.4 Contribution to the Geologic History of the Floridian Plateau. 155 



SUMMARY OF EVENTS OF EARLIER STAGE. 



From the nature of the sedimentation and the faunal characters 

 the physical condition of the Floridian Plateau during this deposition 

 period may be reconstructed with considerable accuracy. 



(i) The Plateau had approximately the same outline as at present, 

 reaching from the northern boundary of the Chattahoochee, 

 which, as previously stated, extends from southwestern 

 Decatur County to the boundary of Burke and Screven 

 counties on the Savannah River, positively south of the 

 latitude of Tampa, and probably to the northern edge of 

 the Florida Strait. In the area now known as Marion County 

 there may have been a small island of Vicksburg rocks. 



(2) The depth of the water was probably at no place north of 



Tampa so great as 100 feet. 



(3) The temperature was tropical, the lowest for the year at least 



as high as 70° F. 



(4) As the temperature was tropical, the movement of the waters 



must have been from the tropics, by a direct or by a return 

 or countercurrent. 



(5) Terrigenous material was deposited over practically the whole 



submerged plateau surface. 



Later Stage. 

 SHORE-LINE. 



Conditions in the later stage of the Apalachicolan deposition had 

 changed considerably from those of the earlier. The physiography 

 of the region was different, and the approximate distribution of land 

 and sea should be determined at the beginning of this section of the 

 discussion. The sediments belonging to the Apalachicola Group subse- 

 quent in age to the Chattahoochee, Hawthorne, and Tampa formations 

 are referred to the Alum Blufif formation. The northern boundary of 

 the Alum Bluff extends from the higher summits in Decatur County 

 northeastward to the Savannah River in southern Screven County. 

 South of this line the sea extended beyond the base of the Florida Penin- 

 sula to an island with a north-and-south axis from Gainesville to Tampa, 

 and an east-and-west axis from Ocala to the west coast. This island, 

 here named Orange Island, may have extended farther westward and 

 comprised territory now beneath the waters of the Gulf. A submarine 

 platform extended southward of this island of Vicksburg and early 

 Apalachicola sediments. The evidence of the deposition of later Apalach- 

 icola sediments southward of Orange Island rests upon the discovery 

 by Matson and Clapp of fossils of the Alum Bluff horizon in the vicinity 

 of Ellenton, on the Manatee River, south of Tampa ^; and upon Dall's 

 previous record of fossils at White Beach, Little Sarasota Bay, of an 

 Oligocene horizon later than that of the Tampa localities.^ 



' Florida Geol. Surv., 2d Ann. Report, p. loi, igio. 



^ Wagner Free Inst. Sci., Trans., vol. iii, pt. vi, pp. 1 568-1 570. 



