THE NONMETALLIC MINEBALS. 367 



deposit in a soft matrix of phosphatic sands, clays, and other material, 

 resulting from the disintegration of the hard rock and constituting the 

 soft phosphates. The deposits underlie sands of from 10 to 20 feet in 

 thickness, and have been penetrated to a depth of 60 feet. The phos- 

 phate deposit proper is white, the bowlders of rounded and irregular 

 outline, varying in diameter from 2 or 3 inches to 10 feet. None of the 

 hard-rock deposits of the Eocene originated in the positions they now 

 occupy. The Miocene hard-rock phosphates, on the other hand, lie 

 in regular bedded deposits in situ, as well as in bowlders. The beds 

 lie horizontal but a few feet below the surface, being covered only by 

 superficial sand. The beds as a rule are but from 4 feet to 5 feet thick. 

 The name soft rock, or soft phosphate, as above indicated, is given to 

 the softer material associated with the hard rock, which in part 

 results from the disintegration of the last named. It is also applied 

 somewhat loosely to any variety not distinctly hard. It therefore 

 varies greatly in color, chemical and physical characteristics, and rarely 

 carries more than 20 to 25 per cent of P 2 O 5 (Specimens Nos. 67304, 

 67319, 67293, 67296, 67297, U.S.N.M.). 



The name land-pebble phosphate includes pebble from deposits con- 

 sisting of either earthy material carrying fossil remains, grains of 

 quartz, and pisolitic grains of lime phosphate, or else of a material 

 resembling in texture and other characteristics the hard-rock phos- 

 phate. The individual pebbles vary in size up to that of the English 

 walnut, are normally white, but when subjected to percolating water 

 become dark gray or nearly black. The exteriors are quite smooth 

 and glossy, colors and textures uniform, and average some 30 to 35 

 per cent P 2 O 5 (Specimen No. 61070, U.S.N.M.). 



The river-pebble varieties differ from the last mainly in mode of 

 occurrence, being found, as the name would indicate, in the beds of 

 streams, where presumably they have accumulated through the wash- 

 ing away of finer and lighter materials. The}' are most abundant in 

 the Peace, Caloosahatcb.ee, Alafia, and other rivers entering the Gulf 

 south of Tampa and Hillsborough bays, though the Withlacoochee, 

 Aucilla, and rivers of the western part of the State, carry also a mix- 

 ture of pebbles, hard-rock fragments, and bones derived from the vari- 

 ous strata through which they have cut their channels. The pebbles 

 of the Western rivers show a very uniform composition, and range from 

 25 to 30 per cent phosphoric anhydride (P 2 O 5 ), or about 65 per cent of 

 phosphate of lime, the impurities being mainly siliceous matter, car- 

 bonate of lime, alumina, and iron oxides (Specimens Nos. 67299, 67298, 

 67355, U.S.N.M.). 



Phosphates the mineralogical nature of which does not seem to be 

 as yet accurately made out occur in the Devonian Shales of Middle 

 Tennessee. They are thus described by Professor Safford: 1 



Engineering and Mining Journal, LVII, April 21, 1894, p. 366. 



