102 THOMAS L. WATSON 



structures. The potash feldspars and quartz of the groundmass 

 are entirely allotriomorphic in crystal outline. The plagioclase 

 feldspar occurs in roughly idiomorphic lath-shaped crystals char- 

 acterized by the polysynthetic twinning, and, as a rule, afford 

 very small extinction angles in basal sections. 



The porphyritically developed minerals (phenocrysts) are 

 composed of both orthoclase with numerous microperthitic struc- 

 tures, and microcline, usually inclosing inequidimensional anhedra 

 of quartz, feldspar and biotite without definite optical orientation. 

 The more basic inclusions, accessory apatite and zircon are also 

 included in the phenocrysts. 



A thin section of one of the feldspar phenocrysts from the 

 Coweta portion of the area, shows the characteristic microcline 

 structure, and contains abundant inclusions of irregularly bounded 

 crystals of feldspar, quartz and biotite ; several rounded disks 

 or ovals of micropegmatitic intergrowths of quartz and feldspar, 

 and prismatic needle-like inclusions of apatite and zircon (see 

 Fig. 2.) A chemical analysis of fragments of carefully selected 

 phenocrysts from hand specimens of the Coweta outcrops of the 

 rock is given in XI a, page 119. 



The biotite occurs as aggregated intergrown shreds with 

 deep brown color, good basal cleavage and strong absorption, 

 partially altered to chlorite and some epidote. The biotite is 

 intergrown with occasional foils of muscovite, and sometimes 

 shows good crystallographic boundaries. In addition to these, 

 prismatic inclusions of apatite and zircon and a few grains of 

 magnetite are present. The effects of dynamo-metamorphism 

 are generally indicated to some degree in the rock by numerous 

 fracture-lines and undulatory extinction common to the larger 

 quartz and feldspar individuals. 



Cubes 1 from the Rockingham, Richmond county, and Mt. 

 Monroe, Iredell county, granite areas in North Carolina, very 

 closely resemble in color, grain and texture the Campbell- 

 Coweta-Fayette counties porphyritic granite, in Georgia. The 



1 Through the kindness of Dr. George P. Merrill, Head Curator, Department of 

 Geology, U. S. National Museum, the writer was accorded access to the Tenth Census 

 Collections of Building Stones in the Museum. 



