IMPURITIES OF PHOSPHATE ROCK. 79 



posits is not necessarily strikingly different from that of the hard 

 rock deposits, except as chemical action has modified the residue, 

 particularly by the formation in many instances of siliceous boul- 

 ders in the hard rock deposits. 



The grade of rock produced from the land pebble deposits 

 under the present methods of mining varies from 66 to 74 per ceni 

 tricalcium phosphate^ while individual samples contain from jy 

 to 78 per cent. 



THE TENNESSEE PHOSPHATE DEPOSITS. 



As further evidence of the complexity of the origin of work- 

 able beds of phosphate, and of the diversity of ways in which the 

 deposits may accumulate, may be mentioned the phosphates of Ten- 

 nessee. 



The broAvn phosphates of Tennessee are very evidently formed 

 in situ from phosphatic limestone. Hayes and Ulrich* find that 

 at least four limestone horizons have given rise to brown phos- 

 phates in Tennessee. The calcium carbonate from the limestone 

 is more or less completely leached out, and is replaced in part at 

 least by calcium phosphate. f The rock is thus enriched and be- 

 comes a workable phosphate. The leaching of the rock usually 

 begins along jointing planes and for this reason unchanged masses 

 of the original limestone in this type of deposit frequently remain 

 as "horses." Two types of deposits are recognized, which are 

 known as "blanket" and "collar" deposits. The blanket deposits 

 are those which extend over a considerable area ; the collar depos- 

 its are formed where the phosphatic limestone comes to the surface 

 around the slope of a hill. The collar deposits are necessarily 

 limited in extent, while the blanket deposits may cover consider- 

 able areas. The brown phosphates, from their manner of origin, 

 have necessarily accumulated in comparatively recent times. 



The blue phosphates, on the other hand, are much older than 

 the brown, having accumulated in their present form during Dev- 

 onian time. It is believed by Hiayes and Ulrich that the blue 

 phosphates were originally formed as residual material from, and 



*Columbia Folio, U. S. Geol. Surv., p. 5, 1903. 



tSome of the brown phosphate of Tennessee is formed, according to Dr. 

 A. F. Rogers, by replacement of crinoidal limestone by calcium phosphate 

 (Personal letter. April i, 1914.) 



