ORIGIN OF THF HARD ROCK PHOSPHATES. 
53 
phates, any better than for the intensely localized hard rock phos¬ 
phates. Upper Oligocene formations, which are throughout more 
or less phosphatic, attain in Florida a thickness of several hundred 
feet. Moreover these formations, except where disconnected by 
erosion, are continuous from the Apalachicola River, in West Flor¬ 
ida, to an undetermined distance beyond the point at which they 
disappear beneath later formations in Central Florida. It is in¬ 
conceivable to the writer that bird guano deposits could have been 
so uniformly scattered over so wide an area and through so great 
a thickness of sedimentary rocks. 
As regards the chemical changes involved in the formation of 
the hard rock phosphate there is much less disagreement among 
the different writers. Fedoux, Darton, Dali, Eldridge, Brown, 
Jumeau and others have recognized that phosphoric acid in solu¬ 
tion in water may and under favorable conditions does replace 
the carbonate of limestones thus forming calcium phosphate. 
Darton recognized the two processes, the first being the replace¬ 
ment of the carbonate by phosphate, and the second the subsequent 
coating over the surface and in cavities by phosphate thrown out 
of solution. Eldridge recognized the formation of boulders by 
replacement of carbonate by phosphate, and by precipitation from 
solution. The evidence of the formation of phosphate by the 
replacement of carbonate by phosphate is entirely incontrovertible, 
since, as has been previously stated, many of the boulders retain 
the original calcareous shells now phosphatized. The evidence of 
subsequent secondary deposition in the cavities is likewise obtained 
from the structure of the rock itself. The formation of boulders 
by precipitation seems probable from the structure of many of the 
boulders. Doubtless, as elsewhere stated, the replacement and 
precipitation have combined in the formation of many boulders. 
The chemical processes involved are more fully discussed else¬ 
where. 
Turning again to the explanation of the hard rock phosphate 
deposits offered by the writer, the key to the solution of the hard 
rock phosphate problems is found, in the writer’s opinion, in a 
study of the geological history of the State. The foundation rock 
in Central Florida is the Vicksburg Limestone of Lower Oligo- 
