A Contribution to the Geologic History of the Floridian Plateau. 161 



western extension; but the thickness of the Jacksonville formation, 400 

 to 500 feet, indicates progressive subsidence during a portion of Miocene 

 time. 



TEMPERATURE. 



Dall has given an excellent statement of the temperature of the 

 Miocene waters of the region (see quotation on page 156). He also says: 



As I have on various occasions insisted, the faunal gap between the upper- 

 most Oligocene (Oak Grove) ' and the Chesapeake or Miocene is the most sudden, 

 emphatic, and distinct in the whole post-Cretaceous history of our southeastern 

 Tertiary, and indicates physical changes in the surrounding region, if not in Florida 

 itself, sufficient to alter the course of ocean currents and wholly change the tem- 

 perature of the waters of our southern coast. (Wagner Free Inst. Sci., Trans., vol. 

 in. pt. VI, p. 1594, 1903.) 



Temperature conditions had within a relatively short time passed 

 from tropical to those of the latitude of Chesapeake Bav. or even the 

 southern coast of Cape Cod and Long Island. 



In an attempt to deduce the temperature of the Miocene waters of 

 Florida, the data presented in Sir John Murray's "On the Temperature 

 of the Floor of the Ocean and of the Surface Waters of the Ocean" ^ 

 have been used. As the Miocene fauna was one of shallow water, the 

 bottom temperature was probably not greatly different from that of the 

 surface. It may also be said that the minimum temperature of the winter 

 months is much more influential in determining the distribution of organ- 

 isms than the maximum temperature of the summer months. For in- 

 stance, according to Map 3 of the paper cited, a summer temperature 

 of 80° to go° F. extends from the Caribbean Sea and Gulf of Mexico 

 northward to New York Harbor, or during the summer a tropical tem- 

 perature extends far northward. 



In the winter conditions are very different. The minimum tempera- 

 ture for the west coast of Florida is between 60° and 70° F.; for south 

 Florida and the east coast, between 70° and 80° F. ; on the south side of 

 Cape Hatteras, 40° to 50° F.; north of Hatteras to Delaware Bay, 30° to 

 40° F. ; north of the last-named locality the temperature may be below 

 30° F. Therefore during the Miocene the minimum winter temperature 

 of the waters was at least as low as between 40° and 50° F. and it may 

 have been as low as between 30° and 40° F.; or between 20° and 30° F. 

 cooler than the present winter temperature of the west coast, and between 

 30° and 40° or even 50° cooler than the present winter temperature of 

 the east coast. 



CURRENTS. 



There was indisputably, as Dall has so often emphasized, a cold 

 current admitted along the shores of the land of embryonic Florida, 

 assuredly as far west as Pensacola Bay. This current could not have 

 been from the Equator, but must have been a southward flowing return 

 or countercurrent from the north ; and in my opinion this cotmtercurrent 



' The Shoal River marl, member of the Alum Bluff formation, has been sub- 

 sequently differentiated. (Vaughan, in Matson and Clapp, Preliminary Report 

 on the Geology of Florida, Florida Geol. Surv., 2d Ann. Report, pp. 104-106, 1910.) 

 * The Geograph. Jour., July, 1899. 



11 



