355 I'he Oligocene of the Southern United States 45 



The Oligocene Series in Detail in the Several States. 

 Florida. 



Vicksburg Group. Vicksbnrg limestone. Orbitoides limestone, 

 with charadleristic Vicksburg fossils, forms a large part of the 

 country rock of northern central Florida. The exadt distribution 

 is not yet definitely known, because the lithological differences of 

 the Vicksburg and later limestones are so trifling that a faunal 

 study is necessary to distinguish them. The rocks throughout 

 the state are also much obscured by the dense sub-tropical vege- 

 tation and b}' superficial sands. The distribution of the Vicks- 

 burg in Florida, based on recent investigation by Dr. W. H. 

 Dall* and others in that state, is sketched on the accompanying 

 map, opposite page 42. In 1850 and 1851, Prof. J. W. Bailey 

 describedf this rock as a white Orbitulite limestone observed be- 

 tween Palatka and Tampa. Recent investigations show that it 

 extends south nearly to the Hillsboro river. To the north it 

 occurs in the region of Gainesville and Ocala, but, at these local- 

 ities it is surrounded by extensive Chattahoochee deposits. At 

 Ocala, the Orbitoides limestone was bored into for 350 feet, and 

 its base not reached. At St. Augustine, as already noted, where 

 this rock lies 212 feet below the surface, borings passed through 

 1,066 feet of the limestone without reaching the base. 



Ivithologically, the Orbitoides limestone is a porous rock of 

 varying purity. It is usually rather soft when quarried, but 

 hardens on exposure to the air. Certain strata are often rendered 

 cherty, or even changed to pure flint, by the infiltration and depo- 

 sition of silex in solution. Others are transformed to gypsum by 

 the acftion on the calcium carbonate of water with sulphur, 

 derived from decaying vegetation, in solution. An analogous 

 chemical acftion results in the formation of phosphatic beds. The 

 later limestones in the state are also subje(5l to these modifications 

 to a greater or less degree, dependent on the purity and solubility 



* Loc, cit., pp. 156-7. 



^ Anier. Jour. Sci., 2nd ser., 1851, vol. 2, p. 86; Sinilhsonian Contr. 

 Knowl., 1S50, vol. 2, No. S, p. 19. 



