SECOND ANNUAL REPORT—GENERAL GEOLOGIC HISTORY. 
163 
present. The number of samples collected was forty-seven and over 
250 slides were examined microscopically. In one of the samples, 
which consisted largely of quartz sand, no tests of diatoms or spicules 
of sponges were seen, but in the other samples remains of these or¬ 
ganisms were very numerous. The conditions governing sedimenta¬ 
tion along the southern end of Florida are probably similar to those 
that prevailed during the Oligocene except that coral reefs were want¬ 
ing in the Tertiary era. The shallow Vicksburg sea was apparently 
comparatively free from sand and mud and the greater portion of 
the sediments consisted of finely divided, calcium carbonate containing 
organic remains. The silica which occurred in this material, being 
amorphous, was readily dissolved by percolating waters and subse¬ 
quently deposited, replacing the calcium carbonate of the limestone. 
This process, appears to have been quite active, for beds of chert a 
fraction of an inch to several .feet in thickness are found to be very 
persistent at certain horizons. In fact, it is the chert beds which form 
the confining strata above the artesian water horizons. The cause for 
these beds being persistent over large areas is perhaps to be found in 
the deposition of large amounts of silica in certain horizons during 
sedimentation, though it is more probable that the silica was originally 
distributed throughout the limestone and the chert beds merely repre¬ 
sent the horizons along which the silica-bearing solutions circulated 
most freely. 
Toward the close of the period of the deposition of the limestones 
of the Vicksburg group, a partial emergence of the land was fore¬ 
shadowed by the increase of terrigenous sediments, and the appear¬ 
ance of fresh-water shells which were doubtless washed into the sea 
from adjacent land areas during the deposition of the Ocala lime¬ 
stone. The extent of this emergence can only be conjectured, but it 
doubtless affected a considerable area north of Lake Okeechobee and 
it probably extended somewhat beyond the present boundaries of the 
State. 
The nature of the movement which closed the period of deposition 
of the Vicksburg group is somewhat obscure, though it was probably 
a continuation of the broad arching of the strata which had been in¬ 
itiated during an earlier epoch when the structural basin of the Gulf 
of Mexico was first formed. The irregularity of the surface of the 
rocks, belonging to the Vicksburg, along the east coast is in part due 
to the deformation which took place at this time, and in part to sub¬ 
sequent erosion. Following this emergence there was a period of de¬ 
nudation when the surface of the land was carved into hills and val¬ 
leys. The evidence of this erosion interval is found in the uneven 
surface upon which the later beds were deposited. When compared 
with mountainous regions the relief of the surface, produced during 
