THE NONMETALLIC MINERALS. 



J61 



62574 and 02781, U.S.N.M.) as duo to the slow accumulation on sea bot- 

 toms of phosphatic organisms (LinyuUf), from which the carbonate of 

 lime was o-radually removed by the leachino- action of carbonated waters, 

 leaving- the less soluble phosphate behind. The white bedded phosphates 

 of Perry County (Specimen No. 520()0, U.S.N.M.), in the same State, are 

 regarded as a product of secondary replacement — that is, as due to phos- 

 phate of lime in solution, replacing the carlwnate of lime of preexisting 

 limestones, as in the case noted above. The source of the phosphoric 

 acid, whether from the overlying C-arboniferous limestones or from the 

 older Devonian and Silurian rocks, is not, however, in this case apparent. 

 Teall has shown ^ that some phosphatic rocks from Clipp<M-ton Atoll, 

 in the northern Pacific, are trachytes in which phosphoric acid has 

 replaced the original silica. The replacement he regards as having 

 been effected through the agency of alkaline (principally ammonium) 

 phosphate which has leached down from overlying guano. A micro- 

 scopic examination of the rock in thin sections showed that the replacing 

 process began with the interstitial matter, then extended to the feldspar 

 microlites, and lastly the porphyritic sanidin crystals. The gradual 

 change in the relative proportion of silica and phosphoric acid, as shown 

 by analyses of more or less altered samples, is shown below. No. I being 

 that of the unaltered rock and II and III of the altered forms: 



Constituents. 



SiOo 



PoOs 



Loss on ignition 



54.0 



8.4 

 3.8 



43.7 

 17.0 

 12.3 



2.8 

 38.5 

 23.0 



From a comparison of th(\se rocks with those of Redonda, in the 

 Spanish West Indies, it is concluded that the latter phosphates have 

 likewise resulted from a similar replacement in andesitic rocks. (Speci- 

 mens Nos. 53148 to 53152, U.S.N.M.) In this connection reference 

 is made to the work of M. A. Gautier,^ in which he describes the 

 formation of aluminous phosphates in caves through the action of the 

 ammonium phosphate arising from decomposing organic matter on 

 the clay of the floor of caverns. (See undei: Occurrences.) 



The guanos, as noted elsewhere, owe their origin mainly to the accu- 

 mulations of sea-fowl excretions. Such deposits when unleached, are 

 relatively poor in phosphatic matter and rich in salts of ammonia. 

 Where, however, subjected to the leaching action of rains the more 

 soluble constituents are carried away, leaving the less soluble phos- 

 phates, together with impurities, in the shape of alumina, silica, and 

 iron oxides to form the so-called leached guanos of the West India 

 Islands. As stated in the descriptions of localities, guano deposits are 



' Quarterly Journal of the Geological Society of London, LIV, 1898, p. 230. 

 ^Formation des Phosphates Naturels d' Alumina et de Fer, Comptes Rendus de 

 I'Academie des Sciences, Paris, CXVI, 1893, p. 1491. 



