24 FLORIDA GEOLOGICAL SURVEY-THIRD ANNUAL REPORT.
the phosphate are ordinarily of frequent occurrence both in pits and in prospect holes.
MATERIALS LYING ABOVE THE PHOSPHATE.
A superficial deposit of pale yellow incoherent sand occurs generally throughout the phosphate section. The thickness of this sand varies exceedingly. Five to fifteen feet may be given as an average as seen in the pits, although a thickness of as much as thirty feet has been observed. The character and manner of occurrence of these sands leads the writer to the belief that they may be residual in origin.
These incoherent sands rest in some localities upon a red clayey sand stratum known to the miners as "hardpan." This
sand stratum contains sufficient clay to give it coherence and stands usually as a vertical wall in mining. This stratum is frequently absent, and when present varies greatly in thickness. The top surface. of this red sand stratum presents irregularities which might be taken to mark an unconformity between this formation and the incoherent sands above. Such irregularities as occur in the top surface, however, present rounded depression rather than sharp irregularities. Moreover the top surface of the red sands frequently conforms to the surface contour. Both the superficial sands and the red sands are, as far as the writer has observed, non-fossiliferous.
RELATION OF THE PHOSPHATE-BEARING FORMATION TO THE UNDERLYING FORMATIONS.
The phosphate-bearing formation rests in this section, wherever observed, upon the Vicksburg limestones. In the northern part of the section the pits are ordinarily worked out to the limestone. affording favorable opportunity for observing the contact. The top surface of the limestone is strikingly irregular, the rock projecting as rounded peaks. The numerous shells and other invertebrate fossils of which the limestone is largely made up are eroded off plane with the surface of the limestone. Passing to the south the limestone lies as a rule at a greater distance beneath the surface, and frequently is not reached by the ordinary processes of mining. It is occasionally reached, however, and wherever seen, throughout this entire section the relation between the phosphate formation and the limestone is the same, that is, the phosphate bearing formation lies upon and ,fills up irregularities in the top sur.face of the limestone. (P1. 2, Fig. I and P1. 5)