Piedmont Plateau of Georgia. — Watson. 213 
Chemical Analyses of the Lithonia Contorted Granite-Gneiss, 
DeKalb County. 
I. 
II. 
III. 
SiOo 
76.00 
75.16 
72.96 
A1003 
13.11 
13.74 
14.70 
FeoOs* 
0.92 
0.91 
1.28 
CaO 
1.06 
0.91 
1.28 
MgO 
0.27 
0.17 
0.07 
K2O 
4.69 
5.05 
4.73 
NaoO 
3.88 
3.76 
4.18 
IkB- 
0.81 
0.32 
0.23 
Total 100.24 100.02 99.43 
I. Biotite granite-gneiss from the Crossle\' quarr\', near Lithonia, 
Georgia. 
II. Biotite granite-gneiss from the Arabia Mountain quarries 3 miles 
from Lithonia, Georgia. 
III. Biotite granite-gneiss from the Southern Granite Co.'s quarrv, 
near Lithonia, Georgia. 
Intrusive nature of the granites. 
The evidence of the igneous origin of the Georgia granites 
is essentially the same as for similar irruptive acid masses 
studied elsewhere. The evidence is from, (i) field rela- 
tions; (2) chemical composition; and (3) mineral composition, 
I. Field relations. 
Stratigraphic Features: The exposures of the gran- 
ites in the field point to rounded or elliptical forms common to 
intruded granite masses. The two granite facies — porphyritic 
and non-porphyritic — may be considered as one, since in many 
of the granite areas the porphyritic structure grades peripher- 
ally, into a non-porphyritic granite facies of the same mineral 
and chemical composition, and are from necessity of the same 
origin. In the case of the foliated phases of the granites, the 
schistosity is not co-incident with that of the surrounding 
schist, but cuts across the foliation of the latter. 
Near many of the larger granite-masses well-defined gran- 
ite dikes, slightly different in texture from, but of the same 
mineral and chemical composition as the mass, occur, which 
would commonly be regarded as apophyses in case the rock is 
regarded as eruptive. [Plate XXIV.] These dikes or 
apophyses project outward from the main rock-mass and cut 
abruptly into the adjacent rocks. A chemical analysis of spec- 
* .K\\ iron estimated as FeaOa. 
