360 EEPOKT OF NATIONAL MUSEUM, 1899. 



deposits are regarded as superficial and limited to those portions of the 

 I'ock affected by surface waters. 



The origin of the amorphous, nodular, and massive rock phosphates 

 can, as a rule, be traced more directly to organic agencies. All things 

 considered, it seems most probable that the phosphatic matter itself 

 was contained in the numerous animal remains, which, in the shape of 

 phosphatic limestones, marls, and guanos, have accumulated under 

 favorable conditions to form deposits of very considerable thickness. 

 Throughout these beds the phosphatic matter would, in most cases, be 

 disseminated in amounts too sparing to be of economic value, but it 

 has since their deposition been concentrated by a leaching out by per- 

 colating waters of the more soluble carbonate of lime. Thus H. Losne, 

 in writing of the nodular phosphates occurring in pockety masses in 

 clay near Doullens (France), argues that the nodules as well as the clay 

 itself are due to the decalcification of preexisting chalk by percolating 

 meteoric waters. 



In this connection it is instructive to note that phosphatic nodules, 

 in size rarely exceeding 4 to 6 cm. , were dredged up during the Chal- 

 lenge?' expedition from depths of from 98 to 1,900 fathoms on the 

 Agualhas Banks, south of the Cape of Good Hope. These are rounded 

 and very irregular capricious forms, sometimes angular and have 

 exteriorly a glazed appearance, due to a thin coating of oxides of iron 

 and manganese. The nodules yield from 19.96 to 23.54 per cent P 2 O 5 . 

 In those from deep water there are found an abundance of calcareous 

 organic remains, especially of rhizopods. The phosphate penetrates 

 the shell in every part, and replaces the original carbonate of lime. 



The nodules are most abundant apparently where there are great 

 and rapid changes of temperature due to alternating warm and cold 

 oceanic currents, as off the Cape of Good Hope and eastern coast of 

 North America. Under such conditions, together with perhaps altered 

 degrees of salinity, marine organisms would be killed in great num- 

 bers, and by the accumulation of their remains would, it is believed, 

 furnish the necessary phosphatic matter for these nodules. It seems 

 probable that the Cretaceous and Tertiary deposits in various parts of 

 the world may have formed under similar conditions. 



Hughes has described * phosphatic coralline limestones on the islands 

 of Barbuda and Aruba (West Indies), as having undoubtedly originated 

 through a replacement of their original carbonic by phosphoric acid, 

 the latter acid being derived from the overlying guano. The phos- 

 phatic guano has, however, now completely disappeared through the 

 leaching and erosive action of water, leaving the coral rock itself con- 

 taining 70 to 80 per cent phosphate of lime. 



Hayes 2 regards the Tennessee black phosphates (Specimens Nos. 



Quarterly Journal of the Geological Society of London, XLI, 1885, p. 80. 

 "Sixteenth Annual Report of the TJ. S. Geological Survey, 1894-95, Pt. 4, p. (520; 

 Seventeenth Annual Report U. S. Geological Survey, 1895-96, Pt. 2, p. 22. 



