ORIGIN OR THE HARD ROCK PHOSPHATES. 
55 
fresh water Pliocene deposits, the Alachua clays, containing 
remains of land vertebrates are found locally around the border 
of the phosphate area. These deposits were formed in small lakes 
and sinks, and similar deposits, doubtless, formed in the phos¬ 
phate area. The red sandy clays which form the surface deposits 
over practically all of the Northern and Central Florida probably 
extended across the phosphate area. 
Assuming the former areal extent of these later formations 
across what is now the phosphate belt of Florida, the solution of 
other problems connected with the hard rock deposits is much 
facilitated. As a result of the action of the weathering agencies 
these formations have disintegrated, their residue forming the 
phosphate matrix. The process of erosion and disintegration has 
been long continued, during which time the general surface level 
has been gradually lowered by the solution and removal of the 
underlying limestone. The lowering of the limestone here as else¬ 
where in limestone countries progresses not uniformly but irregu¬ 
larly, due to the formation of caves, sinks and underground 
channels. This irregular subsidence has resulted in the mixing of 
materials originally distinct. Sinks form in the limestone section 
of Florida by which material at the surface is lowered by the 
sudden caving of the earth. When these sinks are first formed 
the walls are vertical or nearly so. As a result of the caving at 
the sides together with the wash of surface material they fill up. 
By this process long continued the materials of different forma¬ 
tions become intimately mixed. 
The mixing of materials by underground solution and sub¬ 
sidence has been supplemented by stream action. While this area 
is at present practically without streams, yet local streams existed 
during the earlier stages of physiographic development. These 
local streams begin their development as soon as sinks are formed 
and when the stratigraphic conditions are favorable a stream 
enters each sink; working back from the sink the stream estab¬ 
lished in time a normal drainage system. These streams are 
known as disappearing streams since they enter sinks. As has 
