THE ORIGIN OF THE PHENOCRYSTS 



107 



some laths of plagioclase near oligoclase. The large feldspar indi- 

 viduals (phenocrysts) contain inclusions of irregularly bounded 

 crystals of quartz, biotite and other feldspar species. These 

 inclusions are usually round, oval shape in outline. The quartz 

 and feldspar anhedra are variously interlocked as fine and coarse- 

 grained mosaics. The finer-grained mosaics of the two minerals 



Fig. 3. — The Virginia type of foliated porphyritic granite near Chatham, Va. 



represent the peripheral shattered portions of the larger feldspar 

 and quartz crystals. Micropegmatitic structures, intergrowths of 

 quartz and feldspar, are very common. Biotite occurs as grouped 

 shreds and plates, deep brown to occasional green in color, with 

 strong absorption and good basal cleavage. It is partially 

 altered to a dark opaque chlorite and occasional crystals of 

 slightly pleochroic and strongly double refracting epidote. A 

 few scattered grains of magnetite are observed. 



An outcrop, from which some rock has been quarried, of a 

 similar dark-colored, foliated biotite porphyritic granite is found 

 near Chatham depot, Pittsylvania county, Virginia. . The feld- 

 spar phenocrysts are very large in size, 40-50™™ long, roughly 



