REVIEW OF THE GEOLOGY OF FLORIDA 115 
River, and from the Georgia-Florida State line to the Gulf border. 
The formation as exposed on the Apalachicola River includes im¬ 
pure clayey limestones. A rather harder and perhaps more nearly 
pure limestone phase of the formation shows between the Ock- 
locknee and Suwannee Rivers. The Tampa formation is exposed 
on the Hillsboro River and on the Manatee River near Tampa. It 
is likewise a limestone varying in hardness and in purity. The 
thickness of the Oligocene in Florida is difficult to determine, since 
there are no surface exposures that afford a measurement of the 
combined thickness of the formations. The evidence from well 
records as to the thickness of these beds is at present too indefinite 
to be of service. 
MIOCENE. 
The Miocene of Florida includes the Alum Bluff, Jacksonville 
and Choctawhatchee formations. The Alum Bluff formation, for¬ 
merly referred to the Oligocene, as already noted has been placed 
in the Miocene on the evidence of the vertebrate and invertebrate 
fossils. The materials of this formation include calcareous sands 
and sandstones varying to sandy limestones, calcareous clays,, 
fullers earth clays and sands. The conditions under which the 
formation was deposited were evidently shallow water often in the 
presence of conflicting currents. This is especially true of the 
upper part of the formation, in which cross-bedding is not uncom¬ 
mon. Fossil plants are found in this formation at the type locality 
at Alum Bluff. At the fullers earth mines in Gadsden county 
there is found a limited, although extremely interesting land ver¬ 
tebrate fauna, associated with a shallow water invertebrate fauna. 
Farther to the west, on the Choctawhatchee and Yellow Rivers, the 
formation is more distinctly marine and contains an abundant 
marine invertebrate fauna. In the southern part of the State the 
deposits which are believed to represent the equivalent of the Alum 
Bluff formation are distinctly phosphatic. As there developed, 
the formation consists chiefly of marl beds in which is included 
black, brown and white phosphate pebble. Deposits found on 
Black Creek in Clay county and referred to the Jacksonville for¬ 
mation are lithologically very similar to the Alum Bluff formation 
as developed in South Florida, and contain similar phosphate 
pebble. 
