90 FLORIDA GEOLOGICAL SURVEY SIXTH ANNUAL REPORT. 



Flint boulders occur locally in this formation in some abun- 

 dance, and occasionally phosphate pits that are otherwise workable 

 are abandoned on' account of the number of flint boulders encoun- 

 tered. The flint boulders are usually oval or somewhat flattened 

 in shape and are of varying size, some weighing several tons. Some 

 of the boulders are hollow and occasionally the cavity is filled 

 with water ; other boulders are solid, compact and of a bluish color 

 throughout. Limestone inclusions are frequent in this formation. 



Phosphate rock, although the constituent of special economic 

 interest, nevertheless makes up a relatively small part of the forma- 

 tion. The phosphate in these deposits occurs as fragmentary rock, 

 boulder rock, plate rock or pebble. The boulders are often of large 

 size, in some instances weighing several tons, and must be broken 

 up by blasting before being removed from the pit. It is also nec- 

 essary to operate a rock crusher in connection with all hard rock 

 phosphate mines to reduce the larger pieces of rock to a size suit- 

 able for shipping. The relative amount of material that it is 

 necessary to handle to obtain a definite amount of phosphate is al- 

 ways variable with each pit and with the different parts of any 

 one pit. The workable deposits of phosphate lying within this 

 formation occur very irregularly. While at one locality the phos- 

 phate may lie at the surface, elsewhere it may be so deeply buried 

 as not to be economically worked ; while a deposit once located may 

 cover more or less continuously a tract of land some acres in ex- 

 tent, elsewhere a deposit, appearing equally promising on the sur- 

 face, may in reality be found to be of very limited extent. As to 

 location, depth from surface, extent into the ground, lateral extent, 

 quantity and quality, the hard rock phosphate deposits conform to 

 no rule. The desired information is to be obtained only by exten- 

 sive and expensive prospecting and sampling. 



The materials above the phosphate deposits include pale yellow 

 incoherent sand and in some localities clayey sand. The incoher- 

 ent sands are variable, an average thickness being from 5 to 15 

 feet, although as much as 30 feet have been observed. 



In practically all published literature the hard rock phosphates 

 of Florida are referred to as of Vicksburg, Lower Oligocene age, 

 Although resting upon the Vicksburg limestone, the formation is 

 apparently of Pliocene age, for, although fossils older than 

 the Pliocene are present, yet these are derived from the formations 

 that have disintegrated. 



