Piedmont Plateau of Georgia. — Watson. 209 
granite described above. Microcline is possibly subject to 
more variation than in the normal granites, and plagioclase 
is, as a rule, slightly increased. The feldspars are usually 
white opaque in color, with greenish-white and pink tints not 
uncommon. The interstitial feldspar is transitional into the 
phenocrysts in several areas. The quartz frequently crystal- 
lized simultaneously with the feldspars, resulting in the forma- 
tion of ovals of micropegmatitic structures, common in the thin 
sections from all the areas. Biotite is variable in amount but 
occurs in stout plates, deep brown in color and strongly pleo- 
chroic. Apatite is very abundant in the form of stout pris- 
matic inclusions in some slides and almost fails in others. 
The phenocrysts are usually opaque and white in color 
with pink tints common. They vary from 10-50 millimetres 
long and 5-10 millimetres across ; and vary from allotriomor- 
phic to idiomorphic in crystal outline. The idiomorphic type 
prevails, however; flat, tabular parallel to the clinopinacoid 
(010), with (001) and (loi) cleavages well developed. They 
display the usual habit of simple Carlsbad twins. Only the 
potash varieties, orthoclase and microcline, arc porphyritically 
developed, and, as a rule, are intergrown with a second feld- 
spar in microperthitic fashion. They invariably contain in- 
clusions of all the groundmass minerals, which with other evi- 
dence, indicates simultaneous growth with the interstitial com- 
ponents. The inclusions of biotite plates are always macro- 
scopic in size and, in many cases, the included mica is equally 
large as the same interstitial mineral. The in place as against 
iNTRATELLURic Origin for the phenocrysts has been elsewhere 
pointed out. 
Analyses of carefully selected fragments of the feldspar phenocrysts 
from two of the areas yielded the following results: 
I. 
II. 
SiOs 
64.64 
64.40 
AI2O3 
19.64 
18.97 
FesOa 
0.37 
0.37 
CaO 
0.67 
0.59 
MgO 
tr. 
tr. 
K2O 
10.00 
11.40 
NaaO 
3.06 
3.60 
Ignition 
0.22 
0.19 
Total 
98.60 
99.52 
Sp. Gr. 
2.60 
2.55 
(Thoiilet solution.) 
I. Feldspar phenocrvsts from Columbia Co.porphyritic granite mass. 
II. " " ' " Coweta 
