16B 
FLORIDA STATIC GEOLOGICAL SURVEY. 
GENERAL GEOLOGIC HISTORY. 
The absence of pronounced deformation of the rocks and the low 
relief of the surface of Florida make it difficult to obtain continuous 
sections of beds. However, by observations of such exposures as are 
available and the study of samples of strata procured from wells, it 
has been possible to obtain a fairly complete knowledge of the thick¬ 
ness, character and relations of the successive formations from the 
early Oligocene down to the present time. This knowledge permits 
the formulation of a general sedimentary history of the State from 
the deposition of the oldest rocks exposed to the formation of the 
sand dunes and coquina of the present day. 
As a result of the studies already made, certain broad generaliza¬ 
tions relating to the changes that have taken place are possible, one 
of the most important being that the older rocks of the State were 
formed under conditions which were uniform over wide areas and 
which permitted the deposition of several hundred feet of homogene¬ 
ous sediments, while the younger formations were laid down under 
conditions which varied greatly within short distances, and changed 
at frequent intervals. That the period of deposition of the Vicks¬ 
burg group was a time of great uniformity in conditions over 
wide areas is well shown by the. remarkable homogeneity of the beds 
of this age which underlie practically the entire State. These beds 
are uniformly fine-grained and show little variation in chemical com¬ 
position. There is a predominance of limestone, though sand and 
clay occur in small quantities, and the percentage of these impurities 
in the limestone increases in the upper beds of this age. There is 
also an increased percentage of terrigenous material toward the 
northern end of the State, where the proximity of older land, 
afforded opportunity for the entrance of considerable sand and mud 
into the Vicksburg sea. Toward the close of this period of deposi¬ 
tion there appears to have been a shoaling of the seas which permitted 
the entrance of the fresh-water shells and the land-derived sediments 
noticeable in the Ocala limestone of the Vicksburg group. The excel¬ 
lent state of preservation of many of these shells shows that the water 
must have been comparatively quiet during the deposition of the lime¬ 
stone. The inclusion of a small percentage of land-derived sediments 
and in some places of fresh water shells shows that a portion of the 
limestones of Vicksburg age were probably deposited at no great dis¬ 
tance from land. Locally the calcareous sediments appear to have 
contained large quantities of silica, probably in the form of tests of 
microscopic plants (diatoms) and spicules of sponges. 
Recent studies of materials obtained from the sea bottom among 
the Florida keys have shown that there is a large amount of silica 
