66 The Phosphates of America. 



tijne. The rock phosphates appear to be the deeply eroded rem- 

 nants of the phosphatized surface of the middle tertiary limestone ; 

 the conglomerate deposits overlie these limestones unconformably, 

 and in the Gainesville region, at least, appear to be miocene in 

 age, and the river drift deposits are apparently entirely 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 physical 

 counterpart of the brown limonite iron ores of the Appalachian 

 limestone valleys, and the deposits have very similar structural 

 relations. In a number of localities the massive phosphate grad- 

 uates into the limestone, usually by short transitions, and many 

 areas have been discovered in the phosphate belt and under the 

 conglomerate in the Bartow region where the limestone is only par- 

 tially phosphatized. In the mines at Dunellon the massive phos- 

 phate is apparently continuous with the limestones, but there are 

 occasional casts and impressions of the middle tertiary mollusca 

 undoubtedly lying as they were originally deposited. Mr. Darton 

 further says that the origin of the phosphate of lime is not defi- 

 nitely known, but that it seems exceedingly probable that guano was 

 the original source, and that the genesis of the deposits is similar 

 to that of the phosphates in some of the West Indies. Two pro- 

 cesses of deposition have taken place, one the more or less complete 

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

 other a general stalactitic coating on the massive phosphates, its 

 cavities, etc. 



The apparent restriction of the rock-phosphate deposits to the 

 western " ridge " of Florida may have some special bearing on their 

 genesis, but at present no definite relationship is perceived. The 

 aggregate amount of phosphate rock distributed in fragmentary 

 condition in the various subsequent formations is very great, greater 

 by far than the amount remaining in its original position, and it is 

 possible that the area at one time included the greater part, if not 

 all, of the higher portions of the State. As this region apparently 

 constituted a long, narrow peninsula or archipelago during early 

 miocene times, it is a reasonable tentative hypothesis that during 

 this period guanos were deposited from which was derived the ma- 

 terial for the phosphatization of the limestone, either at the same 

 time or soon after. 



Mr. Walter B. Davidson, A.R.S.M., writing in the Engineering 

 and Mining Journal on the probable origin of these phosphates, 



