THE FLORIDA PHOSPHATE DEPOSITS. 
31 
The source of the dark colored water worn flint pebbles and of 
the pebble conglomerate occasionally observed especially in the 
northern part of the field is at present scarcely more than con¬ 
jectural. So far as the writer’s observations have extended, ma¬ 
terials of this character occur more frequently in the Miocene than 
in any other of the formations of the State. The presence of 
mastodon remains indicates admixture of Pliocene material from 
some source. 
The origin of the phosphate is perhaps the most difficult prob¬ 
lem connected with these and, in fact, with phosphate deposits 
in general. In the case of the Florida deposits the writer is in¬ 
clined to the view that the phosphoric acid has been very gradual¬ 
ly concentrated from various formations in which it exists in 
only very small quantities. Enrichment by the addition of phos¬ 
phoric acid is a well known process. Many instances have come to 
light of shells originally calcareous now completely phosphatized, the 
phosphoric acid having replaced the carbonic acid. In many in¬ 
stances the shape and markings of the shell are retained. The bones 
imbedded in the phosphate also are more or less completely phos¬ 
phatized. The formation of the phosphate boulders in situ seems 
evident. The plate and fragmental rock represent boulders 
formed during a preceding stage and subsequently broken, more 
or less transported and finally deposited in their present position. 
The pebble phosphate found among the rock phosphate is prob¬ 
ably largely water worn detritus mechanically accumulated. 
CONDITIONS OF DEPOSITION. 
The variable and mixed character of the formation, the fre¬ 
quent clay lenses, the faint tendency to stratification, the occasion¬ 
al local accumulation of loose or conglomerate material indicate 
to the writer that the material accumulated in shadow water with 
conflicting currents. Much of the material may indeed have been 
scarcely at all transported being residual from formations that 
have decayed in place. The local accumulation of pebble con¬ 
glomerate, however, as well as the local occurrence of clay lenses 
implies conflicting currents in comparatively shallow water. The 
faint tendency to stratification leads to the same conclu¬ 
sions. Such stratification as existed, however, has been much 
distorted by the settling of the formation as the underlying lime¬ 
stone was removed by solution. The conditions of deposition do 
not, in the writer’s opinion, necessarily indicate complete resub¬ 
mergence of this area, although such may have been the case. It 
is extremely probable that the formations which have gone to decay 
