160 Papers from the Marine Biological Laboratory at Tortugas. 



the deposits have been partially removed by subsequent erosion and their present 

 extent is often obscured by considerable thickness of younger beds. (Florida 

 Geol. Surv., 2d Ann. Report, pp. 165, 166, 1910.) 



The question as to whether or no the Miocene sea extended through 

 the Suwanee Strait must for the present remain without a definite answer. 

 The available evidence now permits only the statement that the Strait 

 may have again had ocean currents flowing through it. Except in the 

 Suwanee Strait region, the Miocene shore may be outlined with consider- 

 able definiteness. In west Florida it reached from a short distance north 

 of the present Apalachicola-Miocene boundary, probably not extending 

 to the southern limits of Georgia, into Alabama in the vicinity of 

 Mobile. The problematic condition of the Suwanee Strait has been fully 

 discussed; it is not positively known whether the Strait was open, or 

 whether there was a short peninsula bounded on the east by a shore-line 

 just west of Trail Ridge and projecting as far south as Tampa. Doctor 

 Dall has at my request contributed the following remarks to this 

 discussion: 



In the absence of evidence which would conclusively prove the post-Oligo- 

 cene existence of the Suwanee Strait, one consideration had much weight with me 

 in assuming it as highly probable. This is connected with the presence of the phos- 

 phate beds in the central peninsular region of Florida. There is practically no 

 doubt as to the origin of these beds from the presence in Miocene and perhaps later 

 times of immense rookeries of birds, and perhaps other animals, whose guano was 

 absorbed by the porous limestone underlying their chosen habitat. Now experi- 

 ence shows that such rookeries are invariably separated from possible incursions 

 of carnivorous continental enemies by impassable bodies of water. Otherwise the 

 birds could not maintain themselves, and the occupation of their rookeries for a 

 period, such as was necessary for the formation of the phosphatic deposits, would 

 have been impossible. The erosion of shallow beds of Miocene age under condi- 

 tions which have existed in Florida, over part of the area of the supposed strait, 

 is not an exceptional or remarkable phenomenon; and it is quite possible that 

 more exhaustive exploration than has yet been possible may reveal traces of the 

 missing Miocene sediments. 



In Georgia the shore of the Miocene sea lay somewhat west of a line 

 passing through Doctortown, on the Altamaha River, and Porter's Land- 

 ing, on the Savannah River, but it did not entirely overlap the Apalachi- 

 cola sediments. All of Florida excepting the land areas indicated was 

 submerged. 



DEPTH OF WATER. 



The depth of the sea is shown by both the kind of sediments and the 

 fauna. The sediments are near-shore, shallow-water deposits, and are 

 predominantly terrigenous, although there is some lime in the Jackson- 

 ville formation, probably chemically precipitated, and lime of organic 

 origin, the calcareous remains of fossils. The fossils, comprising such 

 genera as Ostrea, etc., indicate shallow-water conditions. The Floridian 

 Plateau extended to the southern margin of the keys, as shown by 

 the deep well at Marathon, Key Vaca. Probably at no place over the 

 platform did the depth exceed a few fathoms, 25 or 30 seems a safe 

 maximum. The maximum thickness of the Choctawhatchee formation, 

 50 feet, demands no continuous depression of the sea-bottom along the 



