244 The American Geologist. April, \mt 
quite white on drying except of course in the case of some of 
the beds, particularly in the western part of the state, where it 
is mottled red and blue. 
The most wide-spread impurities which occur in the Poto- 
mac clays are scales of mica, usually muscovite, and minute 
crystals of magnetite. Both of these may frequently be seen 
with the naked eye, and the magnetite is sometimes so abun- 
dant as to give a slightly bluish tint to the mass. An average 
sample of the better grades of this clay is fine grained and 
tough, has a characteristic argillaceous odor and a soapy or 
fine mealy feel. Its hardness is about that of chalk. It absorbs 
water readily and disintegrates rapidly, becoming somewhat 
plastic. 
The following tables and descriptive notes show the chem- 
ical and mineralogical composition, physical properties, geo- 
logical age, and manner of occurrence of a few typical clays 
found along the Georgia fall line. 
The analyses and physical tests were made by the writer. 
The samples described here are designated by the name of 
the locality or property, in the vicinity of which they occur. 
The Grisivoldville Clay. Griswoldville is situated on the 
Central railroad about ten miles east of ]\Iacon. 
The white Potomac clays are seen here at many points 
along the railroad, in cuts, and in gullies and old fields in the 
vicinity. While it frequently occurs at the surface it is usually 
buried beneath Lafayette beds, varying in thickness from ten 
to twenty feet. The terrace along which the beds occur is 
about one hundred feet above the Ocmulgee bridge at Macon. 
The hills which rise above this terrace are composed of 
Tertiary strata mantled with Lafayette, and have an average 
hight of one hundred and sixty feet above the plain, or two 
hundred and sixty feet above the terrace upon which the city 
of Macon stands. 
From the railroad cut at the Griswold station a large 
amount of clay has been removed and shipped to Chattanooga. 
Tenn., for manufacture into fire brick and other refractory 
wares. 
The chief characteristic of the clay beds, here, is irregular- 
ity. They frequently grade ofif into white sands and gravels 
and are occasionally cut out by the unconformably overlying 
beds of Lafavette. 
