PEBBLE PHOSPHATE DEPOSITS. 
31 
PROBLEMS TO BE ACCOUNTED FOR. 
Among the problems to be accounted for in connection with 
the origin of the pebble phosphate deposits are the following: (1). 
The source of the phosphate pebble found in the land and river 
phosphate deposits. (2). The source of the materials of the 
matrix, including sand, clay, vertebrate, invertebrate and plant fos¬ 
sils. (3) The conditions under which the formation accumulated. 
(4). The minor problems of the formation, such as variation in 
grade of rock, irregularities in the beds, lack of continuity of the de¬ 
posits, variation in the thickness of the phosphate stratum and in the 
richness of the matrix. While all of the many perplexities met 
with in mining may not be explained in this report, it is believed that 
there is presented here a rational basis from which the causes of 
many of the seemingly erratic variations in the phosphate beds may 
be understood. 
SUMMARY OF THE EXPLANATION OFFERED. 
The explanation offered, briefly summarized, is as follows: The 
land pebble phosphate bed is a conglomerate of pebble, sand and clay 
formed by the sea advancing probably with minor oscillations in 
level across the exposed surface of the great phosphatic marl known 
to the miners as the “bed rock.” The immediate source of the phos¬ 
phate rock as well as the other materials of the matrix is therefore 
from the “bed rock” marl. The sands of the overburden represent 
the part of the formation that was deposited following the accumula¬ 
tion of the pebble conglomerate. Within the phosphate bed the grade 
of rock is enhanced through secondary enrichment brought about by 
the agency of surface waters moving downward afid laterally 
through the formation. The minor variations in structure in the 
beds find their explanation in the varying conditions that prevail in 
shallow water deposits. The fossils in the phosphate deposits are 
derived in part from the earlier formations, and in part represent 
animals that were living at the time the deposits accumulated. The 
original source of the phosphorus is the primitive rocks of the earth’s 
crust through which it is widely disseminated and from which it 
finds its way into sedimentary rocks by diverse processes, some of 
which have been enumerated by the writer in an earlier paper of this 
series.* The river pebble deposits were accumulated in the beds of 
streams during either Pleistocene or recent time, the pebble having 
been washed out of the older formations. 
*Fla. State Geol. Surv., Sixth Ann. Rept., pp. 71-81, 1914. 
