.4 Contribution to the Geologic History of the Floridian Plateau. 165 



elevation of the sea-bottom. There were lagoons of fresh water and probably 

 streams emptying into the sea and in time of flood sweeping their fresh-water 

 population out onto the shoals, where it perished. Part of the bottom became 

 elevated nearly to the surface, oj'ster banks were formed on it, and the compacter 

 parts became water-worn. The absence of shells like Litorina and Nerita seems 

 to indicate that the dry beaches were muddy or sandy rather than rocky. In the 

 course of time elevation so shoaled the water that only species like Venus cancellata 

 and others able -to live between tide marks could remain. This portion of the for- 

 mation constitutes the so-called Venus cancellata bed, though neither of its compo- 

 nent species is peculiar to it. Finally the area became cut ofif almost entirely from 

 the sea and occupied more or less by fresh-water ponds in which the pond snails 

 multiplied in myriads. (U. S. Geol. Surv., Bull. 84, pp. 145, 146.) 



Concerning Venus (Chione) cancellata, Dall says in a preceding 

 paragraph: 



In this connection it may be stated that Chione cancellata is known from the 

 Chipola Old Miocene [Apalachicola Group] marls, in no respect differing from 

 recent specimens, and that it has continued as a conspicuous member of the Florida 

 fauna (except during the epoch when the Ecphora beds [Miocene] were being de- 

 posited) up to the present day. It is a warm-water shell and extended in abundance 

 farther north during Chipola times and the newer Pliocene than during the period 

 when the beds of the Chesapeake Group were being deposited or at present. The 

 last-mentioned periods were and are entirely relatively cooler, and the two former 

 relatively warmer, judging by the fauna. The species has never been entirely absent 

 and at the present day reaches as far north as Hatteras, in the warm-water area. 

 It is also a shallow-water shell, living chiefl}^ between tides when the climate 

 is mild enough. 



Corals are found fossil in the Caloosahatchee marl, although no 

 reefs are known. The genera comprise Dichoccenia, Meandrina {" Pec- 

 tinia"), Cyphastrea, Mceandra (M. areolata is abundant), Siderastrea, 

 etc. These genera indicate shallow water, a maximum of not over 25 or 

 30 feet, more probably not more than 10 or 15 feet, and a tropical tem- 

 perature, 70° F. as a minimum. The assemblage is that of an extensive 

 flat. The corals and mollusks both indicate the same physical conditions 

 prevalent over the areas in which the Caloosahatchee marl was deposited. 



In the area of the Nashua marl conditions nearly the same, but 

 with a slightly lower temperature, must have prevailed, for Chione can- 

 cellata was included in the collections of fossils from every locality except 

 one, and that was a small collection from the east side of St. John's 

 River, 7 miles below Sanford. The opinion may therefore be confidently 

 expressed that seaward, east and south of the Pliocene shore, the Florid- 

 ian Plateau continued as a shallow submarine bank, having practically 

 the same outline as the present Plateau. The cold water of Miocene 

 time had been diverted offshore, or had at least been replaced by warm 

 waters from the tropical regions, and on this bank arenaceous sediments 

 brought from the north entombed the calcareous remains of organisms 

 that lived on it. 



CONFORMITY OF THE PLIOCENE TO OTHER GEOLOGIC BOUNDARIES. 



The conformity of the western boundary of the Miocene sediments 

 to the outline established by the Oligocene, Vicksburg and Apalachicola, 

 formations, and to the present outline of the east and south coasts of 

 the State, is observed by the Pliocene deposits, leading to the inference 



