( 
58 FLORIDA GEOLOGICAL SURVEY-SEVENTH ANNUAL REPORT. 
rence of shark’s teeth and other remains of marine forms in a state of preser¬ 
vation which seems to me to clearly indicate that they were formed since the 
phosphatic pebbles took their shape. They appear indeed to be mere accidental 
and exceedingly modern elements of the deposit. They have evidently experi¬ 
enced no such attrition as has affected the pebbles themselves. 
Dr. W. H. Dali regarded these deposits as of marine origin and 
of Pliocene age.* Messrs. Matson and Clapp, however, while rec¬ 
ognizing the possible estuarine origin of a part of the beds, were 
inclined to regard the Bone Valley formation as chiefly fluviatile.f 
Aside from the papers referred to ‘above few serious at¬ 
tempts have been made to account for the origin of the land pebble 
phosphates. This is in marked contrast to the literature on the hard 
rock phosphates, for the origin of which a multiplicity of conflicting 
and often fantastic theories were offered during the early days of 
mining. With regard to the fluviatile origin of the beds, while the 
deposits were clearly accumulated in shallow water, it has seemed to 
the writer that the characteristics of the formation as given in the 
following pages indicate estuarine or shallow water marine deposits. 
THE LAND PEBBLE PHOSPHATE BED A PEBBLE CONGLOMERATE 
ACCUMULATED UNDER MARINE OR ESTUARINE 
CONDITIONS. 
When the sea advances across an area that previously had been 
dry land the first deposits accumulated are of local origin, being de¬ 
rived from the residual materials of the land surface, together with 
inclusions, fragments, and pieces broken or washed from the rock 
beneath. The material thus accumulated is usually coarse in texture, 
and in that case forms a conglomerate. If subsidence continues the 
conglomerate layer is followed by sediments that, having been trans¬ 
ported a greater distance, and lodged in deeper waters, are finer, 
more uniform in texture, and more thoroughly assorted, than those 
first deposited. The stratum of coarse material at the base of a for¬ 
mation accumulated under these conditions is known as a basal con¬ 
glomerate. 
If the geologic history of southern Florida has been correctly 
interpreted, the sea, probably in early Pliocene time, advanced 
across the upper Oligocene phosphatic marl, and in doing so accumu- 
alted first of all a conglomerate layer, the materials of which, de- 
*Neocene of North America. U. S. Geol. Surv. Bull. 84, pp. 141-142, 1892. 
tFla. Geol. Surv. Second Annual Report, pp. 138-141, 1909. 
