2IO The American Geologist. April, looi. 
(See analyses IX, X, XI, XII, XIII, XIV, and XV of 
Table A.) 
Gradation from the porphyritic facies, peripherally, into a 
medium coarse-textured non-porphyritic granite facies of the 
same mineral and chemical composition is traceable in some o£ 
the areas. The lack of field evidence favoring gradation in 
the other areas is probably due to absence of exposures. As a 
rule, the rocks in the Georgia area, as well as in the southern 
states in general, are covered by a heavy mantle of residual 
decay, and exposures of fresh rock are limited and by no means 
continuous. 
(C) THE GRANITE-GNEISSES.* 
Extensive areas of gneiss of granitic composition abound 
in the Piedmont Plain, and in abvmdance, becomes one of the 
most, if not the most important rock in the plateau-complex. 
The gneisses vary from medium to coarsely crystalline rocks 
with a pronounced banded or schistose structure. The planes 
of schistosity vary from moderately irregular to highly contort- 
ed lines with the banding exceedingly irregular; from very 
thick to very thin layers. They are closely related genetically 
to the massive granites from which they differ only in the 
banded structure, secondarily imparted through dynamo- 
metamorphism. The two types — gneisses and massive gran- 
ites — are essentially alike in mineral and chemical composition. 
The minerals found in one are invariably present in the other. 
Furthermore, the minerals most abundant in the one also pre- 
dominate in the other. Like the massive granites, the acid 
gneisses are all biotite gneisses. Muscovite is an invariable 
associate but is subordinated in every case to the biotite in 
quantity : and hornblende is never present. The acid gneisses 
studied are believed for these and other reasons stated later, 
to be metamorphosed irruptivc granites and are therefore des- 
ignated granite-gneisses. 
Separate descriptions of the individual areas can not be de- 
tailed here, but instead, several of the larger and representative 
ones will be briefly described as illustrating their composition. 
• There may be and probably are gneisses of sedimentary origin in the 
Plateaii-crystalline-complex, btit the areas of gneiss so far studied afford no 
evidence of such origin. The discussion is entirely limited to those areas 
studied. 
