353 I'he Ougocene of the Southern United States 43 



Grand Gulf sandstones. The Grand Gulf sandstones first 

 appear in southern Alabama near the center of the state along the 

 Florida line. The}' widen rapidly beyond the western limits of 

 Florida and extend to the Mississippi, forming the bluff at Grand 

 Gulf. Beyond the Mississippi they reappear in Louisiana, at 

 Sicily Island, and again near Harrisonburg. Crossing the state 

 from that locality in a southwesterly direcftion, the sandstones 

 enter Texas and extend certainly as far as the Brazos river. 



Frio clays. The Frio clays are charadleristic of Texas, but 

 the)^ extend eastward into southwestern Louisiana where they 

 thin out and disappear. 



Conditions of Deposition. 



Physical Geography during the Vicksbiirg period. While the 

 deposition of the Vicksburg limestone was taking place over a 

 wide area of the sea-floor, the waters of the Gulf of Mexico and 

 of the Pacific Ocean were probably confluent across the Isthmian 

 region, as Mr. Alexander Agassiz has shown. Florida was at 

 first entirel}' submerged. At the close of the period, islands near 

 Ocala were upraised, as Dr. Dall has proved by the presence of 

 land shells in the Ocala limestone. The southwestern extremity 

 of the continent at the beginning of the Vicksburg period was, 

 presumably, near the present sites of Macon and Augusta. 



DeJieSlion of the equatorial current. At the close of the 

 Eocene, an elevation of the Isthmian region took place which cut 

 off the Gulf of Mexico from the Pacific Ocean. The equatorial 

 current, that had formerly swept across into the Pacific, was de- 

 fle(5led to the north and northeast, along the shores of the continent. 

 The shore line was upraised, exposing a portion of the Vicksburg 

 limestone. Florida appeared as a chain of islands. The return 

 current, as Dr. Dall has shown, passed between these islands and 

 the shores of the continent through a broad strait for which Dr. 

 Foerste proposes the name of Okefenokee,* as it occupied, in part, 

 the region of Okefenokee swamp between southern Georgia and 



*Chipola Miocene, Amer. Jour. Sci., 3rd ser., vol. 46, 1893, p. 245. 



