Blackwelder — The Geologic Rdle of Phosphorus. 287 



evidently pegmatites. In similar veins some other phosphatic 

 minerals, such as the valuable rare-earth phosphates monazite 

 and xenotime, have been found, but they are decidedly rare. 



Certain veins of much less frequent occurrence than the 

 apatites consist largely or even entirely of the related minerals 

 dahllite or staffelite, — considered by the French mineralogist 

 Laeroix* to be hydrous calcium carbo-phosphates, containing 

 about 39 per cent P„0 6 . The mineral occurs in lamellar and 



Fig. 1. 



Str, 



APATITE IN ""^"=-^5i 

 IGNEOUS ROCK 



r/a/ious veins 



ell e*J?i _--""--.,. 



' ANAMOKPHIC APATITES 



Fig. 1. Diagram illustrating the cycle of changes through which the 

 element phosphorus is believed to pass. Solid phosphatic deposits are shown 

 in bold-faced capitals and by cross-hatching. Arrows indicate the directions 

 in which the more important processes move, but many changes of less 

 importance have been eliminated for the sake of simplicity. 



radiating aggregates of very minute fibrous crystals, rather 

 than in coarse stout prisms like those of apatite. Although a 

 vein of this type near Crown Point, New York, has been 

 described by Emmons,f the only large and well-known exam- 

 ples are those of the province of Estremadura^: in Spain, where 

 they have been recognized for nearly 150 years. Several dif- 

 ferent hypotheses have been advanced to account for these 

 Spanish veins, but most of them are clearly inadmissible. The 

 facts that the veins are persistent with depth, contain quartz, 

 and traverse such slightly altered rocks as quartzite, slate and 

 limestone associated with granitic intrusions, suggests that 

 they have crystallized from ascending magmatic solutions but 

 at a moderate temperature. It is not certain, however, that 

 they have not been produced by waters descending from the 

 surface. 



Under ordinary climatic conditions, rocks near the surface 

 of the earth are subject to chemical decomposition. Percolat- 



* Laeroix, A , Mineralogie de la France, vol. iv, p. 555 etseq. 



f Emmons, E., New York State Geological Survey, Report, 1838, p. 252. 



JFuchs and de Launay, Traite" de Min. et Met., 1893, p. 353. 



