GEOGRAPHY OF CENTRAL FLORIDA 157 



been toward treating it as a mere product of weathering from the 

 sandy clay or rock miderlying it. There are some objections to 

 both hypotheses, however, and the question must be regarded as still 

 unsettled. 



ECONOMIC GEOLOGY 



The most important mineral resource of central Florida is phos- 

 phate rock, which is of two principal kinds, occurring in distinct 

 regions. The "hard rock," which is the highest grade, contain- 

 ing usually from 'jy'^o to 80^ of tricalcium phosphate, occurs in 

 deposits of supposed Pliocene age in the lime-sink region, chiefly 

 in Citrus County and the western part of Marion (and north of our 

 present limits in Alachua). A variety known as "plate rock" was 

 formerly mined near Anthony, which is in the same region but east 

 of the Middle Florida hammock belt. A low-grade by-product 

 known as soft phosphate was formerly discarded in mining, but 

 is now saved in some places and used as a fertilizer in its raw 

 state. 



"Land pebble," containing usually from 65 to yj^o 01* tri- 

 calcium phosphate, occurs in the Bone Valley formation (Pliocene), 

 which covers considerable areas in the flatwoods south of Lakeland 

 and Plant City. A variety known as "river pebble" was formerly 

 dredged out of the Peace River, chiefly south of our present limits. 

 Both the principal types of phosphate deposits are of considerable 

 scientific interest on account of containing many well-preserved 

 vertebrate fossils, representing sharks, crocodiles, armadillos, 

 horses, elephants, mastodons, whales, etc. 



In 1 91 3, the last full year before the export of phosphate was 

 interrupted by the great war, there were 14 companies mining hard 

 rock in Florida (some of them north of the limits of this report, 

 however), and 16 mining pebble phosphate. The total reported 

 production for the State in that year was 489,794 long tons of hard 

 rock and 2,055,482 of pebble, together valued at $9,563,084, or 

 about the same as the farm crops of central Florida in 1909. The 

 hard rock, being of higher grade, brings a higher price, and the only 

 reason the pebble can be marketed in competition with it is probably 

 that the latter can be mined more economically, on account of the 

 deposits being more continuous, the use of hydraulic mining meth- 

 ods, etc. Much of the hard rock at present mined is below ground- 

 water level and has to be taken out with a dredge. Nearly all the 



