THE ORIGIN OF THE PHENOCRYSTS 



I I I 



numerous microperthitic structures. Plagioclase is somewhat 

 abundant. Small-rounded disks or ovals of micropegmatitic 

 structures are common. The larger feldspar and quartz crystals 

 indicate slight peripheral shattering in some of the thin sections. 



Fig. 6. — Photomicrograph of a phenocryst of microperthitic orthoclase from the 

 Greene county porphyritic granite area, showing inclusions of quartz, plagioclase,. 

 biotite, and very small prisms of apatite. Crossed nicols. Magnified 74 diameters. 



The biotite is considerably altered to chlorite and some 

 epidote, and at times carries inclusions. Scattered grains of 

 magnetite and prismatic inclusions of apatite and zircon are 

 present in microscopic proportions. 



The Columbia county porphyritic granite area — Heggie Rock. — 

 This area, marked Ch on the accompanying map, is located 

 near the Fall-line — contact between the Piedmont crystallines 



