GEOLOGY AND PALEONTOLOGY OP THE CANAL ZONE. 267 



of the Ocala is below sea level and is overlain by younger forma- 

 tions, while along the axis of the dome its surface rises from 80 to 

 a little more than 100 feet above sea level. This low dome formed 

 in the upper Oliogocene sea an island or a group of islands to which 

 I have applied the name "Orange Island." The Chattahoochee 

 and Tampa formations were deposited on the western slope of this 

 island but they are not known in central Florida. The subsidence 

 which brought about the deposition of these two formations con- 

 tinued until the Alum Bluff sea advanced entirely across central 

 Florida, where deposits of Alum Bluff age rest on the surface of the 

 Ocala limestone apparently without the intervention of deposits ot 

 intermediate age. 



The portion of the Alum Bluff formation above its basal member 

 contains in central Florida at numerous localities heads of corals of 

 reef facies belonging to the genus Siderastrea. At a place near 

 Nigger Sink, about 8 miles north of Alachua, Florida, there is a 

 Siderastrea reef, which, according to aneroid barometer measurement, 

 is about 35 feet thick. The sediments associated with the Alum 

 Bluff reef corals are greenish, usually phosphatic sands and clays, and 

 impure phosphatic, in places magnesian limestone. The corals are 

 decidedly subordinate in importance to other constructional agents, 

 although they grew on a subsiding basement. 



Alum Bluff sedimentation was succeeded by uplift and subaerial 

 erosion preceding the depression initiating the deposition of the 

 Choctawhatchee Miocene. Although the Miocene Choctawhatchee 

 and Chesapeake faunas comprise about a dozen species of coials of 

 distinctive facies, no reef corals are known as the temperature of the 

 water was evidently too low. 



No Pliocene coral reefs are known, but corals of reef facies are well 

 represented in the Caloosahatchee marl, which is largely composed of 

 molluscan shells. The stratigraphic relation of the Caloosahatchee 

 marl to the Miocene has not been definitely ascertained, but available 

 evidence suggests separation by an erosion unconformity. Whatever 

 this relation may be, the formation was deposited during subsidence. 

 Corals are of slight importance as contributors of material to the 

 formation, as Heilprin long ago pointed out. 



The following table, which is a slightly revised copy of a table 

 previously published, 1 shows the stratigraphic distribution of coral 

 reefs and reef corals from Oligocene to Recent time, and their rela- 

 tion to changing sea level. 



1 Vaughan, T. W., and Shaw, E. W., Geologic investigations of the Florida coral reef tract, Carnegie 

 Inst. Washington Yearbook No. 14, p. 238, 1916. 



