400 Transactions. — Geology. 



Origin of Eock-phosphate. 



Bock-phosphate consists of tricalcium phosphate, which 

 has the formula Ca 3 P 2 5 . It is often called " bone-phos- 

 phate," because it is the substance of which bone is composed. 



The bones of all vertebrate animals contain about 60 per 

 cent, of tricalcium phosphate, while the excrement of some is 

 also rich in the same substance. 



Although invertebrates rarely contain phosphate of lime, 

 there are some notable exceptions— namely, the Brachiopods 

 Lingula and Orbicula, also Conularia, Serpulites, and some re- 

 cent and fossil crustaceans Hence deposits rich in phos- 

 phoric anhydride (P 2 5 ) are found in rocks of all ages, from the 

 Lauren tian up to nearly the Becent period. 



The formation of phosphate-deposits is generally believed 

 to have been due to the leaching or lixiviation of phosphate- 

 bearing rocks by waters containing carbonic and other organic 

 acids, followed by the subsequent concentration of the phos- 

 phate under favourable conditions. In some cases they de- 

 posited their calcium phosphate in caverns formed in limestone 

 or calcareous sandstone, and the subsequent removal by solu- 

 tion of the walls of the caverns, either wholly or partially, left 

 the phosphate in the remaining sands. 



It may be of some interest to note that the apatite beds 

 and veins of Ottawa, in Canada, occur in rocks of Laurentian 

 age. The brown rock-phosphate of Tennessee is believed to 

 have been derived from the weathering of certain phosphatic 

 layers in the Lower Silurian limestone which forms the basin of 

 middle Tennessee. These layers do not occupy an unvarying 

 stratigraphical position, but occur in various horizons in the 

 Lower Silurian formation. * 



The phosphate-deposits in the South of England, in France, 

 and Belgium occur associated with Cretaceous chalk. Those 

 of Algeria and Tunis are of Eocene age, the phosphates oc- 

 curring in nodules in marl or as phosphatic limestone. In 

 Algeria, which has been estimated by M. Chateau, a French 

 mining engineer, to contain from 150,000,000 to 300,000,000 

 tons of phosphate rock, it is considered risky to mine rock 

 under 60 per cent, of the tricalcic phosphate.! 



The celebrated phosphate-deposits in Peninsular Florida 

 occur in detached pockets in the uneven surfaces of an 

 Eocene limestone, and in Western Florida on Miocene lime- 

 stone, under geological conditions which seem almost the 

 same as those existing on the Horseshoe Estate at Clarendon. 



* William Hayes, Annual Rept. U.S. Geol. Survey, 1898-99, p. 633. 

 t " Memoirs of the French Society of Civil Engineers," August, 1897. 



