IGNEOUS ROCKS 309 



and Lenni Mills. Both rocks are greatly decomposed, but the facts in- 

 dicated by the section seem to be that the granite is the older intrusive, 

 and that the serpentine or meta-peridotite intrudes through the granite 

 and lies on top of it as a sheet. The relation of the outcrop of granite 

 and serpentine to the topography also indicates that the latter rock over- 

 lies the granite. 



East of the main body of granitic rock, granite-gneiss emerges from a 

 cover of mica-gneiss on Pennypack creek near Vereeville and at Holmes- 

 burg, where it has long been quarried. 



A granite-gneiss also occurs in west Philadelphia and Fairmount park. 

 It was temporarily exposed in the neighborhood of Powelton Avenue 

 station when the station was removed and the railway cut widened. 



A large body of granite-gneiss constitutes the greater part of the Pied- 

 mond district of Cecil county, Maryland. This has been known as the 

 Port Deposit granite-gneiss. 



Its areal distribution, relations to the gabbroitic eruptions of the dis- 

 trict, and petrography are discussed elsewhere* The relations of gabbro 

 and granite in Cecil county are similar to those farther to the northeast. 

 Dikes of the former intrude in the granite and indicate a greater age for 

 the granite. 



The general lithologic similarity of these isolated outcrops of granite- 

 gneiss suggests a common magmatic origin. 



Character. — The rock of the main mass of granite is medium to coarse 

 grained, typically gneissoid, but often massive away from the periphery, 

 which is characteristically porphyritic. The phenocrysts are light, flesh- 

 colored orthoclases, which show orientation of their longest axes parallel 

 to the strike of the rock and which range from § to H inches in length. 

 The chief constituents of the rock are quartz, feldspar, biotite, and horn- 

 blende. Muscovite is more rarely present. The feldspar is chiefly ortho- 

 clase and microcline and subordinately oligoclase (abganj. Accessory 

 constituents are titanite, apatite, epidote, and actinolite. 



The rock shows the result of pressure in the production either of mi- 

 crocline structure, or of granulation in the feldspar, in the granulation 

 of quartz, and in the orientation of the constituents. Along the contacts 

 with the bounding rock, mica-gneiss, an injection gneiss has been pro- 

 duced by the penetration of the acid magma parallel to planes of fissility 

 of the mica-gneiss. 



In the Pennsylvania Railroad cut, 11 miles southeast of Overbrook 

 station, a contact facies of the granite-gneiss is exposed. The porphy- 

 ritic granite passes abruptly into a close grained aphanitic tough, light, 



* F. Bascom : Op. cit., pp. 90-92, 108, 117-119. 



