ANNUAL REPORT-ROAD MATERIALS. 
4$ 
hills such as that crossed near Evinston and Micanopy 
stand out as evidence of the resistance of flint to the 
weathering agencies, the surrounding limestone having 
disappeared through erosion. This flint-bearing limestone 
is known as the Vicksburg Limestone. It is not to be 
inferred, however, that no other Florida formation con¬ 
tains silica. On the contrary, many of the formations 
are highly siliceous. The Vicksburg Limestone is, how¬ 
ever, the chief flint-bearing formation of Florida. 
Origin of the Flint in the Vicksburg Limestone:—The 
flint occurs as has been stated, in masses irregularly dis¬ 
tributed through the limestone. Well drillers can bear 
witness to the frequency of flints and to their distribu¬ 
tion through the limestone to a great depth. The flint 
masses were clearly not present in the limestone as orig¬ 
inally formed. This formation when not affected by 
chemical change consists typically of a mass of calcarous 
shells of varying size from minute foraminifera to larger 
bivalves and gastropods with which is inter bedded coral 
and other fossils, along with a limited amount of siliceous 
material supplied principally by sponge spicules. Origin¬ 
ally, without doubt the limestone consisted largely of the 
remains of these calcarous shells, the flint masses having 
been subsequently deposited through the agency of under¬ 
ground 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 evidence 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 foram¬ 
inifera and other shells which were originally calcarous 
have been changed to silica. The replacement process is 
by no means confined to the formation of flints. As men¬ 
tioned in the introduction to this chapter, chemical 
changes are constantly going on among the minerals mak¬ 
ing up the rock formations, and replacement of one min- 
4-GeoRepl 
