ORIGIN OF THE HARD ROCK PHOSPHATES. 
31 
from 75 to 100 feet. When of this thickness it is worked to the 
permanent ground water level by the dry pit method of mining, 
and is then mined from 40 to 50 feet below this lev^l by the float¬ 
ing dredge. In the northern part of the area the formation is as 
a rule much thinner, and is worked almost entirely by dry pit 
mining. 
AMOUNT OF HARD ROCK PHOSPHATE. 
It is scarcely possible to give an estimate of the amount of 
hard rock phosphate in Florida that yet remains to be mined. 
This is due to the fact that the deposits are extremely local and 
irregular. While the whole extent of the phosphate bearing 
formation can be mapped with a fair degree of accuracy, the 
deposits of phosphate within the formation can be located and an 
estimate of the amount that is mineable made only after very 
exact prospecting. The cost of such prospecting is such that it is 
seldom undertaken on a, large scale except by the companies 
actually interested in producing the rock. It is true that some 
estimates as to the total tonnage available have been made, but 
these amount to little more than guess work. The amount actually 
mined during the twenty-two years since mining operations began 
in this field is approximately 9,313,071 tons. The output at 
present amounts to about one-half million tons per annum. 
FORMATION NAME. 
The term Dunnellon formation has been applied by the writer 
to the phosphate bearing formation.* These deposits are well 
developed in the vicinity of Dunnellon, in Marion County, and have 
been extensively mined in that section. It was here also that the 
deposits were first discovered and mined. The term Dunnellon is, 
therefore, appropriate. The formation is probably of Pliocene age 
as indicated by the fauna. 
^Florida State Geological Survey, Third Annual Report, p. 32, 1910. 
