2o8 The American Geologist. April, i90i. 
(b) the rORPIIVRITIC GRANITES. 
The porphyritic facies of the Georgia granites are else- 
were described and discussed at some length by the writer.* 
The individual areas are described in detail and their relations 
to the normal granite facies stated. For the purposes of the 
present paper, it is only necessary to note their distribution 
and briefly describe them as a whole, detailing their chief char- 
acters so far as they bear on the present problem. 
More than a half-dozen well-defined separate areas of 
coarsely crystalline porphyritic granites are noted within the 
limits of the Piedmont Plain of Georgia. In every case, they 
are associated with an even-granular facies of the same rock, 
having the same mineral and chemical composition. The por- 
phyritic areas have been designated as follows : The Camp- 
bell-Coweta-Fayette counties' area; the Pike county area; the 
Fulton county area ; the Brinkley place- Holder's Mill area ; the 
Sparta area ; the Milledgeville area ; the Greene county area ; 
the Columbia county area. 
General Descriptiox : Certain general features are com- 
mon to all the porphyritic areas, although the rocks are strong- 
ly contrasted in many. With one exception, they are all mas- 
sive in structure, without, as a rule, trace of the primary or 
secondary foliated structure. The exception is in the Brink- 
ley Place-Holder's Alill area, where the rock has a pronounced 
schistose structure, clearly due to pressure metamorphism. 
The porphyritic granites are prevailingly coarse-grained and 
light to dark gray in color, according to the amount of biotite 
present. 
PETROGRAPHY. 
The groundmass or matrix is a typical coarse biotite granite 
without the foliated structure. It consists of anhedra meas- 
uring 3 to 8 millimetres in size. [Plate XXI, figures i and 2.] 
The principal minerals are quartz, orthoclase, microcline, plagi- 
oclase, biotite, a little muscovite, apatite and zircon. Some 
chlorite and epidote derived from the alteration of the feldspar 
and biotite occur. Quartz is abundant and the feldspars have 
the same general characters as in the even-granular types of 
•/ourna/ or Geo/ogy, 1901, vol. ix, February-March number. A Prelimi- 
nary Report on the Granites and Gneisses of Georgia, Geol.Sur. ofGa., in press. 
