200 The American Geologist. April, i90i. 
The Piedmont plain in Georgia represents the southern 
prolongation of the similar physiographic province in Virginia 
and the Carolinas. It is a well dissected low-land plain of holo- 
crystalline rocks, crossing the state in a south of west course, 
and skirting the southeastern border of the Appalachians with 
a gentle seaward slope, and passing on the southeast beneath 
the Coastal plain sediments. The plateau has a general average 
elevation of 350 to 450 feet above sea level, along the line of 
contact with the Coastal plain formations, and an average ele- 
vation of 1,000 feet along its northwestern border. A few un- 
reduced areas — residuals — of moderate elevation, rise above 
the general level of the plain, which, as a rule, represent parts 
of harder rock-masses remote from the major streams. 
The prevailing rocks forming the Plateau-crystalHne-com.- 
plex are mica-schists, gneisses and massive granites. The 
schists more particularly, are cut by numerous well-defined 
dikes of basic eruptives, diabases and diorytes,* which vary 
from several inches to as many hundred feet in width ; and, 
in one or two cases, have been traced for some miles in length,. 
The mica-schists form a large part of the rock-complex. 
They vary somewhat in color; are thinly foliated rocks, and 
show considerable variation according to locality in the pro- 
portion of mica and quartz and often feldspar. More or less 
feldspar is invariably present. The increase in this constituent 
may possibly mark in places the transition from schists into cer- 
tain gneisses of granitic composition. This can in nowise ap- 
ply, however, to the gneisses thus far studied in this area, since. 
in many instances, sharp contacts are plainly marked between 
the two rocks. The basic dike rocks vary from fine to coarse 
granular in texture ; they are occasionally porphyritic and fre- 
quently banded in structure, and gray to black in color. The 
usual normal rock-types are present, which, as a rule, present 
no unusual features in mineral composition. The dikes are 
limited for the most part, to the enclosing schists and are rare- 
ly actually seen cutting the granitoid rocks. The granitic rocks 
are traceable across the entire width of the state, and are ac- 
cordingly of wide distribution throughout the Piedmont area ; 
* other basic rocks usually laminated or thinly schistose in structure and 
somewhat porphyritic in places, simulating dikes and grouped for the most 
part at present, as ampliibolytes have not been sufficiently studied to warrant 
definite statements. 
