114 FLORIDA GEOLOGICAL SURVEY-TWELFTH ANNUAL REPORT 
dant, although many of the mollusks are preserved only as casts. 
The rock, as a rule, is granular and porous, although in places, by 
replacement, it has become close grained and compact. Masses 
and layers of flint, representing limestone replaced by silica, are of 
frequent occurrence in this formation. Aside from the flint 
masses, much of the rock consists of soft white limestone or marl. 
This formation is exposed to a limited extent on the Chipola River 
near Marianna, and also over a considerable area in central'penin¬ 
sular Florida. According to recent studies by Cushman, the thick¬ 
ness of the formation is very moderate, in. places not exceeding 
40 or 50 feet. 
OLIGOCENE. 
The Oligocene, as limited by recent investigations, includes in 
Florida only the Vicksburg, Chattahoochee and Tampa forma¬ 
tions. The Ocala formation, which had been placed by Dali and 
others in the Oligocene, has been placed in the Eocene upon the 
evidence of the molluscan fauna as determined by Cooke.* The 
Alum Bluff formation, on the other hand, formerly referred to the 
upper Oligocene, has been placed in recent years in the Miocene on 
the evidence of the vertebrates and invertebrates.! 
The Vicksburg formation consists of limestone lithologically 
not unlike those of the Ocala formation. Exposures of the Vicks¬ 
burg formation are found in Florida near Marianna and Chipley, 
in Jackson and Washington counties. The Chattahoochee and 
Tampa formations which apparently are, in part at least, contem¬ 
poraneous in deposition, constitute the upper part of the Oligocene 
as developed in Florida. The Chattahoochee formation is well 
exposed on the Chattahoochee River from the Georgia-Florida 
State line to Rock Bluff, a distance of about twelve miles. Another 
considerable belt of exposures of this formation is found extending 
from the Ocklocknee River to or somewhat east of the Suwannee 
*The age of the Ocala Limestone; U. S. Geological Survey, Prof. Paper 
95, pp. 107-117, 1915, by C. W. Cooke. 
fFossil vertebrates from Florida; a new Miocene fauna; new Pliocene 
species; the Pleistocene fauna; Florida State Geological Survey, 8th Ann. 
Rept., pp. 77-119, 5pls., 1916. by E. H. Sellards. Santo Domingo Type Sections 
and Fossils, Bulletins Amer. Paleontology Vol. 5, No. 30. Correlation table, 
1917. By Carlotta J. Maury. 
