Piedmont Plateau of Georgia. — JVatson. 21 1 
The Lithonia belt of coxtorted gneiss : The Lithonia 
belt of gneiss is continuous over a large part of three contig"- 
uous counties, and lies immediately adjacent on its north and 
west sides to the famous Stone mountain light-gray granite 
area. The belt takes its name from the town of Lithonia, lo- 
cated near the centre of the quarrying industry. The rock has 
been quarried very extensively as blocks and curbing for street 
paving, and has been shipped to the principal cities in the 
south and middle west. 
It is an irregularly banded, highly contorted, biotite gran- 
ite-gneiss, medium light-gray in color, and composed of anhe- 
dra ranging in size from 0.5-2.5 millimetres. [Plate XXII, 
figure I.] The principal minerals are quartz, orthoclase, micro- 
cline, plagioclase, biotite, a little muscovite, apatite, zircon and 
magnetite. Some secondary chlorite, epidote, kaolin and mus- 
covite occur. Idiomorphic crystals of red garnet are very com- 
mon in some of the quarries. The garnets form in some cases 
distinct lenses and layers alternating with bands of the prin- 
cipal minerals, and are also scattered as single crystals through 
the rock. Small areas of black tourmaline are distributed in a 
similar manner through the granite-gneiss to that of the Stone 
mountain granite, with the exception that the areas are by no 
means so frequent in the former rock as in the Stone moun- 
tain type. 
Orthoclase is the predominating feldspar and in part is 
microperthitic in structure. Microcline is subject to mucli 
variation with a general average increase, somewhat larger for 
the gneisses than for the granites. The increase in this con- 
stituent in the foliated granitic rocks over the massive types 
can probably be accounted for on the basis of pressure meta- 
morphism, as numerous pieces of the mineral show some indi- 
cations of a possible induced structure, such as might result 
from excessive strain. A large percentage of this constituent 
however, is undoubtedly primary in origin, while the remain- 
der is somewhat doubtful. Plagioclase is also variable in 
amount and is present in the form of stout laths with charac- 
teristic polysynthetic twinning, affording small extinction 
angles in basal sections. Biotite, as deep brown-colored and 
strongly pleochroic foils, is drawn out along roughly parallel 
lines. It is associated with smaller amounts of muscovite, and. 
