596 Mansfield — Western Phosphates of United States. 



of Permian strata in the regions named farther east. It 

 seems at least reasonable, therefore, to attribute the 

 thickness and richness of the phosphatic strata to long- 

 continued, slow deposition under conditions which 

 excluded for considerable intervals of time the accumula- 

 tion of terrigenous material and of carbonate of lime. 



(4) The ordinary processes of bacterial decay give rise 

 to ammonium phosphate which, according to Clarke, 17 

 has been experimentally shown to react upon mineral 

 gubstances in such manner as to produce phosphates 

 resembling those actually found. Blackwelder 18 states 

 that such experiments have been carried out by several 

 investigators and that the conditions are such as may 

 readily occur on the sea bottom where organic decom- 

 position is in progress. Thus calcareous shells become 

 phosphatized and even such organic material as excre- 

 tory pellets and bits of wood are known to have been 

 altered in the same way. Bones which initially contained 

 about 58 per cent of tricalcium phosphate have their 

 organic matter replaced by phosphatic minerals, thus 

 raising the ratio to 85 per cent or more. 



(5) The oolitic texture so characteristic of much of the 

 Western phosphate is doubtless closely connected with 

 the origin of the rock. In a well presented discussion 

 of the origin of oolites Brown 19 concludes that the older 

 oolitic beds of Pennsylvania were probably all originally 

 laid down as beds of calcareous oolites composed of the 

 mineral aragonite. This mineral being unstable under 

 ordinary rock-forming conditions soon began to change. 

 Where solutions carrying other substances such as silica 

 or iron were present the oolites were more or less com- 

 pletely replaced, as in the case of the siliceous oolites or 

 of the Clinton iron ore. 



(6) Calcareous oolites are now forming at a number of 

 places, notably in the region of the Florida keys and the 

 Bahamas, where they have been studied by Drew 20 and 



17 Clarke, F. W., The data of geochemistry, 3d ed., U. S. Geol. Survey, 

 Bull. 616, p. 523, 1916. 



18 Blackwelder, Eliot, this Jour., vol. 42 (see above), p. 291. 



19 Brown, T. C., Origin of .oolites and the oolitic texture in rocks, Geol. 

 Soc. America Bull., vol. 25, pp. 745-780, pis. 26-28, 1914. 



20 Drew, G. H., On the precipitation of calcium carbonate in the sea by 

 marine bacteria, and on the action of denitrifying bacteria in tropical and 

 temperate seas, Carnegie Inst., Washington, Pub. 182. Papers from the 

 Tortugas laboratory, vol. 5, pp. 9-53, 1914. 



