170 FLORIDA GEOLOGICAL SURVEY-15TH ANNUAL REPORT

 Physical Properties of Brooksville Hammock Clay (Lab. No. 0-59).
 Plasticity, judged by feel........... Excellent.
 W ater of plasticity................. 27.40%
 Pore water ........................ 1.49%
 Shrinkage water ................... 25.93%
 Linear air shrinkage................ 12.1 %
 Volume air shrinkage............... 28.8 %
 Modulus of rupture, average........ 215.3 pounds per square inch.
 Slaking test ....................... 3 minutes.

 Fire tests:
Temperature. Linear Shr. Absorption. Porosity. Color.
 Per Cent. Per Cent. Per Cent.
 950"C. 0.47 16.16 34.80 Brick red.
 1050 0.4 16.64 34.40 Brick red.
 1150 1.6 15.80 33.50 Brick red.
 1190 2.4 16.00 27.75 Brick red.
 1230 2.9 15.49 23.75 Brick red.


 HIGHLANDS COUNTY
 Highlands County lies in the south-central part of the peninsula just northwest of Lake Okeechobee. The Alum Bluff formation underlies the central part of the county from Lake Childs northwestward and contains abundant coarse, red, sandy clay very suitable for sand-clay road material. The remainder of the county is underlain by sand and marls. No clays suitable for burned products are known.


 HILLSBOROUGH COUNTY

 Hillsborough County lies on the western side of the peninsula at the head of Tampa Bay. It is underlain by the Tampa, Alum Bluff and Bone Valley formations. The Bone Valley Gravel carries the Bartow clay which is founo in numerous localities from Plant City southward to the Manatee River. (For a description of the Bartow Clay see Polk County, page 195.)
 The Alum Bluff formation contains some phosphatic and sandy clays, but none having any usefulness for burned products.
 The Tampa formation consists of two distinct strata of light-green plastic clays separated by a limestone. The upper clay is exposed in numerous localities, while the lower one has very few, if any, natural exposures and is found chiefly in excavations and wells. Residual clays from the Tampa limestone are of common occurrence.