16 Sellards — Comanchean Formation. 



From this central area they dip slowly to the east, lying 

 at only a moderate depth on the Atlantic Coast at St. 

 Augustine. To the northeast they dip more rapidly as 

 indicated by the well at Jacksonville, while to the south 

 from the central part of the peninsular they likewise dip 

 very appreciably. 



In a paper published in the Twelfth Annual Eeport of 

 the Florida Survey the writer has sought to use the 

 Eocene formations as an index to structural conditions in 

 peninsular Florida. In this paper the data on the Eocene 

 obtained by Cushman in connection with his study of the 

 well samples is supplemented by data from a number of 

 other wells of which records and samples have been 

 obtained from time to time. From these data it is shown 

 that there is a large belt extending entirely across north 

 Central Peninsular Florida in which the Eocene forma- 

 tions lie either above sea-level (west side of the penin- 

 sula) or at from 10.0 to 200 feet below sea-level (east 

 side of the peninsula). To the north the Eocene forma- 

 tions apparently dip very slightly ; while to the east the 

 dip is somewhat greater. To the northeast as indicated 

 by the wells at Jacksonville the dip is pronounced. Like- 

 wise in passing to the south and southeast from north- 

 central peninsular Florida the dip in the formations is 

 quite appreciable. As early in 1881, Dr. E. A. Smith 

 indicated in a paper and sketch map published in this 

 Journal, that approximately the western half of the Flor- 

 idian land mass is submerged to a shallow depth below 

 sea-level. 2 This conclusion has been supported by subse- 

 quent studies and in addition there has been gradually 

 developed the knowledge of a more complicated structure 

 involving recognition of a broad dome centered toward 

 the west side of the north central part of the peninsula. 

 The anticlinal structure of the Floridian peninsula as a 

 whole has likewise been long recognized, and the structure 

 here referred to may possibly be characterized as a slight 

 doming in the larger structure. The abruptly terminat- 

 ing margins of the Florida land mass, at or near the 100- 

 fathom line in the Atlantic Ocean and in the Gulf of 

 Mexico, suggest faulting along these lines by which the 

 land mass has been lifted as a block as well as folded as a 

 large geo-anticline. 



2 Vol. 21, pp. 292-309. 1881. 



University of Texas, 

 Austin, Texas. 



