A PRELIMINARY REPORT ON CLAYS OE FLORIDA 
129 
Fig. 7. —Black Creek Barge Landing, near Middleburg, Clay County. For¬ 
merly used for shipping brick. Dismantled kiln may'be seen on extreme right. 
clays. Some calcareous and phosphatic clays occur in the southwestern 
portion of the county. 
A clay exposed in the vicinity of Doctors Inlet in the northern part 
of the county and underlying several square miles between the Atlantic 
Coast Line and the St. Johns River has good dry strength and color, but 
has a high air-shrinkage. It may, however, be used for a good grade of 
common brick, drain tile, hollow blocks, etc. The exact thickness of this 
deposit was not determined, but is known to be over six feet in places. It 
is overlain by sand and loam ranging from one to two feet. A sample of 
this clay taken beside the Green Cove Springs-Jacksonville highway, 
about one-half mile east of Doctors Inlet station, has the following 
physical properties: 
Physical Properties of Doctors Inlet Clay (Lab. No. 0 - 50 ). 
Plasticity, judged by feel. . . . 
Water of plasticity..... 
Pore water. 
Shrinkage water. 
Linear air shrinkage....... 
Volume air shrinkage. 
Modulus of rupture, average 
Slaking test. 
Steel hard at cone 1. 
Overfires at cone 9. 
Excellent. 
32.25% 
2.31% 
29.94% 
14.8 % 
36.6 % 
504.2 pounds per square inch. 
10 minutes. 
