington Valley thrust fault explains the absence of limestone 

 exposures between the Cambrian quartzite and the mica-gneiss in the 

 otherwise normal succession of the Paleozoic series. On these grounds, 

 because of stratigraphic relations to presumably Cambro-Ordovician lime- 

 stone, because of passage into Ordovician mica-schist, and in view of the 

 absence of any evidence of unconformity between it and recognized Paleo- 

 zoics, the mica-gneiss is provisionally held to be Ordovician in age. 



Correlation. — The Wissahickon mica-schist and the Wissahickon mica- 

 gneiss are to be correlated with the Berkshire schist of New England, the 

 Hudson schist of New York, the phyllite and the Wissahickon mica- 

 gneiss of Maryland. It is stratigraphically continuous with the latter 

 formation. The Wissahickon mica-schist is H. D. Rogers' "primal upper 

 slate " and the " Cambrian phyllite " of the Second Geological Survey of 

 Pennsylvania. The Wissahickon mica -gneiss comprises, together with 

 intrusive granite and gabbro, H. D. Rogers' " first and second gneiss 

 belts " and the Chestnut Hill, Manayunk, and Philadelphia mica-schists 

 and gneisses, considered pre-Cambrian and Cambrian in age, of the Sec- 

 ond Geological Survey of Pennsylvania. 



Structure of the sedimentary Formations 



The Triassic formations of the Piedmont plateau show a gentle north- 

 west dip and normal faulting. The pre-Cambrian and Paleozoic sedi- 

 ments of the whole width of the Piedmont plateau, ignoring the Triassic 

 formations which conceal a broad central area, apparently form an anti- 

 clinorium of the first order, which has brought to the surface pre-Cam- 

 brian gneiss along a central axis striking northeast, flanked by Cambrian 

 quartzite, Cambro-Ordovician limestone, and Ordovician mica-schist. 

 The surface outcrop of these formations is interrupted parallel to the 

 strike and controlled in width by the minor transverse folding, which, 

 alternately bending the axis of the anticlinorium in a low trough or 

 raising it in a low arch, bring successively younger or successively older 

 formations to the surface. 



There is, then, an anticlinorium of the first order compounded of anti- 

 clinoria and synclinoria of the second order. These are in turn com- 



