734 THOMAS LEONARD WATSON 
road. It is subordinate to biotite but is in sufficient amount to desig- 
nate the granite a hornblende-bearing biotite granite (p. 742). 
The great preponderance of biotite granites over others in South 
Carolina is shown in the following tabulation: 
I Biotites oranite sea oce wae ee aon eon 40 
i Muscovited granite eye eit ant I 
III. Muscovite-bearing biotite granite.......... 8 
IV. Hornblende-bearing biotite granite......... I 
VDiotite eranite-sneisss. nears eee 14 
VI. Muscovite-bearing biotite granite-gneiss.... 4 
The feldspars, as indicated in the table of chemical analyses 
below, include nearly equal mixtures of potassic (orthoclase and 
microcline) and sodic-lime (plagioclase) varieties. Microcline is 
equal to and occasionally greater in amount than orthoclase in some 
sections, and in others it entirely fails. Microperthite, intergrowths 
of orthoclase with a second feldspar (albite), is very constantly present. 
Plagioclase near oligoclase (soda-lime feldspar) is present in large 
amount, and in most cases is equal to or greater in amount than potash 
feldspar. Examination of the analyses below brings out this fact of 
high soda content, which is confirmed by the optical studies of thin 
sections of the granites. 
Besides the principal minerals enumerated above, there occur 
accessory apatite, zircon, magnetite, sphene, and rutile, together with 
secondary chlorite, epidote, a light-colored mica, and occasionally 
some other minerals. ‘These are the usual accessories in granite. 
CHEMICAL COMPOSITION 
The following analyses give a fair idea of the range in composition 
of the granites described below. 
The chemical composition is characterized by a percentage of 
SiO, ranging from 68.70 to 70.70 per cent., with an average for 17 
analyses of 69.46 per cent., which is about the normal silica per- 
centage for granites. Of the 18 available analyses of the South 
Carolina granites, five show more SiO, than is indicated in the maxi- 
mum percentage (Col. VI, 70.70 per cent.) of the table given on p. 735, 
ranging for the five from 70.90 to 73.26 per cent. The iron oxides, 
magnesia, and lime show some variation, but on the whole each is 
