THE ORIGIN OF THE PHENOCRYSTS 



109 



Five miles north of the central porphyritic mass are outcrops 

 of a coarse but close compact-grained granite, containing only 

 a very small amount of biotite. The feldspars show pronounced 

 pink and greenish tints. 



About four miles south of Greensboro, on the north side of 

 Beaver-Dam Creek, is an outcrop of practically the same granite. 

 The quartz is decidedly dark in color and of the smoky variety ; 



Fig. 4. — The Greene county porphyritic granite area. 



the feldspars are flesh colored, and the rock contains but little 

 mica. 



The porphyritic facies of the rock consists of a coarse-grained, 

 light-gray groundmass of quartz, feldspar and biotite, in which 

 are embedded large, flat tabular feldspar phenocrysts. The por- 

 phyritic feldspars average 30-50 mm long and io-i5 mm broad; 

 and indicate the usual elongation parallel to the clinopinacoid 

 (010), and Carlsbad twinning. The phenocrysts are deep pink 

 to perfectly white in color, and are usually cloudy and opaque in 

 appearance. 



