164 Florida state: geological surve:y. 
the interval of erosion, was insignificant, though it may have been 
sufficient to be comparable with that of the same region today. 
The erosion interval was followed by a period when the sea once 
more encroached upon the land to such an extent that a large part of 
Florida was probably submerged. During this time considerable 
thicknesses of clay, sand and calcareous mud were laid down in the 
shallow water. In the east and south central portions of the peninsula 
the clay and sand predominated during the earlier part of this epoch, 
while farther north and west similar deposits characterized the later 
stages. The calcareous materials, which are found now in the form 
of marls and limestones were especially important in the area now 
drained by the Apalachicola River, but they were also deposited in 
smaller quantities farther south and east. Throughout the period 
represented by the Apalachicola group the conditions governing depo¬ 
sition appear to have differed considerably in neighboring localities, 
but there was no such abrupt variation as may be found along the 
present coast. The changes from sediments of one character to those 
of another were frequently rapid, and during the entire time there 
was more or less intermingling of different kinds of sediments giving- 
rise to the marls, impure limestones, shales, and sands of this epoch. 
The fullers earth beds which occupy considerable areas in the north- 
central part of the State represent short intervals of uniform condi¬ 
tions controlling sedimentation, but these alternated with conditions 
which permitted the deposition of sands, and ordinary clays. In gen¬ 
eral, the rocks of the Apalachicola group appear to have been marine, 
but during the latter part of the epoch some non-marine sands were 
laid down in the Apalachicola valley. At this time, the central por¬ 
tion of the Florida peninsula appears to have been an island which 
was separated from the mainland to the north by a shallow passage 
known as the Suwanee Strait . 1 
The Hawthorne formation which is largely clay, appears to have 
been deposited in the eastern part of this strait, while farther west 
the deposition consisted of limestone and marl. At certain localities 
the conditions were especially favorable for the development of 
organic life and hence some beds are very fossiliferous, notable exam¬ 
ples being the “silex bed” at.Tampa, the Chipola marl, the Oak Grove 
sand and the Shoal River marl members of the Alum Bluff formation. 
Associated with the other fossils in the silex bed there was probably 
a large quantity of amorphous silica in the form of sponge spicules 
and tests of microscopic plants. Subsequent solution and redeposition 
of this silica has given rise to the silex. Silica is also present in the 
1 Dali, Wm. H., Neocene of North America, U. S. Geol. Surv., Bull. 84, 
1892, p. 111. 
