242 The Americaii Geologist. April, i899 
posits are outlined as occupying a triangular area in the cen- 
tral western half of the state, the northern boundary being the 
fall line, and extending from Columbus, on the west, to Macon 
in the center of the state, the western boundary being the 
Chattahoochee river and the southern or eastern half extend- 
ing from a point in the vicinity of Fort Gaines to Macon. 
A careful study of the base of this formation, that is, *.he 
Potomac, which is composed of clays, sands, and gravels, has 
convinced the writer that its beds continue in an easterly direc- 
tion across the state, following approximately, the fall line, and 
passing into South Carolina through Augusta. It furnishes 
clay of great economic value. 
Associated with 'the Potomac clays are the clay products 
resulting from the decomposition of the gneisses and schists of 
the Piedmont belt, which now furnish clays, in situ, and in low 
places where the washings from the hills accumulate, suitable 
for many common uses, and also a variety of clays belonging 
to the Tertiary period as well as those occurring in the Lafay- 
ette mantle, which are derived in part from the Piedmont 1)ell 
and in part from the underlying strata. From them may l)e 
made a high grade building brick, and a comparatively low 
grade refractory ware. As a rule, however, these clays are 
sandy and full of impurities and their use is rarely more than 
for local purposes. They furnish, however, material which is 
often used to mix with other clays, especially such as are to 
be found in the same vicinity, and which are by themselves 
too "fat" or plastic. 
On some of the terraces, particularly in the vicinity of the 
rivers such as the Savannah,. Oconee, Ocmulgee, Flint and 
Chattahoochee are clay beds belonging to the Columbia forma- 
tion, which furnish a material now largely used in the manufac- 
ture of bricks. 
The most interesting of the fall line clays and by far the 
most important from a commercial standpoint, are those of tlie 
Potomac group. Some of the other clays, however, are re- 
markable in many respects. For example, a certain Tertiary- 
clay, which is hard and tough and cuts with a knife much like 
wax, has a specific gravity so low that it readily floats on water. 
Others are noteworthy for their extraordinary colors, great 
plasticity, large amount of shrinkage on drying and .the enor- 
