MINERAL INDUSTRIES. 
31 
throughout all of the central peninsular section of the State from 
Columbia County on the north to Sumter County on the south and 
from the Suwannee River and the Gulf coast to east Alachua and 
Marion counties. Much of the hard rock phosphate rests upon this 
flint-bearing limestone, and from the phosphate pits great quantities 
of the flint may be obtained'. This flint-bearing limestone is known 
as the Ocala formation. It is not to be inferred that no other Flor¬ 
ida formation contains silica for, on the contrary, many of the for¬ 
mations are highly siliceous. The Ocala limestone, is, however, 
the chief flint-bearing formation of Florida. 
The flint masses were clearly not present in the limestone as orig¬ 
inally formed. This formation when not affected by chemical change 
consists typically O’f a mass of calcareous shells varying in size from 
minute foraminifera to larger bivalves and gastropods with which 
are interbedded coral and other fossils, along with a limited amount 
of siliceous material supplied principally by sponge spicules. Orig¬ 
inally, without doubt, the limestone consisted largely of the remains 
of these calcareous shells, the flint masses having been subsequently 
deposited through the agency of underground water. Water in its 
round of circulation through surface and deeper formations takes 
silica as well as other substances into solution. In the course of its 
circulation through the limestone, the silica in solution in the water 
replaces the calcium carbonate of the limestone. The direct evi¬ 
dence that the flint masses are formed by the replacement process is 
to be had from the examination of a piece of flint. In this it will 
be seen that the foraminifera and other shells which were origi¬ 
nally calcareous have been changed to silica. The replacement pro¬ 
cess is by no means confined to the formation of flints. As already 
mentioned,, chemical changes are constantly going on among the 
minerals making up the rock formations, and replacement of one 
mineral by another is one of the important phases of chemical 
change. 
Siliceous gravels are widely scattered over the State and in a few 
localities occur in sufficient abundance to be used as a road material. 
Gravel pits have been opened at Interlachen and at Grandin in Put¬ 
nam county and in Escambia and Jackson counties. 
