THE FLORIDA PHOSPHATE DEPOSITS. 3

The source of the dark colored water worn flint Debbles and of the pebble conglomerate occasionally observed especially in the northern part of the field is at present scarcely more than conjectural. So far as the writer's observations hav e extended, materials of this character occur more 'frequently in the Miocene than in any other of the formations of the State.- 1 he presence of mastodon remains indicates admixture of Pliocene material from some source.
The origin of the phosphate is perhaps the most difficult problenm connected with these and, in fact, with phosphate deposits in general. In the case of the Florida deposits the writer is inclined to the view that the phosphoric acid has been very gradually concentrated from various formations in which it exists in only very small quantities. Enrichment by the addition of phosphoric 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 instances the shape and markings of the shell are retained. The bones imbedded in the phosphate also are more or less completely phosphatized. The formation of the phosphate boulders in situ seems evident. The plate an d fragumental. 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 probably largely water worn detritus mechanically accumulated.
CONDITIONS OF DEPOSITION.
The variable and mixed character of the formation, the frequent clay lenses, the faint tendency to stratification, the occasional local accumulation of loose or conglomerate material indicate to the writer that the material accumulated in shallow 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 conglomerate, however, as well as the local occurrence of clay lenses implies conflicting currents in comparatively shallow water.- The faint tendency to st ratification leads to the same conclusions. Such stratification as existed, however, has been much distorted by the settling of the formation as the underlying limestone was removed by solution. The conditions of deposition do

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