PEBBLE PHOSPHATE DEPOSITS. 
53 
come compact and close grained, probably in the form of small 
boulders which are broken up in drilling. Boulders of this type may 
be either calcareous or flinty. While fossils are not abundant it is 
probable that this phosphatic marl is of Upper Oligocene age. The 
marl is with little doubt the parent formation from which by re¬ 
working and concentration the pebble phosphate conglomerate of 
the Bone Valley formation was formed. 
3. Beneath the phosphatic marl is found limestone in which all 
the wells terminate. At Tiger Bay the first 50 feet of this limestone, 
360 to 410 feet from the surface, consists largely of a mass of small 
pieces having the appearance of broken fragments of fossils. Below 
410 feet the prevailing phase is a light colored limestone, often gran¬ 
ular in appearance, containing many foraminifera. Occasional 
strata are found which are hard and compact and usually of a brown¬ 
ish color, the material being finely powdered by the drill. Locally 
also flint masses occur which represent silicified limestone and may 
or may not be encountered in any particular well. Locally also the 
limestone is partially crystallized, this phase being due to solu¬ 
tion and redeposition by underground water. At Christina the 
limestone beneath the marl is reached at a depth of about 120 feet, 
and foraminifera become abundant at about 320 feet. At Fort 
Myers the non-phosphatic limestones are reached at about 700 feet. 
The rock in which foraminifera are abundant is believed to repre¬ 
sent the Ocala limestone and it may be that all of the. non-phosphatic 
limestones beneath the phosphatic marl are of that formation. 
The generalized sketch (fig. 4) has been drawn to illustrate 
the geologic structure as found on a line through the State from 
north to south passing through the pebble and hard rock phosphate 
section. The relation of the phosphate deposits to the underlying 
formations is well shown by this sketch. The foundation rock is 
the Ocala limestone which comes to the surface over a limited area 
in the central peninsula section. The hard rock phosphate deposit 
rests directly upon this limestone. To the south and also to the 
north the limestone dips below the surface and disappears beneath 
the later formations. The upper Oligocene formations lie upon the 
Ocala limestone in extreme northern and southern Florida. The 
Bone Valley formation, including the land pebble phosphate, in turn 
rests upon these upper Oligocene deposits. In extreme southern 
Florida Pliocene and Pleistocene marls and sands form the surface 
materials. 
