188 FLORIDA GEOLOGICAL SURVEY-I5TH ANNUAL REPORT

 The Callahan Brick and Tile Company works a deposit, probably marine, ranging from eight to twenty feet in thickness and having an overburden of six or eight inches. This deposit is underlain in places by a marl. About twenty acres have been worked and about twenty acres more have been proven. The deposit probably underlies. a much more extensive area.
 The plant is located on the Seaboard Air Line Railway near its intersection with the Atlantic Coast Line Railroad. The product is shipped to points in south and west Florida. Both a common and a face brick are produced.
 This clay may be used for common structural materials where a vitrified product is not essential. The fired product retains a porous texture at cone 15. Its physical properties are:
 Physical Properties of Callahan Clay (Lab. No. 0-29).
 Plasticity, judged by feel........... 25.60%
 W ater of plasticity........ ........ 0.42%
 Pore water ........................ 25.18%
 Shrinkage water .................. 11.70%
 Linear air shrinkage............... 29.85%
 Volume air shrinkage.............. 805.2 pounds per square inch.
 Modulus of rupture, average........ 3 days.
 Slaking test ....................... Excellent.
 Fire tests:
Temperature. Linear Shr. Absorption. Porosity. Color.
 Per Cent. Per Cent. Per Cent.
 -950*C. 0.7 15.85 39.75 Brick red.
 1050 1.3 15.70 39.00 Brick red.
 1150 1.8 14.15 31.48 Brick red.
 1190 2.2 12.60 30.25 Brick red.
 1230 2.2 11.95 28.00 Brick red.
 1310 2.2 11.58 26.75 Brick red.

 A brick plant was formerly operated by the Callahan Brick and Tile Company about three-quarters of a mile south of Callahan. The clay worked is the same as is now being .worked at Callahan.
 The region northward between Callahan and the St. Marys River is practically all underlain by a slightly sandy clay.
 A brick plant was formerly operated at the "Old Brick Yard Landing" on the St. Marys River, which is about forty miles west of Fernandina. Here there is an overburden of six to eight inches of soil, two feet of weathered, reddish clay, then a considerable depth of a grayish, plastic clay. Wells nearby indicate that this clay has a thickness of approxi-