ORIGIN OR THE HARD ROCK PHOSPHATES. 
37 
problems to be accounted for. 
Among the problems that must be accounted for in connection 
with the hard rock phosphate deposits of Florida are the follow¬ 
ing: (1.) The source of the miscellaneous materials that make 
up the formation, including sands, clays, flint pebbles, vertebrate 
and invertebrate fossils, silicified wood, flint boulders, limestone 
inclusions and phosphate rock in its varying forms. (2.) The 
intimate admixture in the formation of these diverse materials. 
(3.) The processes by which phosphate and flint boulders have 
formed. (4.) The limitation of the hard rock phosphate forma¬ 
tion to a characteristic well marked physiographic type of country. 
(5.) The localization within the formation of phosphate rock to 
such an extent as to form workable deposits. (6.) The forma¬ 
tion of the plate rock deposits. 
SUMMARY OF THE EXPLANATION OFFERED. 
The explanation offered, briefly summarized, is as follows: It 
is believed that the Upper Oligocene and probably some later 
formations, now' found on the surrounding uplands, formerly 
extended directly across the section that is. now the hard rock 
phosphate fields. The disintegration of these formations supplied 
the miscellaneous materials of which the deposits are made up. 
The mixing of the materials was brought about in part by stream 
action, which has resulted in a reworking and reaccumulation of 
the residual material from these formations, and in part by the 
local irregular subsidence such as is constantly going on in a lime¬ 
stone country. In some parts of the phosphate fields the lower¬ 
ing and mixing of the materials by solution of the underlying 
limestone has been the predominating factor, while elsewhere the 
reworking of the materials by stream action has predominated. 
It is probable that local bodies of water existed also in which the 
materials reaccumulated. The immediate source of the phosphoric 
acid is the phosphate, which was widely disseminated through the 
overlying formations. The fossils now found in the formation 
include those that were residual from the formations that have 
disintegrated, and those that were incorporated in connection with 
