A PRELIMINARY REPORT ON CLAYS OF FLORIDA 133

 COLUMBIA COUNTY
 Columbia County lies in the northeastern part of the State. The Alum Bluff formation is exposed at the surface.in the northern twothirds of the county and the Ocala formation in the southern one-third. Sandy surface clays of uncertain age cover a large part of the county. In some localities this material is desirable for sand-clay road material. South of the region about Lake City no clays of value are known as is also the case in the north and northeastern parts of the county. About six miles northeast of White Springs a very soft white shale is exposed on the Hamilton County side of the Suwannee River. For a detailed description of this exposure see page 165. This clay no doubt underlies adjacent portions of Columbia County as well.
 A green jointed clay overlain by four feet of brown, gray and white mottled sandy clay is exposed about two miles south of Lake City on the Gainesville road. The contact between these two clays is indistinct and the upper portion is probably a residual mantle. The thickness of the lower clay is unknown. It has a high air shrinkage and cracks badly in drying and is therefore unsuited for use in manufactured products.
 The region between Lake City and White Springs is underlain by one or more beds of a greenish jointed clay. These beds are variable in thickness and are interbedded with sands and sandy clays. In places this clay contains calcareous concretions and in other places it contains some phosphatic material, it has a high air shrinkage, warps and cracks badly, and its plasticity is too low to wclrk satisfactorily in a stiff mud machine. For a typical section of this clay see page 114.
 At a brick plant formerly operated about three miles southwest of Lake City a gray sandy clay occurs and was formerly used for common brick. Its low plasticity, low bonding strength and poor working qualities make it undesirable for any kind of manufactured clay product. It is still distinctly soft and porous at cone 15. It has the following physical properties:
 Physical Properties of Lake City Brick Yard No. r Clay (Lab. No. o-36).
 Plasticity, judged by feel............ Poor.
 W ater of plasticity ................ 17.45%
 Pore water ........................ 0.23%
 Shrinkage water ................... 17.22%
 Linear air shrinkage................ 3.6%
 Volume air shrinkage............... 7.92%
 Modulus of rupture, average........ 121.0 pounds per square inch.
 Slaking test ....................... 30 hours.