30 FLORIDA GEOLOGICAL SURVEY-EIGHTH ANNUAL REPORT. 
ROCKS OF FLORIDA CLASSIFIED ACCORDING TO CHEMICAL 
COMPOSITION. 
Flint and chert, 
Siliceous rocks-Sandstone, 
I Infusorial earth. 
Calcareous rock 
Argillaceous rock 
Carbo-Hydrates 
Ferruginous rocks 
Phosphatic rocks _ 
Shell limestone, 
J Crystallized limestone, 
Oolitic limestone, 
Marl. 
-Clay and shale, 
-Muck, peat, lignite. 
_Bog iron ore. 
Pebble and rock phosphate. 
SILICEOUS ROCKSFLINT, CHERT AND GRAVEL 
Flint is chemically an oxid.e of silica, SiO, 2) with more or less 
accompanying impurities. It is a variety of the mineral quartz, oc¬ 
curring massive and non-crystallized or miore accurately very imper¬ 
fectly crystallized (crypto-crystalline.) The term “chert” is often 
used interchangeably with flint. Properly chert is an impure flint 
or flinty rock. Flint and chert are lacking in cleavage. They break, 
as do the other varieties of quartz, with conchoidal fracture. A flint 
rock when crushed breaks into sharp cornered pieces of varying size. 
Thp mineral quartz, of which flint is a variety, has a hardness of 
seven on a scale in which the hardest mineral, diamond, is ten. The 
varieties of quartz vary in hardness slightly according to the im¬ 
purities that they contain. Silica is one of the least soluble of 
minerals and among the most resistant to decay. 
Flint and chert occur mostly as masses or “horsebacks” in the 
limestone formations. A good illustration of the manner of occur¬ 
rence may be seen in phosphate pits or in some of the lime pits at 
Ocala. In some of the sinks on Thompson’s farm two miles east of 
Sumterville will be seen flint masses exposed by the natural decay 
of the limestone. The flint masses appear to conform to no rule as 
to size and extent. Flints may form a ridge running through the 
limestone; or again they occur as rounded or elongated masses. Oc¬ 
casionally the flint forms as a thin stratum lying horizontally. This 
flint bearing limestone lies at no great distance from the surface 
