250 
FLORIDA STATE GEOLOGICAL SURVEY. 
This formation when not affected by chemical change consists typical¬ 
ly of a mass of calcareous shells of varying size from minute fora- 
minifera to larger bivalves and gastropods with which is interbedded 
coral and other fossils, along with a limited amount of siliceous ma¬ 
terial supplied principally by sponge spicules. Originally, 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 car¬ 
bonate of the limestone. The direct evidence that the flint masses are 
formed by the replacement process is to be had from the examina¬ 
tion of a piece of flint. In this it will be seen that the foraminifera 
and other shells which were originally calcareous have-been changed 
to silica. The replacement process is by no means confined to the 
formation of flints. 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. 
The compact and partly crystallized limestones which occur locally 
as previously mentioned under Building Stone is probably due to a 
somewhat similar process. The change in the case of the limestone 
involves a rearrangement of the constituent molecules, resulting in 
a harder and more compact rock. 
Shell deposits, both recent and fossil, are numerous in the State. 
The use of shell from the coquina rock for building purposes has al¬ 
ready been mentioned. Among notable buildings from these shells 
may be mentioned the Ponce de Leon hotel at St. Augustine. The 
calcareous shell mass as found at this locality may contain a small 
admixture of siliceous sand blown in by the winds. Recent shell de¬ 
posits occur at many places along both the Atlantic and the Gulf 
coasts. Shell mounds piled up by the Indians are likewise numerous 
on and near the coast. Some occur inland also, those of the St. Johns 
River from Jacksonville to Sanford being notable examples. Deposits 
of fossil shells are likewise numerous; although as a rule less free 
from impurities than are the accumulations of recent shells. The local¬ 
ities at which many shell exposures occur have been mentioned in the 
preceding pages of this report. 
