66 
FLORIDA STATE GEOLOGICAL SURVEY. 
ground solution. The many basins now occupied by lakes have 
been formed by subsidence due to solution. Following the 
formation of the basins the surrounding uplands are gradually 
lowered, the tendency being to fill up the basins and to reduce 
the land surface once more to a common, although lower level. 
An examination of the accompanying map, on which the lake 
region is separately indicated, bears out the view that this region 
represents the further southeastward migration of the limestone 
country of the peninsula. 
It is not necessary to assume that the hard rock phosphate 
belt has passed through a stage of development identical with 
that of either the lake region or the Middle Florida Hammock 
Belt. Differences in the thickness and character of the forma¬ 
tions, or of the drainage, or other conditions may have modified 
the results in this region. Certain it is, however, that the lime¬ 
stone region of Central Florida is encroaching on the non-lime¬ 
stone areas to the east. Whether or not what is now the hard 
rock phosphate belt passed through the typical lake region topo¬ 
graphy, it is at least a reasonable inference that lakes more or 
less extensive existed in the earlier stages of the development of 
this area. 
ECONOMIC RELATION. 
The economic bearing of the observation that the hard rock 
phosphate is confined to a particular physiographic type is im¬ 
portant. Although within the area careful and expensive pros¬ 
pecting is necessary to locate the individual deposits, yet to pros¬ 
pect for hard rock phosphate outside of the particular physio¬ 
graphic type of country with which the hard rock phosphates are 
associated is recognized as useless. No hard rock phosphate is 
to be expected, for instance, in the lake region nor elsewhere in 
the non-limestone areas of Florida, nor in the Middle Florida 
Hammock Belt, except possibly in such local areas as have by 
more rapid erosion passed into the stage in which hard rock 
phosphate accumulates. 
