PEBBLE PHOSPHATE DEPOSITS. 
57 
ties characteristic of and common to exposed land surfaces. This 
period of emergence and erosion continued apparently during the 
greater part, if not all of the Miocene period. Near the close of the 
Miocene or early in the Pliocene came a period of submergence per¬ 
mitting the sea to cover large areas of Florida that had previously 
been dry land. In connection with and following this Pliocene sub¬ 
mergence was accumulated the Bone Valley formation, the first or 
lowest member of which holds the land pebble phosphates. With 
these facts regarding the geologic history of southern Florida in 
mind it becomes possible to understand and account- for the pebble 
phosphate deposits, and to explain many of the peculiarities of the 
formation. 
THE LAND PEBBLE PHOSPHATE DEPOSITS 
With regard to the genesis of the land pebble phosphate deposits 
Eldridge in 1893 wrote as follows :* 
The resemblances in texture, color, fossil casts, and general appearance 
which the pebbles of this deposit occasionally bear to the hard-rock type are in 
a measure suggestive of their derivation from a limestone of pre-Pliocene times, 
possibly older Miocene. On the other hand the prevailing white color, the often 
earthy appearance of the fresh surfaces, the lower percentage in phosphate of 
lime, the softness and the condition of preservation of included fossils suggest 
their origin from a marl or at least a very earthy friable limestone. In either 
case they may be the rolled fragments- of pre-existing beds, a possibility enhanced 
by the character of their matrix and by the occasional presence of well-rounded, 
white quartz pebbles. 
Shaler was of the opinion that the land pebble phosphates of 
Florida should be placed in the category of Residual Ablation De¬ 
posits. He says, as quoted by Eldridge ( 1 . c.) : 
In the case of the pebble phosphates it is evident that the fragments have 
been to a certain extent swept from the uplands into the valleys. Thus on the 
low divides between the Alafia river district and the neighboring portions of 
Florida, the phosphatic pebbles may form but a thin' layer or be altogether want¬ 
ing, while in the valleys the accumulations of pebbly material may have a thick¬ 
ness of 30 feet or more. This segregative process has probably in part been 
accomplished by the work of the streams themselves, but it is in my opinion 
mainly due to the action of the sea during the time or times when this part of 
the peninsula has sunk beneath and risen above the ocean level. The action of 
the sea in this concentrative work appears to be indicated by the frequent occur- 
*A Preliminary Sketch of the Phosphates of Florida, by George H. Eldridge, 
Am. Inst. Min. Eng. Trans, xxi, pp. 196-231, 1893. 
