Blackwelder — The Geologic Role of Phosphorus. 295 



water. In humid regions, however, bacterial fermentation 

 decomposes these compounds, and the soluble resultants, in- 

 cluding phosphoric acid, are removed by rain water so rapidly 

 that no appreciable residue is left. 



On those arid islands, however, which are situated under the 

 trade winds and "horse latitudes," neither fermentation nor 

 solution is favored, and hence the guano accumulates from 

 year to year. The well-known deposits on the islets off the 

 coast of Peru, in the Leeward Islands of the Caribbean Sea, 

 and on many of the East Indian islands, serve as examples. As 

 compared with its ratio in the fishes and other marine animals, 

 the concentration of the phosphorus in the freshly deposited 

 excrement of the fish-eating birds is about the same, — averag- 

 ing but little more than one per cent P,0 5 . In the thoroughly 

 dry guano of the desert islands off the Peruvian coast, where 

 almost no chemical change has taken place, it contains 10-16 

 per cent P 2 6 , as well as a noteworthy quantity of nitrogenous 

 and other organic compounds. 



On the other hand, where underground water has access to 

 the older portion of the deposit, the guano is more or less fer- 

 mented, probably by such micro-organisms as the bacteria, with 

 the result that the nitrogeneous matter is largely converted 

 into nitrates and ammonia, while the phosphorus forms calcium, 

 magnesium and ammonium phosphates. The occasional rains 

 dissolve out the more soluble ammonium phosphates and nearly 

 all the nitrates, leaving the relatively insoluble alkaline-earth 

 phosphates to form a residue of solid " stone guano." The 

 latter contains from 28 to as much as 39 per cent P„0 5 , 

 largely in the form of hydrous acidic and basic calcium phos- 

 phates, closely, and probably chemically, associated with more 

 or less lime-carbonate, The commercially exploited guanos on 

 Baker island in the tropical Pacific, and many others, appear 

 to have passed through this type of alteration. 



The strongly phosphatic solutions thus derived from the 

 guano sink downward through the underlying rocks and pro- 

 duce characteristic alterations in them. Where the rock is 

 limestone, it is somewhat rapidly converted into a mass of 

 calcium phosphates, in which the mineral species are various, 

 although collophanite seems to predominate. On Christinas 

 Island in the western part of the Pacific Ocean, Willis* found 

 that coral limestone had been changed to calcium phosphate to 

 a depth of from 2 to 3 feet within 20 years. In the laboratory, 

 Colletf immersed a coral skeleton in a weak solution of 

 ammonium phosphate, with the result that the coral was 60 

 per cent phosphatized in only two months. Still more remark- 



* Willis, J. L., Ottawa Naturalist, vol. vi, p. 18, 1892. 



t Collet, L. W., Proc. Eoyal Soc. Edinburgh, vol. xxv, p. 882. 



