IMPURITIES OF PHOSPHATE ROCK. yj 



the relatively insoluble and resistant elements of the earlier forma- 

 tions. 



The phosphate itself is derived from the Alum Bluff sands, the 

 later and thicker of the two formations that have disintegrated. 

 This formation, the Alum Bluff, in some places reaches a thickness 

 of several hundred feet, and has a large areal extent reaching from 

 west Florida, through northern and central Florida, into southern 

 Florida. Throughout its entire thickness, and throughout its whole 

 areal extent this formation is distinctly phosphatic, although in 

 no instance is the phosphate in this formation sufficiently concen- 

 trated to form workable deposits. 



While these formations, the Chattahoochee and the Alum Bluff, 

 were disintegrating in the area that is now the hard rock phosphate 

 region, the calcium phosphate from the Alum Bluff formation 

 was gradually being taken into solution by ground water and was 

 being redeposited at a lower level in the earth, thus forming the 

 workable hard rock phosphate deposits. In this process the replace- 

 ment of the original limestone by calcium phosphate was an im- 

 portant factor, and these deposits afford excellent illustration of 

 the formation of phosphate rock by the replacement process, the 

 shells of the original limestone in many instances retaining their 

 form, although changed chemically to calcium phosphate. In addi- 

 tion to replacement, other processes are observed, prominent among 

 whi^h is the formation of the phosphate by precipitation from solu- 

 tion in a manner similar to the formation of calcium carbonate 

 deposits in caves. This process is evidently secondary, and, being 

 now operative, is to be observed in the phosphate boulders them- 

 selves, in which all existing cavities are being gradually tilled by 

 the accumulation of calcium phosphate. By this process pinnacles 

 are formed hanging from the roof of the cavities, while succes5i\'e 

 layers of phosphate are spread out over the floor of the cavities. 

 This method of formation of phosphate deposits has given rise 

 to very high grade phosphate rock, the Florida hard rock grading, 

 under present methods of mining, 77 to 80 per cent ti'icalcium 

 phosphate, while individual specimens contain 84 to 85 per cent. 



While the origin of the hard rock phosphate in its present form 

 is thus clearly evident, there yet remains a large field of investiga- 

 tion to determine the chemical processes by which the phosphate 

 is first taken into solution, and is subsequently redeposited. Some 



