4 BULLETIN 312, U. S. DEPARTMENT OF AGRICULTURE. 



Most of the rock is hauled to Tampa and Port Tampa and there 

 shipped by water to Europe or to the east coast of the United States. 

 The rock belongs to a more recent period than the hard-rock phos- 

 phate and is much more regular in its occurrence. As a whole the 

 rock consists of medium-sized, light-gray pebbles somewhat softer 

 than and not of as high grade as the hard-rock phosphates. Its con- 

 tent of tricalcium phosphate runs from 68 to 75 per cent and it con- 

 tains as a rule less than 4 per cent of iron and aluminum oxides. 



The hydraulic method of mining is practiced almost entirely in the 

 pebble regions, and on account of the uniformity of the deposits and 

 the ease with which the material can be handled the rock can be pro- 

 duced very cheaply. Pebble phosphates are by far the most exten- 

 sively mined of all the American deposits, and up to the year 1914 

 the output from these fields steadily increased. The consumption 

 of the product has heretofore been about equally divided between 

 this country and Europe. 



TENNESSEE BROWN-ROCK PHOSPHATE. 



The brown-rock phosphate of Tennessee occurs in the central part 

 of the State, extending in a general north and south direction from 

 the northern to the southern boundary line. The most important 

 deposits so far exploited occur in Sumner, Davidson, Williamson, 

 Hickman, Maury, Lewis, and Giles Counties. The deposits are 

 reached by the Louisville & Nashville and the Middle Tennessee 

 Railroads and the Nashville, Chattanooga & St. Louis Railway. 

 The deposits in Davidson and Sumner Counties have easy outlet to 

 the Cumberland River. 



The brown rock is of Ordovician age and in general consists of beds 

 of brown porous plates of varying thickness overlying the original or 

 slightly altered phosphate limestone from which it is derived. Fre- 

 quently the beds of brown rock are much disintegrated and require 

 special machinery to separate the phosphate from the impurities with 

 which it is mingled. The brown-rock phosphate, as separated by 

 mechanical means, contains from 72 to 78 per cent tricalcium phosphate 

 and from 3 to 5 per cent of iron and aluminum oxides. Practically 

 all of the brown-rock phosphates is now consumed in this country. 



TENNESSEE BLUE-ROCK PHOSPHATE. 



The important deposits of blue-rock, or Devonian, phosphate in 

 Tennessee He along Leatherwood Creek, in the western part of Maury 

 County, south and east of Centerville in Hickman County, on both 

 sides of Swan Creek in Hickman County, and in the eastern part of 

 Lewis County near Gordonsburg. The mines are reached by the 

 Louisville & Nashville and the Middle Tennessee Railroads and the 

 Nashville, Chattanooga & St. Louis Railway. The Duck River is 



