420 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
erosion of the valleys lias reached the surface of this easily soluble 
limestone. The white phosphate therefore, although its connection 
with any particular bed is in a measure accidental and not essential, 
as is the case with the brown phosphate, may be expected to occur in 
greatest quantity at the horizon of these particular beds. Hence the 
latter, identified by their peculiar fossils and physical characteristics, 
will afford a guide of some value in further prospecting. 
Since this phosphate was deposited in caverns, it can not be expected 
to have very great lateral extent. As stated in the previous report, 
it is essentially a pocket deposit, although the possibilities for exten- 
sive deposits are much greater than were recognized at the time that 
report was made. The examination of one deposit, no matter how 
thoroughly it is exposed to view, will not enable the prospector to 
make definite estimates as to the extent of any other deposit. Doubt- 
less similar natural exposures may lead to inferences of some value, 
but they can not be depended upon to take the place of actual pros- 
pecting. Each deposit must be examined itself, and the amount of 
rock which it contains, as well as its character and the depth of over- 
burden, must be determined by systematic exploitation, similar to that 
which has been emploj'ed by the Perry Company at Wilsdorf s Branch. 
The necessity for this thorough examination of each individual deposit 
can not be dwelt upon too stronglv. 
Probably the greater part of the white phosphate in this district 
will be obtained by open workings. The character of the overbur- 
den — unconsolidated clay in which movement takes place with great 
ease — is such that underground working will be attended with the 
greatest difficulty. Substantial and expensive timbering would be 
required wherever the phosphate was removed, and doubtless even 
then a large proportion of the rock would be wasted. The specific 
gravity of this rock is so much greater than that of the brown phos- 
phate that a very much greater overburden can be removed with 
profit for the same thickness of bed. Where the phosphate bed has a 
thickness averaging 3 feet it is probable that 10 or 18 feet of over- 
burden can be removed with profit while the rock commands present 
prices. The phosphate which is disseminated through the overlying 
clay can be saved easily with the proper plant, which should include 
screens and washers. Owing to the much greater density of this rock, 
the matter of drying is less important than with the brown phosphate. 
It would, however, probably pay to dry the rock before shipment, and 
this might be done b}^ open-air burning, or more economically with an 
ordinary rotary drier. 
POSSIBLE EXTENSIONS OF THE FIELD. 
Considerable interest attaches to the distribution of the white phos- 
phate and the possible extension of the productive territory. If the 
conclusion outlined above is correct — namely, that the phosphate 
deposits were accumulated in caverns in the limestone — consideration 
