76 FLORIDA GEOLOGICAL SURVEY SIXTH ANNUAL REPORT. 



While the round of circulation of phosphate minerals is thus 

 capable of demonstration as a normal process comparable to that 

 of other common minerals, yet the actual processes of the accumu- 

 lation of large workable deposits of phosphate rock are in many 

 ways complicated. 



THE FLORIDA PHOSPHATE DEPOSITS. 



The complexity of origin of the phosphate rock, and the man- 

 ner m which impurities are included in the formation, is well illus- 

 trated by the Florida phosphate deposits. Of these there are two 

 distinct types known respectively as the hard rock and the land 

 pebble phosphates. These differ materially in their location, origin 

 and manner of occurrence. The hard rock phosphates lie in a belt 

 along the Gulf side of the peninsula, extending in a general north 

 and south direction roughly paralleling the Gulf coast for a dis- 

 tance of about 100 miles. The land pebble phosphate deposits are 

 found farther south, lying chiefly in Polk and Hillsboro counties. 



The hard rock phosphate deposits rest upon a thick and very 

 pure, light-colored, porous and cavernous limestone known as the 

 Vicksburg formation, which is of Lower Oligocene age. At the 

 present time, in that section of the State in which the hard rock 

 phosphate occurs, no formation other than the phosphate itself lies 

 on top of this limestone. It has, however, been demonstrated by 

 the combined observations of several geologists that certain forma- 

 tions, of which only a residue remains, formerly extended across 

 the area that now holds the hard rock phosphate deposits. The 

 formations referred to are the Chattahoochee Limestone and Alum 

 Bluff sands, both of which are* of Upper Oligocene age. These 

 formations are now found bordering the hard rock phosphate area. 

 The proof of their former extent has been given elsewhere and 

 need not be repeated at this time.* The hard rock phosphate depos- 

 its are made up largely of the residue of these formations which 

 have disintegrated in situ and accordingly consist of a mixture 

 of materials of the most diverse character including sands, clays, 

 limestone fragments, pebbles and water worn flints, vertebrate, in- 

 vertebrate and plant fossils; in fact a heterogeneous mixture of 



*Origin of the Hard Rock Phosphate Deposits of Florida, by E. H. Sel- 

 lards. Fifth Annual Report, Florida State Geological Survey, pp. 23-80, 1913. 



