88 FLORIDA GEOLOGICAL SURVEY SIXTH ANNUAL REPORT. 



pact, and, so far as ol^served, not conglomeratic. The dark color 

 is believed to be due to the smaller amount of white material and 

 possibly to a larger amount of iron and manganese. The two beds 

 are separated by a thin layer of manganese iron ore. In addition 

 to the developed locality more or less phosphatic material occurs 

 as nodules and pebbles in the Devonian shales and sandstones.-'' 



The phosphate rock that has been produced in Arkansas is said 

 to average about 65 per cent tricalcium phosphate. No phosphate 

 rock was produced in Arkansas during the year 1913.! 



TENNESSEE. 



In Tennessee several more or less distinct types of phosphate 

 occur, only two of which, however, blue rock and brown rock, are 

 being mined at present. The blue phosphate occurs as a bedded 

 deposit within, but forming the lowest member of the Chattanooga 

 formation of Devonian age, and resting directly upon Ordovician 

 limestone. The blue phosphates are said to vaiy in grade from 

 about 30 to 85 per cent tricalcium phosphate. Iron and aluminum 

 in the better grade of rock aggregate less than three per cent. The 

 beds vary in thickness from zero to 50 inches, although the bed 

 furnishing high grade rock rarely exceeds 20 inches in thickness. 



The brown rock of Tennessee is at the present time of greater 

 commercial importance than the blue rock. This rock occurs in 

 irregular deposits lying near the surface and resting on limestone. 

 The overburden consists usually of residual material including clay, 

 some phosphate and a limited amount of sand. Its thickness is 

 extremely variable, although averaging 8 or 10 feet. The phosphate 

 consists of a shelly rock, breaking up into pieces of \'arying size, 

 and of small variegated pebbles which in mass have a dark brown 

 color. Locally the small pebble rock predominates, forming a 

 scarcely coherent mass having a brownish color. A phase of the 

 material, known locally as "muck" dqDosit. consists of a mass of 

 small pebbles, often not exceeding a pin head in size. The under- 

 lying limestone has an irregular top surface, and not infrequently 

 projects into the phosphate stratum. The limestone is dense and 



*The Phosphate Deposits of Arkansas, by John C. Branner, Amer. Inst. 



Min. Eng. Trans, xxvi, pp. 580-598, 1896. 



tMineral Resources of the United States, Calendar vear 1913, — part 2, 

 p. 285, 1914. 



