THE ORIGIN OF THE PHENOCRYSTS 



103 



phenocrysts in the Carolina rock are usually allotriomorphic, 

 occasionally idiomorphic, in crystal outline ; and, as a rule, are 

 large in size, and contain numerous plates of included biotite. 

 A similar though darker colored rock, owing to a greater amount 

 of biotite present, occurs in Aiken county, South Carolina. The 



Fig. 2. — Photomicrograph of a phenocryst of microcline from the Coweta county 

 porphyritic granite area, showing inclusions of quartz, plagioclase and biotite. The 

 inclusions of plagioclase and biotite are considerably altered. Crossed nicols. 

 Magnified 74 diameters. 



porphyritic feldspars in the Aiken rock present the same char- 

 acteristics as the phenocrysts of the Georgia and North Carolina 

 porphyritic granites. 



The Hancock county porphyritic granite area. — A coarse-grained 

 porphyritic granite outcrops one quarter of a mile east of Sparta 



