hayes.] TENNESSEE WHITE PHOSPHATES. 419 
however, in getting from such deposits as are being worked on Wils- 
dorfs Branch a uniform product which will run between 79 and 81 per 
cent of lime phosphate. 
ORIGIN OF THE DEPOSITS. 
As stated in a former report, the conclusion arrived at from exami- 
nation of the surface outcrops was that the lamellar variety had been 
formed by deposition from solution in cavities in the limestone. 
Observations recently made on more extended exposures amply con- 
firm this conclusion. They show, moreover, what could not be deter- 
mined from the surface outcrops, that the cavities in which deposition 
took place were very extensive, forming, in fact, large caverns in the 
limestone. It appears that the phosphate was deposited in a some- 
what uniform and continuous layer on the floors of these caverns, in 
general following their undulations, but more was deposited in the 
depressions than on the elevations. Phosphate was also deposited in 
less regular cavities in a limestone above the stratum in which the 
main cavern formed. When this limestone was dissolved these masses 
settled down with the residual clay in which they are now embedded. 
During this readjustment, brought about by the solution of the lime- 
stone, the phosphate was repeatedly fractured and recemented, giving 
it a brecciated structure. The phosphate was doubtless deposited. in 
these caverns from quiet water, but they also contained at times 
rapid streams which carried sand and gravel and formed alluvial 
deposits. The latter differ distinctly from those formed b}^ surface 
streams under ordinary conditions. Since the stream was more or 
less confined above by the roof of the cavern, as well as at the sides, 
the water was sometimes under hydrostatic pressure. Under such 
conditions the laws which govern ordinary stream transportation and 
deposition do not apply, and the deposits possess certain charac- 
teristics which clearly indicate the conditions under which they were 
formed. 
EXTENT OF THE DEPOSITS. 
Since the lamellar variety of the white phosphate was deposited in 
limestone caverns, it will be found only where the conditions were 
favorable for the formation of caverns. It need not be expected above 
the top of the Silurian, since the Carboniferous limestone in this region 
contains so large a proportion of chert and other impurities that it 
probably never gives rise to the formation of caverns. It should fur- 
ther be noted that certain horizons in the Silurian limestone are much 
more soluble, and hence better adapted to cavern formation, than 
others. This is the characteristic of certain beds of Upper Silurian 
limestone which have a granular crystalline structure and are com- 
posed largely of crinoid stems. Wherever these beds are exposed by 
stream cutting they are apt to be cavernous. At numerous points in 
this region the streams sink and flow for considerable distances in 
underground channels, and this is most often the case where the 
