368 REPOKT OF NATIONAL MUSEUM, 1899. 



There are two distinct beds of the phosphates, one above a stratum 

 known as the black shale; the other below the shale. The one ubovo is u 

 bed or layer of concretionary masses, balls, and kidney and knee-shaped 

 forms from the size of walnuts to that of a man's head (Specimen No. 

 52059 from Hickman County). These are sometimes loosely disposed 

 in a greenish or bluish shale, and sometimes tightly packed together 

 like so many cannon balls in a layer 8 or 10 inches thick. Ordinarily 

 the layer has less thickness, often, in fact, being represented by only 

 a few scattered concretions. But, thick or thin, it may be said to be 

 universally present, its kidneys serving to indicate the place of the 

 black shale and the underlying bed when these are concealed by debris 

 or soil. 



The other phosphate, that underlying the shale, and the more impor- 

 tant of the two, is, in its best presentations, a well-defined, continuous 

 stratum of dark-bluish or bluish-black rarely grayish rock, with fine 

 or coarse grain. Its regularly stratified character and its dark color 

 make it look like a bed of stone coal. 



The geographical distribution and general geology of these phos- 

 phates has been worked out in considerable detail by C. W. Hayes, 

 to whose papers reference has been already made (p. 360). Accord- 

 ing to this authority the phosphates occur in four distinct varieties: 

 (1) Black nodular phosphate; (2) black bedded phosphate; (3) white 

 breccia phosphate, and (4) white bedded phosphate. The first two of 

 these are of Devonian age, the third is a secondary and comparatively 

 recent deposit, while the fourth, perhaps also of secondary origin, is 

 interbedded with rocks of Carboniferous age. The black nodular 

 variety contains from 60 to 70 per cent of phosphate of lime, and is 

 found in commercial quantities only in the region of the black bedded 

 phosphate in western middle Tennessee. The black bedded variety, 

 which is the only one that has thus far proved of commercial importance, 

 is confined, so far as at present known, "to an oval area southwest of 

 Nashville, having Centerville about in its center." It also covers 

 portions of Hickman, Williamson, Maury, Lewis, Wayne, Perry, and 

 Decatur counties. 



Sections showing the relation of the phosphates to the adjacent for- 

 mations are given in Dr. Hayes's paper. The beds vary in thickness 

 from a fraction of 1 to 8 or 10 feet, the average run of the rock being 

 about 50 per cent phosphate of lime. The white bedded and white 

 breccia phosphates are limited to small areas in Perry County. Their 

 contents of phosphoric acid (P 2 O 5 ) is low, varying from 14 to 15 per 

 cent, and as yet their value for other than local purposes is to be deter- 

 mined. (See especially Specimens Nos. 52058, 52060, 52061, U. S. N. M. ) 



England. Deposits of phosphates sufficiently concentrated for com- 

 mercial purposes lie near the upper limit of Cambro- Silurian strata in 

 North Wales. According to Davies, the phosphatic material occurs 



