150 Papers from the Marine Biological Laboratory at Tortugas. 



GEOLOGIC HISTORY OF THE FLORIDIAN PLATEAU. 



EVENTS OF VICKSBURGIAN TIME. 

 THE VICKSBURGIAN SUBMERGENCE. 



During Vicksburgian time remarkable uniformity of marine condi- 

 tions prevailed throughout an extensive area of what are now the south- 

 ern United States, from central Louisiana, across Mississippi, Alabama, 

 and Georgia, and similar sedimentation was also taking place on the 

 Floridian Plateau, practically to its southern extremity, should the 

 material from the deep well at Key West, studied by Hovey, be trust- 

 worthy. A well record from Palm Beach, given on page 127, shows 

 that Vicksburg rocks were there encountered between 915 to 1,000 feet 

 below the surface. At Key West, according to Hovey,' Orbitoides first 

 appears in abundance at a depth of 900 feet. 



DEPTH AND TEMPERATURE OF THE WATERS. 



It is important to determine the approximate depth of the Vicks- 

 burgian Sea ; as it is the oldest geologic formation known on the Floridian 

 Plateau, light will be thrown on the age of the Plateau. The formation 

 of an opinion on this subject may be made possible by data from two 

 sources: (i) the material composing the sediments; (2) the character 

 of the fauna. 



In Florida, the Vicksburg Group has been tentatively divided into 

 three formations, as follows: in west Florida, the Marianna limestone; 

 on the Peninsula, "Peninsular" and Ocala limestones. Recent investi- 

 gations in Georgia render it probable that only one formation should be 

 recognized, for in that State the Ocala can be definitely identified, and 

 no demarcation of the Marianna or "Peninsular" at present seems pos- 

 sible. In this discussion, which is an account of physical events, the 

 "Peninsular" and Ocala limestones are spoken of collectively by the 

 group-name Vicksburg. The Vicksburg limestones are predominatingly 

 calcareous, as the terminology suggests, but they are not pure, consider- 

 able proportions of both clay and silica being present. Matson makes 

 the following statement: 



These beds are uniformly fine-grained and show httle variation in chemical 

 composition. There is a predominance of limestone, though sand and clay occur 

 in small quantities, and the percentage of these impurities in the limestone increases 

 in the upper beds of this age. There is also an increased percentage of terrigenous 

 material toward the northern end of the State, where the proximity of older land 

 afforded opportunity for the entrance of considerable sand and mud into the Vicks- 

 burg sea. Toward the close of this period of deposition there appears to have been 

 a shoaling of the seas which permitted the entrance of the fresh-water shells and 

 the land-derived sediments noticeable in the Ocala limestone of the Vicksburg 

 Group. The excellent state of preservation of many of these shells shows that the 



' Mus. Comp. Zool., Bull., vol. xxviii, p. 67, 1896. 



