36 AGRICULTURAL ANALYSIS 



lias evidently been formed by deposition of solutions, successive 

 layers being slightly different in color and texture. In numerous 

 cases the deposition seems to have taken place in a rather smooth 

 cavity which was but partly filled with the deposited solution, so 

 that deposition took place only on the bottom. 



40. Origin of the White Phosphates. According to Hayes, the 

 white phosphates of Tennessee originated as follows : 21 



From the nature of the deposits of white phosphate, their rela- 

 tions to other formations of the region, and the physical charac- 

 teristics of the several varieties of the rock, there can be little 

 doubt as to their mode of deposition. It seems reasonably certain 

 that the rock is entirely a secondary deposit, accumulated subse- 

 quently to the deposition of the carboniferous, Devonian and Silu- 

 rian formations, with which it is now associated. The latter were 

 laid down on the sea bottom as horizontal beds of sand, mud and 

 shells, having great lateral extent. They were buried beneath 

 other beds of sediment many hundred feet in thickness, which 

 have since been removed by erosion. The black phosphate, as 

 has already been explained, is one such sedimentary bed which 

 was deposited when the conditions were favorable for the accumu- 

 lation of lime phosphate on the sea bottom. It was afterwards 

 deeply buried by later deposited sediments, and has been brought 

 to light by elevation of the sea bottom and erosion of the over- 

 lying strata. 



Entirely different is the formation of the white phosphate. 

 The lime phosphate of which these deposits are composed was 

 doubtless originally extracted from sea water by organisms and 

 accumulated together with other sediments, either segregated in 

 beds and concretions or disseminated through limestones and 

 shales. When these rocks were brought near the surface by up- 

 lift and erosion they were attacked by percolating surface waters, 

 which contain carbonic and other organic acids. These acids 

 readily dissolve carbonate of lime, and to some extent also phos- 

 * l The Tennessee Phosphates, by C. W. Hayes. Abstract from the iyth 

 Annual Report of the Geological Survey, 1895-6, Part II, Economic Geology 

 and Hydrography : 356. Also, Extract from the 2ist Annual Report of the 

 Geological Survey, 1899-1900, by C. W. Hayes, Part III, General Geology, 

 Ore and Phosphate Deposits : 479. 



