54 MANUFACTURE OF FERTILIZING MATERIALS 



lime to phosphate of lime. An instance of this is 

 cited in which limestone in one of the South Pacific 

 islands was believed to have been changed to 

 phosphate to a depth of several feet within a 

 period of twenty years. The phosphoric acid in 

 this instance was leached by rainwater from re- 

 cently deposited guano. The belief has been ex- 

 pressed that the phosphoric acid of the phosphates 

 has been derived directly from bird guano. The 

 local character of the bird rookeries determines 

 the local occurrence of phosphate rock. 



Whether the hard rock phosphates of Florida 

 resulted from a superficial and heavy deposit of 

 soluble guano, or from the concentration of phos- 

 phate of lime already widely and uniformly dis- 

 tributed throughout the mass of original rock, or 

 from both of these sources is regarded as a difficult 

 question. The writer does not believe, however, 

 that the bird-guano theory will account for these 

 widely disseminated phosphates any better than 

 for the intensely localized hard rock phosphates. 

 The key to the solution of the hard-rock phosphate 

 problems is found, in the writer's opinion, in a 

 study of the geological history of each particular 

 section of the world's deposits. 



However, the evidence of the formation of phos- 

 phate by the transformation of carbonate of lime 

 into the phosphate of lime is entirely incontroverti- 

 ble, since many of the boulders retain the original 

 calcareous shells now phosphatized. The evidence 

 of subsequent secondary deposition in the cavities 



