34 FLORIDA GEOLOGICAL SURVEY-THIRD ANNUAL REPORT..
absent, the phosphate lying at the surface. Usually the sand contains sufficient admixture of clay to give it coherence. Under these conditions it oxidizes red near the surface. While this is the prevailing phase of the sand it is nevertheless subject to considerable variation from place to place. Not infrequently the sand is firmly cemented forming the so called "hardpan" which gives much trouble in prospecting and frequently necessitates blasting in mining. (Plate 3, Fig. 2.) In places the sand has a calcareous or phosphatic cement. Locally it varies also to an indurated rock with innumerable small cavities which gives a vesicular appearance to the mass. A sample of this rock was found to contain 15.56% phosphoric acid (equivalent to 33.97% tri-calcium phosphate).
The phosphate bearing member contains vertebrate remains including both marine and land animals. Most of the bones are more or less rolled and water worn although occasional whole skeletons are found. In the sands above the phosphate, fossils are rare. The writer has obtained, however, through the kindness of Mr. M. A. Waldo, Manager of the Dominion Phosphate Company a single tooth of the mastodon preserved as a cast in the phosphatic sands of the overburden. Aside from a few casts near the bottom of the phosphate bed invertebrates have not been found in this formation.
MATERIALS LYING ABOVE THE PHOSPHATE FORMATION.
As in the case of the hard rock section the surface material consists of incoherent pale yellow sand. The depth of this sand is variable, ranging from four to ten or more feet. A very definite and often irregular line separates these loose sands from the formation beneath. (P1. 3, Figs. I and 2, and P1. 4, Fig. 3.) This line Matson interprets as an evident unconformity.* This may be true although the fact must not be overlooked that seeming unconfoimities in materials lying near the surface may in reality represent only lines of decay. The writer is inclined to regard the loose surface sands in this section as residual, the irregular line representing the line of complete disintegration of the original sandy formation. A similar explanation has been offered previously by the writer for the surface sands of Gadsden Countyt as well as for the sands overlying the hard rock phosphate formation. (ante P. 24.)
*Florida Geol Survey. Second Annual Report, p. 139, 1909.
tFlorida Geol. Survey. Second Annual Report, p. 263, I9O9.