ORIGIN OR THE HARD ROCK PHOSPHATES. 
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regarded as resulting from the disintegration of the hard rock, 
although a part of the soft phosphate may be merely phosphatic 
clays. 
LOCALIZATION OF THE HARD ROCK DEPOSITS. 
The localized nature of the hard rock deposits within the 
formation is with little doubt explained by the variable character 
of the materials in which it occurs. As has been previously 
stated, the deposits of phosphate boulders are to some extent 
associated with local clay lenses. Such an association is a priori 
natural since clay interferes with the free circulation of the per¬ 
colating water. On the other hand, when the matrix is chiefly 
sands with uniform and open texture, through which the water 
moves readily, the conditions are not favorable for the chemical 
deposition of phosphate. However, occurrence of the rock can 
not be expected to follow too closely the structural conditions as 
now observed since, as has already been explained, the whole phos¬ 
phate producing section has been subjected to erosion by solution, 
which permitted irregular and intermittent local subsidence, thus 
thoroughly mixing the materials and moving them more or less 
from their original location. 
LIMITATION OF THE HARD ROCK PHOSPHATES. 
There yet remains the problem of the limitation of the hard 
rock phosphate to a particular and well recognized physiographic 
type of country. That the phosphate beds are so confined has 
long been apparent to those actively engaged in prospecting for 
and mining phosphate as well as to those who have investigated 
the deposits from a scientific standpoint. The accompanying map 
from the Fourth Annual Report of the Florida Geological Survey 
outlines in a general way the several physiographic types of the 
limestone section of Central Florida. In the light of what has 
previously been written, together with the legend, the map is 
largely self-explanatory. Four well defined physiographic types 
are recognized as follows: The Gulf Hammock Belt, The Hard 
