.4 Contribution to the Geologic History of the Floridian Plateau. 129 



The matrix of these fossils is light olive-green quartz sand with 

 some calcareous material. A few oolitic granules are present. Similar 

 material continues to 640 feet, becoming coarser at the lower levels. 

 Between 620 and 640 feet there are quartz pebbles 0.375 inch long. 



Depth 640 to 660 feet: Orbitolites complanata d'Orb.; Stylophora sp.; 

 Porites sp. 



Geologic Horizon: Apalachicolan Oligocene. The matrix is a whitish 

 limestone in which are small cavities. 



Depth 680 to 700 feet: Pecten fragments, probably P. perplanus 

 Morton. 



Geologic Horizon: Not definitely determinable, but Vicksburgian 

 Oligocene is suggested. 



A record of the Buck Key well (of W. H. Knowles) given from 

 memory by the driller, James Sykes, supplemented by samples saved 

 at odd depths, furnishes the following section: 



Description. 



Feet. 



Sand and shells 



Brown crystalline limestone with cherty streaks and sand grains 



White quartz sand, with marl and shell fragments 



Brownish sandy limestone with shell fragments 



Dark greenish marl 



White quartz sand, with shell bed at i 50 feet 



Medium dark greenish marly sand, with shell beds, and streaks of lighter marl 



White to brownish, and soft to hard, limestone, with a few shell casts; hard brownish 

 limestone contains many siliceous grains 



490 60s 



The correlation of the geologic formations penetrated in these wells 

 is a difficult matter, but we know sand is abundant below 155 feet in 

 depth on Key Vaca and we may be confident that Pliocene and Miocene 

 sands extend as far southward as that key. The quantity of siliceous 

 material contributed to southern Florida appears to have reached its 

 maximum in the Miocene period and since then to have diminished inter- 

 ruptedly. The Pleistocene limestones of the mainland rest on an 

 arenaceous foundation. 



The presence of Miocene sands as far south as Key Vaca possesses 

 a geologic interest in that they indicate that the great Floridian plat- 

 form existed in Miocene times, and that sand which must have come 

 from the north, as no southern source is known, was being carried to 

 that region during that period. 



Silica derived from sponge spicules and diatoms is universally 

 present in the near-shore marine deposits, but not in sufficient quantity 

 to form of itself important deposits. 



Calcium Carbonate. 

 The origin of the material of the calcareous deposits presents a 



more complicated problem than that of the siliceous, 

 inorganic and organic. 

 9 



Its source is both 



