294 E. H. SELLARDS 



is more distinctly marine and contains an abundant marine inverte- 

 brate fauna. In the southern part of the state the deposits which 

 are beheved to represent the equivalent of the Alum Bluff formation 

 are distinctly phosphatic. As there developed the formation con- 

 sists chiefly of shell marl beds in which is included black, brown, and 

 white phosphate pebble. 



Deposits found on Black Creek in Clay County and referred to 

 the Jacksonville formation are lithologically very similar to the Alum 

 Bluff formation as developed in south Florida, and contain similar 

 phosphate pebble. 



The Choctawhatchee formation is later in age and overlies the 

 Alum Bluff formation. It is chiefly a loosely cemented shell marl, 

 formed in shallow water and often quite sandy. The surface 

 outcrop of this formation occupies a narrow belt extending in a 

 general east-west direction from west of the Choctawhatchee to 

 somewhat east of the Ocklocknee rivers. In determining the thick- 

 ness of the Miocene it is necessary to rely on well samples. At 

 Jacksonville the Miocene apparently extends from near the surface 

 (about 2,2, feet) to possibly as deep as 500 feet, giving for the 

 formations of this period a thickness approximating 450 feet. 

 Probably at least 400 feet of the section of the well at Fort Myers 

 is likewise to be referred to the Miocene, while in the well at 

 Okeechobee the Miocene apparently approximates 400 feet in 

 thickness. 



Phosphate in the Miocene. — ^Phosphate rock in considerable 

 quantity appears for the first time in the Lower Miocene^ (Aliun 

 Bluff formation), the phosphates of both the Alachua and Bone 

 Valley formations having been derived from this formation; the 

 hard rock chiefly by chemical, the land pebble phosphate chiefly by 

 mechanical, segregation.^ While the processes of concentration to 



' In a well at Apopka, Florida, described in the Twelfth Annual Report of the Florida 

 Geological Survey, Comanchean foraminifera have been identified by Cushman from 

 limestones or marls with which is included a considerable quantity of pebble phosphate 

 through a thickness of about 170 feet. Whether the phosphate belongs in this lime- 

 stone or has fallen down from a higher level remains to be determined. At no other 

 locality is pebble phosphate found in the Comanchean. 



^ E. H. Sellards, "Origin of the Hard Rock Phosphate Deposits of Florida," 

 Florida State Geol. Surv., jth Ann. Rept., 1913, pp. 23-66; "The Pebble Phosphates of 

 Florida," ibid., 7th Ann. Rept., 1915, pp. 25-116. 



