86 FIRST PRINCIPLES OF AGRICULTURE. 



South Carolina Rock Phosphates. — These are found 

 both on the land and in the beds of rivers in the vicinity 

 of Charleston, S. C, and are sometimes called " Charles- 

 ton Phosphates." The deposits vary in thickness from 

 one to twenty feet, through which the phosphate is dis- 

 tributed in the form of lumps or nodules, ranging in 

 weight from an ounce to over a ton. These nodules 

 are irregular, non-crystalline masses, often full of holes, 

 which contain clay or other non-phosphatic materials. 

 That obtained from the river is called " river phosphate," 

 or "river rock;" and that from the land, "land phos- 

 phate," or " land rock." The two varieties do not dif- 

 fer materially in composition, particularly in the content 

 of phosphoric acid. 



The rock contains from twenty-six per cent to twenty- 

 eight per cent of phosphoric acid. Its uniformity, in 

 connection with the fact that it contains but small per- 

 centages of compounds of iron and alumina, minerals 

 which prevent its best use by the manufacturer, makes 

 it a highly satisfactory source of phosphoric acid. 



The river rock is secured by dredging; that from the 

 land is largely dug. In either case, it is washed to 

 remove the adhering matter, and then dried, when it is 

 ready for grinding or shipment. South Carolina rock 

 phosphate, when very finely ground, is called "floats." 

 It is sometimes used upon the land in this form. 



These deposits were first worked in 1868, though the 

 presence of phosphate at this point was known at a 

 much earlier date. 



Florida Phosphates. — The presence of phosphate in 

 commercial quantities in Florida was discovered in 1888, 



