LAMELLAR PHOSPHATE 35 



grading into each other imperceptibly, so that the distinctions which 

 were supposed to exist tend to disappear on more careful exam- 

 ination. The result of this gradual merging has led to the drop- 

 ping of the original classification and the bringing of the white 

 phosphate into one group with a few slightly different varieties. 

 Whatever may have been the original form of this rock, the 

 phosphatic deposit is evidently secondary and is intimately asso- 

 ciated with the rocks of the carboniferous period. The sections 

 of this variety of phosphatic rock exhibit under the microscope 

 masses of silica in which are bedded rhombohedral crystals, some- 

 times very small and widely scattered, but perfect and sharply 

 defined. In the granular portions of the rock the crystals are 

 larger, appearing as sections of rhombohedrons which are not 

 perfectly independent, but are segregated into irregular groups. 

 These crystals, which have the form of calcite, have been en- 

 tirely changed in their structure by the secondary deposit. Phos- 

 phate of lime, in other words, has practically taken the place of 

 the carbonate of lime in these crystals. The following analyses, 

 made under the direction of Monroe, show the composition of this 

 form of Tennessee white phosphate : 20 



ANALYSES OF TENNESSEE WHITE STONY PHOSPHATE. 



i 2 3 4 5 6 



Silica, SiO 2 61.34 49.43 54.30 54.88 50.18 56.46 



Lime, CaO 20.30 26.40 22.87 22.76 25.57 22.01 



Phosphoric acid, P 2 O 3 12.55 15-12 14.86 15.30 15.21 13.15 



Corresponding to: 



Lime phosphate, Ca, P 2 O 8 , and.. 27.40 33.00 32.45 33.40 33.20 28.60 



Lime carbonate, CaCO 3 9.75 15.21 9.36 8.23 13.45 11.56 



38. Breccia Phosphate. The breccia is the most abundant 

 variety of the white phosphate. It occurs in irregular masses com- 

 posed of slightly angular fragments of carboniferous chert im- 

 bedded in a matrix of phosphate of lime. The phosphatic matrix 

 before exposure to the weather is of a white or slightly reddish 

 color and somewhat harder than compact chalk. 



39. Lamellar Phosphate. Another variety of the phosphate is 

 the white lamellar, consisting of even, parallel layers or plates. It 



20 i7th Annual Report of the U. S. Geological Survey, Part II, 1895-6 : 

 539- 



