976 A TREATISE ON METAMORPHISM. 



The amount of P 2 5 in the limestones used for building purposes rather 

 than the average of all limestones is taken, as probably more nearly repre- 

 senting the average amount of phosphorus in these rocks, since it is well 

 known that in the belt of weathering the phosphorus is leached out. 



Therefore it appears that the amount of P 2 5 in the shales is about 

 three-fourths of that present in the original rocks ; in the sandstones is about 

 one-third; and in the limestones is nearly doubled. If one multiplies the 

 amount of P 2 6 in each of the sediments by their estimated quantities, and 

 adds them together, we have the following equation: 



.17X.65+.07X.30+.42X.05=.1525. 



This shows a deficiency of about one-third of P 2 5 , as compared with 

 the original rocks. A portion of this deficiency is undoubtedly accounted 

 for by the phosphate rock deposits, such as those of South Carolina, Florida, 

 and Tennessee, and by the guano deposits. These represent the economic 

 products of segregating processes which have increased the proportional 

 amount of phosphorus many fold. 



All agree that the first stage of the segregation of phosphates in the 

 sedimentary rocks is accomplished through the agency of animals. For the 

 o'uanos this first concentration is made by sea birds. For the more exten- 

 sive phosphate deposits the first concentration of phosphorus was by inver- 

 tebrate animals, such as brachiopods aud crustaceans, and by vertebrates, 

 such as sharks and saurians. Very coninionly this first concentration is in 

 limestones. The further concentration of the phosphorus of guanos and 

 that of phosphatic limestones and other rocks is by underground water. 

 The circulations producing the concentration and the forms of the resultant 

 deposits are multifarious, but the general principle applicable to most cases 

 appears to be that the phosphates are dissolved by descending waters in 

 the belt of weathering and tin-own down on reaching the belt of cementa- 

 tion.' 1 Usually the latter reaction takes place in the upper part of the belt 

 of cementation, so that the phosphates are segregated at or just below 

 the level of ground water. The precipitation of the phosphates is especially 



« Penrose, R. A. F., jr., Nature and origin of deposits of phosphate of lime: Bull. U. S. Geol. Survey 

 No. 46, 1888, pp. 1-143. Dall, W. H., and Harris, G. D., Correlation papers — Neocene; phosphatic 

 deposits of Florida: Bull. U. S. Geol. Survey No. 84, 1S92, pp. 134-140: Eldridge, Geo. H., A prelimi- 

 nary sketch of the phosphates of Florida: Trans. Am. Inst. Min. Eng., vol. 21, 1893, pp. 196-231. 

 Hayes, C. W., The Tennessee phosphates: Seventeenth Ann. Kept. U. S. Geol. Survey, 1895-96, pt. 2, 

 1896, pp. 513-550. 



