302 F. BASCOM — PIEDMONT DISTRICT OP PENNSYLVANIA 



important or characteristic constituent. A micaceous chlorite is uni- 

 formly distributed through the rock, interspersed with wavy lamella? of 

 muscovite. Plagioclase, biotite, magnetite, ilmenite, apatite, and pyrite 

 are accessory constituents. In the neighborhood of an intrusive gabbro 

 on the south muscovite occurs in large areas, giving a somewhat spangled 

 appearance to the rock, and garnets, staurolite, and tourmaline are de- 

 veloped. The lowest member of this formation is decidedly calcareous 

 and also more silicious than the upper members. In some localities 

 there is interbedded a quartz-schist (or, more rarely, a quartzite) which 

 closely resembles the Cambrian quartzite. The mica-schist of the south 

 Chester Valley hills overlies the Chester Valley limestone. The structure 

 of the hills is evidently synclinal, though cleavage and fissility are very 

 pronounced and stratification is often completely obscured. Parallel 

 structures prevail on the limbs of the syncline and cross-structures in the 

 trough. 



This formation is typically exposed in the ravine cut through the south 

 Chester Valley hills by Gulf creek (see plate 52). 



The mica-schist is considered to be Ordovician in age on the ground 

 of its stratigraphic relations with Cambro- Ordovician limestone. That 

 it overlies the limestone without faulting and probably without uncon- 

 formity is indicated by the persistence of the geodiferous silicious bed 

 along the contact of the two formations, and by the lithologic gradation 

 which may be seen between the limestone and schist. 



Thickness. — The structure of the formation in the south Chester Valley 

 hills indicates a thickness not exceeding 500 feet. 



Distribution of the mica-gniess. — The mica-gneiss of the southeastern area 

 extends north and south, from Trenton, where it passes under cover, to 

 and into Cecil county, Maryland.* West and east it extends from Buck 

 ridge to the Delaware, expanding to the southwest and, where Buck ridge 

 disappears as a marked topographic feature, it appears to grade across 

 the strike into the mica-schist. 



The width of exposure thus varies from less than a mile near Trenton 

 to more than 40 miles on the Maryland boundary. The formation has 

 been intruded by large bodies of granitic, gabbroitic, pyroxenic, and 

 peridotitic material, lending it a massive and igneous aspect in the neigh- 

 borhood of these intrusives. Aside from these contacts, the mica-gneiss 

 is manifestly a stratified formation of a more or less heterogeneous 

 character. 



This heterogeneity is both local and regional, and, together with the 

 presence of igneous intrusives, has been the occasion of the separation 



*F. Bascom : The geology of the crystallines of Cecil county. Cecil County, Maryland Geol. 

 Survey, pp. 87-90, 103-108 ; also Atlas. 



