IMPURITIES OF PHOSPHATE ROCK. 8l 



deposits formed from guano, the phosphate is taken in solution 

 by rain water^ and after being carried to a lower level is redepos- 

 ited, replacing the carbonate of the limestone. The rapidity with 

 which this process may be carried on is illustrated by an instance 

 cited by Dr. Albert R. Ledoux* in which limestone on one of the 

 South Pacific islands was observed to have been changed to phos- 

 phate to a depth of several feet within a period of twenty years, 

 the phosphoric acid in this instance being leached by rain water 

 from recently deposited guano. 



MINING. 



Phosphate rock is mined either by open pit or by underground 

 mining. Those deposits having a removable overburden are mined 

 by the open pit method, underground mining being resorted to only 

 for deposits interstratified with other formations, so that the over- 

 burden cannot be removed. 



UNDERGROUND MINING. 



The deposits worked in America by underground mining in- 

 clude the blue rock of Tennessee, the Arkansas deposits, and for 

 the most part the extensive deposits of the western United States, 

 which are as yet but little developed. In underground mining, 

 ordinarily, operations begin at the surface outcrop of the phos- 

 phate straturn, the first rock being uncovered by stripping off the 

 overburden. When the overburden can no longer be removed eco- 

 nomically, drifts are run into the bank and the phosphate rock 

 removed, support being given to the roof, when necessary, after 

 the phosphate is taken out. This method of mining is similar to 

 that used in mining coal seams. In the Arkansas and Tennessee 

 m.ines the phosphate rock is first drilled and blasted. It is then 

 broken up by pick and loaded into tram-cars to be drawn from the 

 mine. • 



OPEN PIT MINING. 



By far the greater part of the phosphate rock produced in 

 America is obtained at present by open pit mining, in which the 

 overburden is first removed from the rock. The purity of the rock, 



*Trans. N. Y. Acad. Sci., Vol. 9, p. 85, 1890. 



