L. G. Johnson — Phosphate Fields of Florida. 499 



swamps by which it is connected with Orange Lake. The 

 elevated ridges of this section are, like the Flatwoods region of 

 Lake Nnnan, Miocene. The depressions usually expose Vicks- 

 burg rocks, silicin'ed. These outcrop in many old sinks upon 

 Payne's Prairie and about the various ponds. None however 

 on the hills, and none at the northern end of Orange Lake 

 and Lockloosa Lake, its northeastern extension. The southern 

 end of Orange Lake, and Tuscawilla Lake — separated from it 

 by hammock ridges 100 feet high — expose Eocene rocks. 



Orange Lake — like so many others which have an Eocene 

 bottom — has a subterranean outlet. And less than a mile 

 north of this " sink," it has within its basin, a " rise " — one of 

 the great springs, so common in the Yicksburg formation of 

 Florida. The lake has its overland outlet also in time of high 

 water, through Orange Creek into Oklawaha River. Over all 

 the foregoing region it is evident that great denudations have 

 taken place, and that from 40 to 100 feet of Miocene marls 

 and later sands have been removed : a history only to be under- 

 stood upon the Hilgard hypothesis of an elevation of 600 feet 

 and over, in Post-Tertiary times. 



It is in the elevated region about Gainesville, the " Land- 

 pebble " phosphates of Alachua occur. The derivation of 

 these pebbles can hardly have been from beds of compact 

 stone, whatever theory may obtain for other parts of the 

 State. For several reasons : 



First. The elevation assigned the pebble beds of this region 

 is 150 feet above tide water. Islo known beds of compact 

 hard rock occur above 75-100 feet. 



Secondly. The pebbles are largely fossil casts or fragments 

 of such casts; and it may well be assumed they had their 

 origin in a fossiliferous marl, which by disintegration and 

 removal of the calcium carbonates, left behind the less soluble 

 phosphates. The aggregation of phosphatic matter in fossil 

 forms contained in marls is well known. According to Dr. 

 Eugene A. Smith, all the shell casts in Cretaceous rocks, as 

 well as in those of Tertiary age, of Alabama and Mississippi, 

 are phosphatic. 



Third. Compact phosphate is not composed of fossils ; nor 

 are its beds especially fossiliferous ; though phosphatic rock 

 containing fossils may locally occur by metamorphosis of the 

 original limestones upon which phosphate beds were deposited. 



P. The "Plate-rock"" Deposits of Eastern Marion County. 



The term " plate-rock " is the mining term applied to frag- 

 mentary thin flat pieces of "Hard-rock" phosphates. It anal- 

 yzes as high as any, namely : about 80 to 85 per cent tribasic 

 calcium phosphate. 



