312 F. BASCOM — PIEDMONT DISTRICT OF PENNSYLVANIA 



It is an important formation in Maryland, Delaware, and southeastern 

 Pennsylvania, where it intrudes alike into pre-Cambrian Baltimore 

 gneiss and into Paleozoic limestone and mica-gneiss. 



In the Pennsylvania belt its maximum development is in the south- 

 east. From the Susquehanna river and from the region about Wilming- 

 ton, Delaware, it extends northeast, forming the main mass of Buck 

 ridge, where it shows itself at the surface in exceedingly irregular areas. 

 It is intimately associated with the Baltimore gneiss, and has so affected 

 this formation along contacts as to produce an appearance of gradation 

 from banded gneiss to massive gabbro. 



There are many instructive exposures of gabbro and Baltimore gneiss. 

 They all suggest the intercalation, along the periphery of the gabbro' 

 mass, of the former rock between the beds of the latter and the subse- 

 quent folding of both materials. The penetration of the gneiss by the 

 gabbro is quite irregular. Thin sheets swell into large masses, which, 

 on exposure, weather into spheroidal boulders. Because of this pecul- 

 iar injection of the gneiss by gabbro, gabbro boulders may appear spo- 

 radically in areas where gneiss is the prevailing formation at the surface, 

 and vice versa. 



At Glen Mills, a few rods south of a large quarry in gabbro, an aban- 

 doned quarry shows gneiss and gabbro interbedded, yet the gneiss does 

 not appear as a surface formation. 



Under these conditions it is impossible, in fixing the boundaries of 

 these two formations, not to include some gneiss within the gabbro area, 

 while gabbro boulders may be seen within the gneiss area and gabbro 

 injections appear in rock cuts, though not prevailing at the surface. 

 This is particularly true of the areas northwest of Media. 



The Baltimore gneiss and the gabbro have not before been separated 

 in the Pennsylvania Piedmont. They alike give rise to a relatively 

 elevated rolling country, with irregular, rounded eminences. 



Dark-colored boulders of disintegration, with rusty exteriors, strew 

 the fields and afford almost the only indication of the underlying rock. 

 The gabbro boulders are exceedingly tough, except along contacts with 

 the gneiss, where the development of hornblende and mica renders them 

 more schistose and more easily attacked mechanically. 



Character. — The gabbro is a medium-grained massive rock,* possessing 

 either a bronzy-gray or a greenish -gray color, depending on the character 

 of the ferromagnesian constituent. 



Quartz, pyroxene, and feldspar, may usually be determined in the 

 hand specimen. The gabbro is a typical hypersthene-augite-plagioclase- 



*For full petrographic description of the Cecil County gabbro and norite, see Bascom, op. cit., 

 pp. 121-132. 



