156 Papers from the Marine Biological Laboratory at Tortugas. 



SUWANEE STRAIT AND ORANGE ISLAND. 



Orange Island was separated from the mainland to the north by a 

 broad strait, the Suwanee Strait of Dall.' That marine conditions pre- 

 vailed across this area is proven by the occurrence of Alum Bluff species 

 of fossils at White Springs on the Suwanee River, where Matson and 

 Clapp obtained specimens of Ostrea rugifera Dall, Pododesma scopelus 

 Dall, and Pecten madisonius var. sayanits Dall.^ 



For the details of the Alum Bluff formation across northern Florida 

 the report of Matson and Clapp in the Second Annual Report of the 

 Florida Geological Siirvey may be consulted. 



DEFORMATION. 



It is evident from the preceding remarks that during Apalachicolan 

 time there was differential earth-movement in the Floridian region. As 

 all of the sediments were laid down in shallow water, the sea-bottom 

 must have been subsiding to receive the considerable thickness known to 

 be present in west Florida and through the Suwanee Strait; while the 

 area represented by Orange Island was a region of uplift. These changes 

 in physiography were accompanied by changes in sedimentation, in 

 climate, and in the fauna. 



TEMPERATURE. 



The basal bed of the Alum Bluff formation on the Apalachicola and 

 the Chipola rivers is a yellow clay marl, the Chipola marl member, replete 

 with excellently preserved fossils, indicating a tropical temperature. The 

 yellow color is noteworthy, as it is predominant in the older stage of the 

 Apalachicolan time. The climatic conditions have been the subject of 

 detailed consideration hy Dall.^ During the latter part of Apalachicolan 

 time the waters gradually cooled, ultimately becoming temperate. These 

 changes are most appropriately described by Dall, whose account is here 

 quoted: 



As indicated by the changes in the fauna, the physical changes attending 

 the close of the Oligocene were at first slow, allowing a certain element of transition 

 to appear in the Oak Grove or uppermost Oligocene fauna. At the last the)' appear 

 to have been sudden, at least the change in the fauna on the Gulf coast was abso- 

 lute and complete. The change W'as not only in the species and prevalent genera 

 of the fauna, but a change from a subtropical to a cool temperate association of 

 animals. Previously, since the beginning of the Eocene, on the Gulf coast the 

 assemblage of genera in the successive faunas uniformly indicates a warm or sub- 

 tropical temperature of water, and the sediments uniformly show, from the Jack- 

 sonian upward, a yellowish tinge due to oxidation. In the Oak Grove sands come 

 the first indications of a change towards the gra)' of the Miocene marls. With the 

 incursion of the colder water the change becomes complete. Not only do northern 

 animals compose the fauna, but the southern ones are driven out, some of them 

 surviving in the Antilles to return later. Some change along the northern coast 

 permitted an inshore cold current to penetrate the Gulf, depositing on the floor of 

 the shoal Suwanee Strait, separating the island of Florida from the continental 

 shore, a thin series of Miocene sediments, which were also carried as far south as 

 Lake Worth on the east coast of Florida and Tampa on the west coast, as shown 

 by artesian borings. (Op. cit., pp. 1549, 1550.) 



' U. S. Geol. Surv., Bull. 84, p. iii, i8g2. 



^ Florida Geol. Surv., 2d Ann. Report, p. 100. 



'' Wagner Free Inst. Sci., Trans., vol. in, pt. 11, pp. 1574-1575, 1903. 



