68 FLORIDA GEOLOGICAL SURVEY-SEVENTH ANNUAL REPORT. 
INCOHERENT SURFACE SANDS AND SOIL. 
The loose surface sands are believed to have been derived by 
disintegration from the phosphatic sands beneath. * Accordingly 
they are found to be variable in thickness depending upon the local 
conditions. As a rule an irregular line produced by decay separates 
the loose sands from the more or less indurated sands beneath. At 
places, however, the phosphatic sand passes gradually into the sur¬ 
face sand and soil. 
HARDPAN IN THE SUB-SOIL. 
Hardpan which is frequently present in the sub-soil consists as 
a rule of sand held together more or less firmly by either organic 
matter or iron oxide acting as a cement. The hardpan is prevail¬ 
ingly dark brown or chocolate colored and may be from two to 
several feet in thickness. 
VESICULAR AND CALCAREOUS SAND ROCK. 
Occasionally the phosphatic sand rock of the overburden is found 
to be vesicular and calcareous. This phase of the formation was ob¬ 
served in the pit of the Dominion Phosphate Company, and in one 
of the pits of the Medulla Phosphate Company. 
OVERBURDEN ABSENT OR REDUCED IN THICKNESS. 
It is not unusual to find the overburden locally much reduced in 
thickness, and in some instances entirely absent. This is due evi¬ 
dently to surface wash and erosion which has removed the sands 
of the overburden, and in some instances has cut into the phosphate 
beds. Elsewhere the overburden increases in thickness and finally 
reaches a point beyond which the phosphate beds under present con¬ 
ditions cannot be economically worked. 
BOG IRON ORE IN THE OVERBURDEN 
Local small deposits of bog ore are not uncommon. These 
are found near the surface and as a rule mark the location of small 
ponds. An unusual phase of iron rock lying practically at the sur¬ 
face is that seen in pit No. 4 of the Phosphate Mining Company 
one half mile northeast of Mulberry where the rock although hard, 
contains many cavities. 
