294 F. BASCOM — PIEDMONT DISTRICT OF PENNSYLVANIA 



The igneous intrusives cut across the belts of sedimentary material 



more or less irregularly, but in general show a northeast-southwest trend. 



The sedimentary series of the Piedmont complex fall into four groups : 



Ordovician : Wissahickon mica-schist and mica-gneiss. 



Cambro-Ordovician : Chester Valley limestone. 



Cambrian : Chickies quartzite. 



Pre-Cambrian : Baltimore gneiss. 



PRE-CAMBRIAN ROCKS: BALTIMORE GNEISS 



Distribution. — This formation appears in the central portion of the belt 

 and traverses it obliquely to the northeast. It is very thoroughly injected 

 by gabbro, which, with the Baltimore gneiss, constitutes the flat-topped 

 highland known as Buck ridge. North of Chestnut hill' the gneiss belt 

 becomes very narrow, though still persistent and expanding again to 

 the northeast. It is bounded by faults throughout most of its extent. 

 The formation is best exposed on the east bank of the Schuylkill between 

 Lafayette and Spring Mills. 



In Cecil county, Maryland, the Baltimore gneiss occurs in a narrow, 

 wedge-shaped area (1 mile by 3 J) on the Susquehanna river 2 miles 

 northwest of Port Deposit. It expands southwestward in Harford 

 count}'. 



Character of the formation and stratigraphic relations. — The Baltimore 

 gneiss is a medium grained, thoroughly crystalline aggregate of quartz, 

 feldspar, and biotite, and is characterized by a pronounced banding, 

 which may be very fine and intensely plicated. A gritty feel and a 

 pseudo-porphyritic texture further characterize the formation. To the 

 alternation of layers of biotite with quartz layers or quartz-feldspar 

 layers is due the finely gneissic character of the rock. Biotite occurs in 

 minute plates and is never developed in such dimensions or in such 

 excess as to render the formation schistose. Associated with the biotitic 

 layers are hornblende, epidote, titanite, garnets, and more rarely stauro- 

 lite or augite. Hornblende is sometimes as prominent a constituent as 

 biotite, and, like it, arranged in layers. Rounded apatites are also 

 present. The feldspar is microline, orthoclase, and acid plagioclase of 

 about the composition of oligoclase. The porphyritic texture is due to 

 the presence of lenticular areas of quartz and of feldspar irregularly inter- 

 spersed along layers. These lenses are without crystal boundary and 

 distinctly pebble-like in character. They are sometimes a marked feature 

 of the rock and indicate an original conglomeratic character. 



The fresh character of the crystallization and absence of pressure 

 effects on the crystals, indicating that strain was relieved by recrystal- 

 lization, the rounded apatites, the [quartz and feldspar pebbles, the sort- 



