Piedmont Plateau of Georgia. — JWitsoii. 223 
a moderatel}' thick veneering of perfectly smoolb, somewhat 
vellowish-colored sericitic material, frequentl)^ more or less 
grooved and striated from the movement. 
Two sets of intersecting material differing widely in texture 
and mineral composition, and in origin, are common to the 
granitic rocks. These are: (i) True granite dikes, which vary 
in width from a few inches to many feet, cutting the rocks in a 
nearly vertical position ; are dark blue in color, and fine even- 
granular in texture; and are composed of the same minerals 
in the same proportions, and have the same chemical composi- 
tion as the even-granular granites. [Plate XXIV.] Since 
this class of intersecting material is not found in the more mas- 
sive adjacent granites it is regarded in the nature of apophyses 
from these massifs. (See page 214 for chemical analysis.) 
(2) True pegmatitic dikes and veins of variable dimensions arc 
common to the three rock phases and are composed of coarsely 
crystallized feldspar and quartz, with varying small propor- 
tions of the two micas, biotite and muscovite. [Plate XXIL] 
These are described in some detail on page 215. 
AGE RELATIONS OF THE GEORGIA GRANITIC ROCKS. 
No sedimentary rocks whose age is definitely known occur 
in the Plateau-crystalline-complex to admit of the definite de- 
termination of the age of the granites. In the absence of such, 
we can only hope to arrive at their relative ages by careful 
study of the contacts and the relative amounts of dynamic met- 
amorphism the rocks have suffered. A careful field examina- 
tion of the granitic rocks in the Plateau region of the state, 
shows considerable contrast in the amount of foliation or band- 
ing secondarily induced by pressure. All thin sections of the 
rocks studied showed some evidence of dynamo-metamorphism. 
This evidence is detailed above. The strongly banded gneisses 
differ from the massive phases of the rocks, simply in the pro- 
nounced schistose structure. In some areas of the massive 
granites an imperfect partial gneissoid structure is visible, 
which becomes rather noticeable in the Stone mountain area. 
While no case has been observed in the field, where the 
highly contorted gneissoid phase was definitely traced into the 
massive type, owing perhaps, if they exist, to lack of exposures 
there are a few areas where conditions very strongly point to 
