﻿4 BULLETIN 18, U. S. DEPARTMENT OF AGRICULTURE. 



GEOLOGICAL OCCURRENCE AND ORIGIN. 



The phosphate-bearing stratum belongs to the Tertiary period, but 

 geologists differ considerably regarding the exact age of the phosphate. 



Tourney 1 is of the opinion that the phosphate nodules are derived 

 from the fragments of the Eocene marl on which the beds rest. This 

 author, however, was unacquainted with the true nature of the phos- 

 phate. Holmes, 2 Shepard, 3 and Chazal 4 also think the nodules are 

 waterworn fragments of the Eocene marl enriched by the leaching 

 out of the carbonate of lime and absorption of phosphoric acid from 

 solution. These authors assign the phosphate stratum to the post- 

 Pliocene formation. Chazal points out that it is hardly likely that 

 the nodules have derived their phosphoric acid from the animal 

 remains with which they are mingled, since these remains themselves 

 have been enriched by phosphatization after deposition. Levat 5 and 

 Brown 6 — the latter quoting extensively from the former — agree with 

 the above authors concerning the origin of the phosphate, but say it 

 occurs at the Miocene horizon. Pratt 7 thinks it belongs to an even 

 more recent formation and that it is derived from the feces and 

 remains of both terrestrial and marine animals intermingled with 

 disintegrated coral and deposited in the form of a calcareous and 

 phosphatic mud. He considers the present beds the result of fresh- 

 water rivers cutting through the phosphate strata and separating the 

 more from the less valuable material. This author thinks the forma- 

 tion of these beds is still going on. Dall, 8 from an examination of 

 the fossils, states without hesitation that the phosphate is derived 

 from rocks of Miocene age and thinks it doubtful if the underlying 

 marl belongs to the Eocene. 



The phosphate occurs in the form of nodules and bowlders embed- 

 ded in a matrix of sand, clay, and calcareous mud. The beds vary 

 from a few inches to 3 feet in thickness, with an average thickness of 

 approximately 1 foot. 



The nodules average from 30 to 50 per cent of the phosphate 

 stratum, and the beds will yield from 300 to 1,500 tons of phosphate 

 per acre, with an average of about 850 tons. The beds, as a rule, do 

 not follow the coutour of the land surface, but lie nearly horizontal. 

 The overburden, therefore, varies considerably from place to place. 



Although only the upper stratum is mined, phosphate nodules are 

 found at more than one horizon. The following table of Prof. 



i Geology of South Carolina, pp. 164, 165 (1848). 



2 Phosphate Rocks of South Carolina, pp. 27-31 (1870). 



3 South Carolina Phosphates, pp. 22-24 (1880). 



i A Sketch of the South Carolina Phosphate Industry (1904). 

 ■> Industrie des Phosphates et Superphosphates, pp. 83-84. 



6 Eng. Assoc, of the South Trans. 15, pp. 58-60 (1904). 



7 Native Bone Phosphates of South Carolina, pp. 24-28 (1868). 

 s Amer. Jour. Sci., ser. 3, p. 296 (1894). 



