318 F. BASCOM — PIEDMONT DISTRICT OF PENNSYLVANIA 



vanced in the main mass of Castle rock, which still shows the character 

 of the original type, an augite-enstatite rock or pyroxenite. Associated 

 with this pyroxenite is a pure green serpentine. 



The serpentine of the fourth or most northwesterly intrusive extends 

 with sporadic outcrops from the Schuylkill river southwestward. South 

 of Paoli it expands into a considerable area, which is continuous to West 

 Chester and has a sporadic extension to the state line and into Mary- 

 land * The rock of this belt is chiefly a massive dark-green or a light- 

 green granular serpentine. Fibrous serpentine is associated with it; also 

 talc, quartz, magnetite, and limonite. The microscopic structure of the 

 serpentine indicates that the original rock type was olivinitic — that is, 

 a peridotite. 



The genetic relationship of the serpentine (meta-pyroxenite and raeta- 

 peridotite) to the gabbro is to be seen in the petrographic character of 

 the original types and in the field relations which it sustains to the 

 gabbro. The first shows it to be a differentiation product of the gabbro 

 magma ; the second shows it to be either peripheral in character or a 

 subsequent intrusive in spaces left by the contraction of the cooling 

 gabbro magma. 



The areas which have been mapped as serpentine country are by no 

 means underlain by serpentine exclusively nor even by rocks in which 

 serpentine is the predominating constituent. In some localities the 

 serpentine has been completely removed and only silicious ironstones or 

 "honeycomb rock" remains. In other localities serpentine is scarcely 

 developed, and pyroxenite or peridotite are the underlying materials. 

 In still other localities talc, anthophyllite, or chlorite are the prevailing- 

 alteration products, producing soapstone, an anthophyllite rock, or a 

 chlorite schist. An occurrence on the tributary to Green creek, 11 miles 

 southwest of Chelsea, is a pure anthophyllite-steatite rock, in which the 

 former mineral occurs in large crystals. A great variety of minerals, in 

 addition to or replacing serpentine, may develop from the alteration of 

 basic igneous rocks. Talc, asbestos, anthophyllite, tremolite, hornblende, 

 actinolite, epidote, chlorite, clinochlore, vermiculite, pectolite, magnetite, 

 hematite, limonite, calcite, breunnerite, magnesite, quartz, especially 

 drusy quartz, chalcedony, opal, chromite, and corundum have been 

 found associated with the serpentine of the Pennsylvania Piedmont 

 district. 



META-GABBRO 



Distribution. — Penetrating the Baltimore gneiss and the Wissahickon 

 mica-gneiss are numerous dikes of basic igneous material. In many 



*For the geology and petrography of the "State Line" serpentines, as the serpentines of the 

 extreme southern portion of this belt are designated, see Bascom, op. cit,, pp. 93-94, 134-135, 



