REDISTRIBUTION OF PHOSPHORUS. 977 



likely to occur in limestone. Eldridge suggests that under such circum- 

 stances the precipitation is brought about "by the simple interchange of 

 bases between the phosphate and carbonate of lime thus brought together, 

 or by the lowering of the solvent power of the waters through loss of car- 

 bonic acid. The latter would happen Avhenever the acid was required for 

 the solution of additional carbonate of lime, or when, through aeration, it 

 should escape from the water. The zone of phosphate deposition was 

 apparently one of double concentration, resulting from the removal of the 

 soluble carbonate thus raising the percentage of the less soluble phosphate, 

 and from the acquirement of additional phosphates of lime from the over- 

 lying portions of the deposit." a 



The precipitated phosphate is likely to be deposited in nodules. This 

 material is more resistant than the containing limestone. Through erosion 

 by streams or ocean there may be a further concentration of the phosphates 

 due to the greater resistance of the phosphate to both solution and 

 mechanical wear as compared with limestone. 



The bedded Devonian phosphates of Tennessee are in part, according 

 to Hayes, an important exception to the above, the phosphatic material of 

 the beds being regarded by him as largely concentrated when originally 

 laid down. 6 



Whether or not the sedimentary rocks rich in phosphates not considered 

 in the analyses of shales, sandstones, and limestones are sufficient to account 

 for the deficiency in these rocks in phosphorus is undetermined. In this 

 connection it should be remembered that the vein phosphates of Canada, 

 Norway, and similar phosphatic rocks, in which a large amount of phos- 

 phorus is concentrated probably in consequence of original igneous or 

 j)egmatitic processes, are also excluded from the original rocks, and in so 

 far as these deposits occur they offset the concentrations of the sedimentary 

 phosphatic deposits. 



The redistribution of phosphorus by the ordinary metamorphic processes 

 has as yet been little studied. In the iron-ore deposits in which phosphorus 

 is a very important economic compound such studies have been begun, but 

 as yet no g-eneral conclusions have been reached. 



"Eldridge, Geo. H., cit . , p. 216. 



6 Hayes, C. W., and Ulrich, E. O., Description of the Columbia [Tennessee] quadrangle: Geologic 

 Atlas U. S., folio 95, IT. S. Geol. Survey, 1903, pp. 4-6. 

 MOK XLVII — 04 62 



