104 JV. H. Darton — Geology of Florida phosphate deposits. 



pebbles and along the larger streams there are many thick and 

 extensive pebble deposits. Peace Creek drains the Bartow- 

 Fort Meade conglomerate region and flows over many great 

 placer deposits, some of which are now extensively worked in 

 De Soto connty. The Withlacoochee, near Dunellon, and 

 Alifia Creek northeast of Bartow also contain extensive accum- 

 ulations of pebbles. These deposits consist of rock-phosphate 

 pebbles usually from an inch to one-quarter inch in diameter, 

 mixed with more or less sand and usually with bone fragments 

 and occasional flint pebbles from the limestones. 



Age and geologic history. — The three geologic formations to 

 which the phosphates belong are distinctly separate stratigraph- 

 ically, and represent a long interval of geologic time. The 

 rock phosphates appear to be the deeply eroded remnants of 

 the phosphatized surface of the middle Tertiary limestone ; the 

 conglomerate .deposits overlie these limestones unconform- 

 ably and in the Gainesville region at least, appear to be Mio- 

 cene in age, and the river drift deposits are apparently en- 

 tirely subsequent to the great mantle of Pleistocene white and 

 gray sands which covers the entire peninsula to a greater or 

 less depth. 



Excepting in its light color the rock phosphate is a physi- 

 cal counterpart of the brown limonite iron ores of the Appala- 

 chian limestone valleys and the deposits have very similar 

 structural relations. I have found at a number of localities 

 that the massive phosphate graduates into the limestone 

 usually by short transitions and many areas were discovered 

 in the phosphate belt and under the conglomerate in the Bar- 

 tow region where the limestone is only partially phosphatized. 

 In the mines at Dunellon the massive phosphate is apparently 

 continuous with the limestones, but unfortunately at the time 

 of my visit there were no continuous exposures from rich 

 phosphate to the walls of the basin, and the bottom was not yet 

 reached, so I was unable to establish a graduating sequence at 

 that locality. There are, however, in the massive phosphate, 

 occasional casts and impressions of the same middle Tertiary 

 mollusca undoubtedly lying as they were originally deposited. 



The origin of the phosphate of lime is not definitely known, 

 but it seems exceedingly probable that guano was the original 

 source and the genesis of the deposits similar to that of the 

 phosphates on some of the West Indies. Two processes of de- 

 position have taken place, one the more or less complete re- 

 placement of the carbonate of lime by phosphate of lime, and 

 the other a general stalactitic coating on the massive phosjmates, 

 its cavities, etc. 



