GEOGRAPHY OF CENTRAL FLORIDA 157

been toward treating it as a mere product of weathering from the sandy clay or rock underlying 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 phosphate rock, which is of two principal kinds, occurring in distinct regions. The "hard rock," which is the highest grade, containing usually from 77% to 8o% 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 77% oi tricalcium 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 1913, 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 methods, etc. Much of the hard rock at present mined is below groundwater level and has to be taken out with a dredge. Nearly all the