740 INDEX TO THE STRATIGKAPHY OF NORTH AMERICA. 



In Florida the Vicksburg group is tentatively subdivided into three formations — the 

 Marianna, "Peninsular," and Ocala limestones. The Marianna limestone occurs in Florida 

 mostly west of Apalachicola River; the "Peninsular" limestone constitutes that portion of the 

 Vicksburg group underlying the Ocala limestone in Peninsular Florida and is largely con- 

 temporaneous with the Marianna limestone. Matson and Clapp^^"* give the following 

 description : 



"The Marianna limestone is thought to be the stratigraphic equivalent of the upper part 

 of the bluff at Vicksburg, Miss., and some of the wells in west Florida enter beds of sand and 

 clay which probably represent older horizons, but the stratigraphic relation of the formation 

 to these older beds can not be determined. In west Florida, where this formation is recognized, 

 it is unconformably overlain by beds belonging to the Apalachicola group or by post-Pliocene 

 formations. 



"The Marianna and 'Peninsular' formations consist of soft, porous white or light-gray 

 limestone, sometimes resembling marl, especially when partly decomposed. Some bands of 

 darker-colored dense limestone are reported at various locahties where these formations have 

 been penetrated by drilling, and nodules and layers of chert are common throughout them, 

 but chert beds are especially prominent in certain horizons. The chert beds are usually darker 

 in color than the limestone and range in thickness from a fraction of an inch to 12 or 15 feet. 

 In some localities as many as six or seven successive beds of chert have been encountered in a 

 single well. In general the heavier layers are more persistent, and it is usually the chert which 

 forms a nearly water-tight cap to the artesian-water beds in these formations. Certain horizons 

 are abundantly fossiliferous, containing innumerable specimens of Orbitoides and shells of 

 mollusks, such as Pecten poulsoni, etc. At several locahties the rock is so soft that it can be 

 cut into blocks with a saw, and upon exposure to the weather these blocks harden rapidly, 

 making a very fair quality of building stone. Beds of sand, sometimes 10 feet or more in 

 thickness, are reported in some of the wells which penetrate this formation. In general, these 

 sands appear to be most numerous in the northwestern part of the State. 



"The thickness of the 'Peninsular' limestone and the Marianna limestone appears to be 

 exceedingly variable. The thickness given by Foerste, from his investigations of the Vicks- 

 burgian hmestones in southwestern Georgia and the adjacent part of Florida, is 220 feet, and 

 probably this may be regarded as the approximate measure of the thickness of the Marianna. 

 On the basis of well borings Dr. Dall estimates the thickness to be over 350 feet at Gainesville, 

 212 feet at Lake Worth, and 1,068 feet at St. Augustine. From recent examinations of well 

 borings by Drs. Vaughan and Bassler, limestone of Vicksburg age is known to have a thickness 

 of over 225 feet at Quincy, 250 feet at Alachua, and 325 feet at Bartow. Apparently there is 

 a marked thickening of these limestones from the exposures of Georgia and Alabama south- 

 ward. Of all the estimates given above the one at Gainesville is probably the most reliable, 

 because the well is cased to the bottom." 



Rocks belonging to the Vicksburg group occur in deep wells on Key Vaca and Key West. 

 The Ocala is described by Matson and Clapp ^^°^ as follows: 



"The Ocala limestone hes stratigraphically between the 'Peninsular' and the beds here 

 designated Apalachicola group. LithologicaUy it bears a strong resemblance to the underlying 

 'Peninsular' limestone, with which it also has a close faunal relation. These facts have led to 

 the conclusion that the two formations are conformable, and it has also been suspected that the 

 Ocala limestone represented a local phase of the 'Peninsular.' While the two formations are 

 probably conformable, the extensive distribution of nummuUtes of the Ocala limestone shows 

 that it represents a widespread change in conditions and is not to be classed as a mere local 

 phase of the underlying beds. 



"The Ocala limestone, as already noted by Johnson, is sometimes wanting, so that the 

 overlying formations rest directly upon the 'Peninsular.' This relation was noted at several 

 localities which will be mentioned in discussing the younger formations. At present it is 

 sufficient to note its absence and to suggest that since the Ocala limestone does not appear to 

 be a local phase of the 'Peninsular,' there is probably a stratigraphic break between the rocks 

 belonging to the Vicksburg and Apalachicola groups in the central part of the peninsula. 



