308 F. BASCOM — PIEDMONT DISTRICT OF PENNSYLVANIA 



Cresheim branch of the Pennsylvania railroad near Laverock. Here 

 the gneiss is vertical to 60 degrees southeast. Elsewhere the gneiss when 

 exposed is either so decayed or so thoroughly penetrated by the gabbro 

 as to obscure stratification and the record of the folding to which it has 

 been subjected. 



The fault on the southeast limb of the anticlinorium, or the Hunting- 

 ton Valley fault, is thought to hade * to the northwest and has thus 

 thrust the Baltimore gneiss on the eroded surface of the Paleozoics. The 

 mica-gneiss, like the mica-schist, susceptible to crumbling, shows much 

 secondary folding. This fact, together with the absence of recognizable 

 beds, obscures the primary folding, and while there are evidences of two 

 synclinoria and one anticlinorium southeast of the Huntington Valley 

 fault line, they can not be distinctly traced in the Philadelphia district. 

 The presence of intrusives and the cover of Pleistocene materials obscure 

 the structure of the mica-gneiss. On the Wissahickon, where the gneiss 

 is well exposed, the dips are often very gentle and predominatingly north- 

 west (see plate 54). The cleavage dip remains constant to the southeast. 



Igneous Rocks 



classifica tion 



The igneous rocks of the belt are intrusive in character. They may be 

 classified lithologically as granitic, including several distinct masses, and 

 as gabbroitic, including the gabbro, hypersthene gabbro, norite, pyrox- 

 enite, peridotite, meta-gabbro, meta-pyroxenite, and meta-peridotite, or 

 serpentine, the diabase, and meta-diabase. 



OR A NITE-GNEISS 



Distribution. — This is an igneous intrusive of a granitic character, which 

 is well exposed on the west side of the Schuylkill. Striking east in the 

 direction of some scattered outcrops of granite, the main mass of granitic 

 material disappears somewhat abruptly on the east bank of the river at 

 the Mis of the Schuylkill, but expands southwestward toward the Dela- 

 ware river, where it disappears as a surface formation beneath a cover of 

 gravels. Its presence beneath the gravel is attested by numerous large 

 quarries in the formation, on Crum and Ridley creeks, in the neighbor- 

 hood of the Delaware. West of this body of granite-gneiss a similar rock 

 occurs southeast of Lima and can be traced to Glen Riddle. The Wil- 

 mington and Baltimore Central divisions of the Pennsylvania railroad 

 gives a section through this granite and serpentine between Glen Riddle 



*The writer is indebted to Mr George \V. Stose, of the U. S, Geol. Survey, for suggestions as to 

 the direction of hade. 



