PETROLOGY OF SOUTH CAROLINA GRANITES TAG 
of biotite. Quartz, 1 to 7™™ in size, inclines to moderate dark smoky 
color, and biotite, 0.25 to 5™™ in areas up to 10™™ in size. 
Its principal minerals are orthoclase, microcline, plagioclase 
(oligoclase), quartz, biotite, sphene, apatite, zircon, and iron oxide, 
with secondary chlorite, epidote, a little colorless mica, and kaolin. 
The thin sections examined showed soda-lime feldspar (oligoclase) 
in excess of the potash feldspar (orthoclase and microcline). A 
chemical analysis has not been made of this granite nor of the similar 
one from the North Carolina area, but the feldspar content is appar- 
ently somewhat above the average for normal granites. 
GRANITE-GNEISSES 
Gneisses of granitic composition comprise one of the principal 
rock-types in the South Carolina crystalline region. Many of these 
were derived from original massive granites, and are invariably of the 
mica type, usually biotite; sometimes subordinate muscovite is 
associated with biotite. ‘The granite-gneisses are essentially identical 
“in mineral composition (table of analyses, p. 735) with the massive 
granites, from which the former differ chiefly in the banded structure 
secondarily induced into them by metamorphism. The principal 
minerals are quartz, orthoclase (largely microperthite), microcline, - 
plagioclase (oligoclase), biotite, a little muscovite, apatite, zircon, 
sphene, and secondary chlorite and epidote. Recrystallization and 
orientation of the essential minerals result in a schistose structure. 
The gneissic banding may be regular or irregular and contorted, 
usually the latter; the bands are of varying thicknesses, and are com- 
posed of alternating ones of light- (chiefly feldspar and quartz) and 
dark- (chiefly biotite) colored minerals. Like the massive granites, 
the gneisses show similar variation in color and texture (even-granu- 
lar to porphyritic). 
The Beverly granite-gneiss—This type, a contorted biotite granite- 
gneiss of medium- to dark-gray color and medium-grained texture, 
resembles quite closely the granite-gneiss of the Lithonia and Odessa- 
dale areas in Georgia,* and the Rockyface Mountain area, Alexander 
County, in North Carolina.? It is more coarsely crystalline than the 
« Georgia Geological Survey, 1902, Bull. No. 9A, pp. 78, 79, 125 f. 
2 North Carolina Geological Survey, 1906, Bull. No. 2, pp. 160 f. 
