30 
FLORIDA GEOLOGICAL SURVEY. 
fact that the resisting power of the nodules is greater 
than that of the surrounding material, or chemically by 
the replacement process. The hard rock phosphate area 
has not in the view of several geologists, been contin¬ 
uously exposed since its first appearance above water in 
Oligocene time. Later formations, originally present, 
are believed to have been largely removed by erosion. 
Phosphate, usually of a low grade, occurs in the Upper 
Oligocene formations in several localities between the 
Suwannee and the Apalachicola Rivers and in several 
localities in east central Florida. The Miocene deposits 
usually contain some phosphatic material, as do also the 
Pliocene. These later formations, if originally present 
over the Vicksburg Limestone in the hard rock phosphate 
section may have served as a source of phosphate which, 
with the decay of these rocks, accumulated either chemi¬ 
cally or mechanically at a lower level. Several other 
possible sources of phosphoric acid have been suggested. 
Among these may be mentioned the probability of the 
addition of phosphoric acid to the limestone from the 
rookeries of birds. 
The land pebble phosphate occurs in the Pliocene forma¬ 
tion and represents apparently a stratified deposit of 
chemical origin. 
Future of the Phosphate Industry:—An estimate of the 
total amount of phosphate in Florida is difficult. The 
deposits are local and scattered and their extent is to be 
determined only by extensive prospecting. It is safe to 
say, however, that the industry, both in the extent of 
production and in the method of mining, Is still in its 
infancy. Practically all of the high grade phosphate pro¬ 
duced is now exported to foreign markets. This condi¬ 
tion is unfortunate. Phosphate is one of the earliest of 
soil constituents to be exhausted, and it is apparently 
only a matter of time until the American demand will 
exceed the supply. More regrettable is the waste which 
accompanies mining. While the phosphate companies 
operate as economically as present conditions will per¬ 
mit, a large loss in phosphate salts is known to occur. 
