16 FLORIDA GEOLOGICAL SURVEY—FOURTH ANNUAL REPORT. 
phosphatic material also is dissolved, by the surface waters and 
removed. The resulting soil is sandy, and is possibly not always 
distinguished from sandy soils resulting from other formations. 
PLIOCENE. 
The Pliocene is represented by several more or less well marked 
formations as follows: The Caloosahatchee Marl, the Nashua Marl, 
the Alachua Clays, the Bone Valley formation and the Dunnellon 
formation. The first two of these are marine formations and con¬ 
tain many well preserved marine fossils which serve to determine 
definitely their age. The remaining three, the Alachua Clays, the 
Bone Valley and Dunnellon formations, are less definitely deter¬ 
mined. The Alachua Clay is apparently of fresh water origin, hav¬ 
ing been deposited in and around the borders of the many lakes 
which existed in the central part of the State. It is therefore a dis 
connected formation and the different deposits were not necessarily 
contemporaneous. In fact these lake deposits are doubtless a phase 
of lake formation and refilling which began in the Pliocene or 
earlier, and has continued to the present. The'Bone Valley forma¬ 
tion includes the land pebble phosphate deposits. These have been 
commonly referred to the Pliocene. They consist of pebble phos¬ 
phate imbedded in a phosphatic clay. 
The Dunnellon formation was described in the preceding report.* 
The materials of the formation are miscellaneous in character, and 
include sands, clays, gravel and pebble, flint boulders, limestone 
inclusions, and phosphate rock. The phosphate is of vast importance 
to agriculture, and its presence in the Dunnellon and Bone Valley 
formations lends especial interest to these deposits. In regard to 
the hard rock phosphate of the Dunnellon formation, the writer 
expressed the view in the paper referred to that the phosphoric acid 
has been very gradually concentrated from various formations in 
which it existed in very small quantities. In support of this 
view is the notable fact that the hard rock phosphate boulders, some 
of which are of immense size, have unquestionably formed and are 
forming in situ. The plate and fragmental rock represents boulders 
formed during a preceding stage and subsequently broken, more or 
less transported, and finally deposited in their present position. In 
support of this view of the origin of the phosphates it is a notable 
fact that hard rock phosphates in Florida occur only in sections 
which have been subjected to prolonged disintegration and erosion. 
*A Preliminary Paper on the Florida Phosphate Deposits, by E. H. Sel- 
lards, Third Annual Report, Fla. Geol. Survey, 17-41, 1910. 
