GEOLOGY OF FLORIDA 297 



counties; nor the Bone Valley formation, which rests upon the 

 Miocene of Polk and Hillsboro counties. The alluvial Pleistocene 

 deposits are likewise omitted from the map, and also the limited 

 exposures of Pliocene ( ?) on the St. Marys River. 



STRUCTURAL CONDITIONS IN THE FLORIDIAN PLATEAU 

 * THE RELATION OF THE FL0RH)A PENINSULA TO THE COASTAL PLAIN 



An unusual structural feature in coastal plains geology is the 

 great Floridian plateau which, projecting from the mainland, 

 separates the Atlantic Ocean from the Gulf of Mexico. The times 

 of origin of this plateau may not be determinable, although it is 

 now known to have been in existence and to have formed a large 

 shallow^- water area as early at least as the Lower Cretaceous or 

 Comanchean period. Although affected by diastrophic agencies, 

 including elevation and depression, this plateau has continued as a 

 structural feature from the Comanchean or earlier to the present 

 time. 



For some years the Florida Geological Survey has been collecting 

 well logs and well cuttings with a view to determining so far as 

 practicable the structural conditions underlying peninsular Florida. 

 This work has progressed slowly, although some data have now 

 accumulated that have a very important bearing on this problem. 

 Early in 19 18 cuttings containing numerous foraminifera from a 

 deep well then recently completed in Sumter County were sent to 

 Dr. T. W. Vaughan, of the United States Geological Survey, by 

 whom they were referred to Dr. J. A. Cushman. Dr. Cushman's 

 identification of the foraminifera of these samples led to the very 

 unexpected result that they indicated the presence of Comanchean 

 (Lower Cretaceous) formations. Subsequently the foraminifera 

 were identified by Dr. Cushman from the cuttings of about fifteen 

 wells in Florida. In addition there are a number of other wells 

 that from logs or samples give approximate data. 



SKETCH MAP OF STRUCTURAL CONDITIONS IN FLORIDA 



Upon the basis of all data available at this time there has been 

 constructed a sketch map (Fig. i) which represents approximately 

 what is known of structural conditions in the peninsular section of 



