310 



P. BASCOM — PIEDMONT DISTRICT OP PENNSYLVANIA 



gray rock, which breaks with a conch oidal fracture. Quartz, feldspar, 

 colorless augite, and titanite are the chief constituents. The feldspar is 

 bytownite (afyanj. The structure is microgranulitic. The exposure is 

 some 500 feet in width, and no other exposure of this type has been found. 

 This may be a dike : if, on the other hand, it represents a contact zone, 

 the more rapid cooling and earlier crystallization which would take place 

 along the periphery of the intruding granite may explain its finer texture 

 and the more basic character of its feldspar. 



The formation furnishes a good building stone, and has therefore been 

 extensively quarried. There are quarries at the falls of the Schuylkill 

 on the west bank of the river, at Overbrook, southwest of Overbrook on 

 Indian creek, at Kelleyville, and along the lower courses of Ridley and 

 Crum creeks (see plates 55, 56). In the Ridley and Crum Creek quarries 

 the rock is non-porphyritic, of an even, medium grain, light colored, and 

 is characterized by the presence of both micas. This rock has not been 

 separated by the previous surveys from the Manayunk and Philadelphia 

 gneisses. 



Two analyses have been made of the Port Deposit granite. They are 

 as follows : 



I. II. 



Si0 2 73.69 66.68 



A1 2 3 12.89 14.93 



Fe 2 3 1.02 1.58 



FeO 2.58 3.23 



MgO 0.50 2.19 



CaO 3.74 4.89 



Na 2 0.. 2.81 2.65 



K 2 1.48 2.05 



HO 106i H *°+ L09 



HaU 1 - 0b \ H 2 - 0. 16 



Ti0 2 0.50 



P 2 5 0.10 



MnO 0.10 



BaO 0.08 



SrO. .... Trace. 



Li 2 * Trace. 



99.77 100.23 



I. Biotite-granite or quartz-monzonite. Analysis made by William Brownell in 

 the chemical laboratory of Johns Hopkins University. 



II. Hornblende-biotite-granite or quartz-monzonite. Analysis made by W. F. 

 Hillebrand in the laboratory of the United States Geological Survey. 



G. P. Grimsley : Journal of Cincinnati Society of Natural History, volume xvii, 

 1894, pages 88-89. 



