KiXEOUS ROCKS 317 



intrusive. The interrupted character of the exposures, the variation in 

 width of the belt, the massive nature of the rock, the contact phenomena, 

 and the displacement of the mica-gneiss which have accompanied its 

 intrusion are field evidences of an intrusive origin. Petrographically 

 the rock shows alteration from the igneous type peridotite, an olivine- 

 pyroxenite rock. It is in general characterized by the remains of olivine 

 crystals, which are now largely altered to serpentine, by serpentine, which 

 is often the chief constituent of the rock, by steatite (talc), which is an 

 alteration product both of the original pyroxenic constituent and of the 

 serpentine, and by calcite, quartz, and the iron oxides. The three last- 

 named constituents not only occur in subordinate amount with serpen- 

 tine as by-products of serpentinization, but as the final products of 

 alteration they may constitute the entire rock mass. Breunnerite is also 

 sometimes present as a by-product. 



Locally the rock varies greatly. In some localities, notably near 

 Lafayette, on both sides of the Schuylkill, it is a fairly pure, gray-green 

 steatite (soapstone) (see plate 60), in others (near Mill creek) it is a mass- 

 ive blue-green serpentine or a reddish-yellow silicious rock, and in still 

 others ( a Black Rock quarry ") the original rock type can still be traced. 

 In this case dark-green serpentized olivine crystals mottle the light-gray 

 steatite which forms the body of the rock (see figure 3, plate 61). Pene- 

 tration twins of olivine, producing cross and stellate forms, have been 

 found in this dike (see plate 61, figures 1 and 2). The formation is 

 penetrated by a network of joints, and on the joint plane a blue-green 

 opaline serpentine is sometimes deposited. 



The second dike, upon the southeast flank of the gabbro mass, extends 

 from the east bank of the Schuylkill to 2 miles southwest of Chester 

 creek. It widens southward, reaching a maximum width of II miles. 

 There are numerous sporadic outcrops southeast of the main areas. 



As in the case of the other dike, contact metamorphism has affected 

 the surrounding rock. The mica-gneiss has altered to an actinolite- 

 schist or to a spangled muscovite-schist or has become excessively gar- 

 netiferous. Petrographically the rock shows alteration from an original 

 type which differed from that of the first dike in the possession of a 

 larger amount of pyroxene and a smaller amount of olivine. This orig- 

 inal rock type seems to have been a pyroxenite in which enstatite was 

 the prevailing pyroxene. Associated with the serpentine are enstatite, 

 tremolite, and anthophyllite in abundant development. The usual ac- 

 cessory constituents are present. 



The third dike shows few exposures. The most conspicuous of them 

 is located southeast of Edgemont. Here the rock lies piled up in pic- 

 turesque masses, which have won for the locality the name of Castle 

 rock (see plate 62). Serpentinization and steatitization are not far ad- 



