•_ ,( .m; Blackwelder — The Geologic Role of Phosphorus. 



able cases have been reported in which refractory volcanic 

 rocks such as andesite and trachyte have been phosphatized by 

 solutions descending from guano beds. 



For many years it was generally supposed that the rich and 

 important phosphate deposits of Florida and the Carolina coast 

 had been produced by solutions from guano beds percolating 

 down into limestones, and thus changing them into calcium 

 phosphate ; but now it is fairly well established that the phos- 

 phatic solutions were derived not from guano, but from 

 marine clays, containing 1-5 per cent P 2 6 . It has been 

 demonstrated that these clay beds originally overlie the lime- 

 stone but have been rather generally stripped off by erosion in 

 more recent time. Although poor in phosphates the clays are 

 hundreds of feet thick and have, therefore, yielded a vast 

 amount of phosphoric acid. In other countries, there are many 

 illustrations of the phosphatized limestone type, and in most 

 cases, as in Florida, the source of the phosphorus was not 

 guano, but a lean phosphatic clay or chalk. The plateau of 

 southern France, the Lasne valley in Germany, and south- 

 western Belgium furnish well-known examples. 



Where the slightly phosphatic original rock was chalk or 

 limestone, a variation of the process has been brought about 

 because the lime carbonate is relatively more soluble than the 

 lime phosphate. Therefore, during the slow process of solution 

 by rainwater descending from the surface, the calcium phos- 

 phate, although actually decreased in total quantity, has been 

 relatively concentrated by differential solution. Cases of this 

 kind have been reported from southern England, northern 

 France, and Belgium. 



Phosphate beds of both of these secondary types are irregular 

 in thickness, and rest upon a most uneven and cavernous sur- 

 face of the corroded limestone beneath. The deposits often 

 contain phosphatized bones and shells of animals really belong- 

 ing to a geologic age much more recent than the limestone of 

 which they appear to form a part. These have slumped in 

 from the surface and been thus mixed with what is in reality 

 only a special type of residual soil. 



Although the concentration of phosphorus thus brought 

 about by the phosphatization and differential solution of lime- 

 stones is never quite equal to that in the leached guanos, it may 

 rise to over 36 per cent P 2 6 , a ratio which indicates almost 

 pure collophanite. Sellards* finds reason to think that the 

 amorphous and probably colloidal mineral collophanite is 

 gradually converted into a fibrous crystalline mineral (staf- 

 felite ? ) which has about the same composition. If this opinion 



*Sellards, E. H., 5th Ann. Eep. Florida State Geol. Survey, 1913, pp. 

 37-66. 



