IMPURITIES OF PHOSPHATE ROCK. 87 



ning of mining in these deposits dates almost to the beginning of 

 the use of mineral fertilizers. The first mineral phosphate to be 

 used by the modern methods of treating with sulphuric acid were 

 made in England in 1841, while plans for using the South Caro- 

 lina phosphates were made as early as i860, and actual mining 

 began in 1S67. 



The grade of rock produced in the South Carolina fields, for 

 rock properly washed and dried, permits a guarantee of about 60 

 per cent tricalcium phosphate. The age of the South Carolina - 

 phosphates^ as is often the case with this class of deposits, is hard 

 to determine, owing to the mixed character of the beds, for, while 

 the phosphates rest upon the Eocene marl, they are themselves of 

 later date. 



ARKANSAS. 



The Arkansas phosphates that are being worked are found as 

 bedded deposits within shales lying between limestones. The over- 

 lying formation, the St. Clair limestone, according to Purdue,* is 

 of Silurian age, while the underlying formation, the Polk Bayou 

 Limestone, is of Ordovician age. The shales, within which the 

 phosphate occurs, are believed to be the top member of the Ordo- 

 vician. The phosphate beds are variable in character, ranging 

 frorn those that are brown and sandy and of low grade to those 

 that on the fresh surface are blue gray, apparently without sand, 

 and of uniform texture and color. Two beds are usually present. 

 The upper bed, the only one being worked, is described by Purdue 

 as a compact, homogeneous, light gray rock. The color is said 

 to be due to small white particles resembling fragments of bone 

 that are thoroughly mixed with a grayish dark material. The 

 gray material is made up of particles of varying size, some so 

 small that they can be seen only with the lens, while others are as 

 large as one-fourth inch in diameter. These particles are more or 

 less angular, some of them distinctly so, making the stone con- 

 glomeratic in character. Near the surface exposure of the rock, 

 the lime has been leached out by surface waters and the rock ap- 

 pears black in color. This rock is richer in phosphate than un- 

 weathered material. 



The lower bed of phosphate, which is not being worked, is sim- 

 ilar in character to the upper, though darker in color, more com- 



*U. S. Geol. Survey, Bull. 215, pp. 463-483, 1907. 



