PHOSPHATIC ACCUMULATIONS SUPERFICIAL. 81 



The liue of deraarkation between phosphate and limestone is generally 

 very sharp though always irregular, and there is no difficulty,, with but ordi- 

 nary attention, in thoroughly separating the rock. 



The yield of phosphate from the individual cylindrical pockets varies 

 usually from two to ten tons as the depth varies from five to twenty feet; 

 but in many places, where the depth was increased to thirty feet by the 

 coalescence of several small pockets, the yield has been very much greater, 

 one such hole on the lower flat having contained nearly 300 tons, while 

 another in the upper flat furnished upwards of 1,000 tons. 



Experimental shafts were sunk at several points on the island to determine 

 whether the phosphate occurred below the general line of the surface pockets. 

 One such shaft, started on the top of the upper terrace 248 feet above sea- 

 level, was carried down 251 feet, and when abandoned held from 12 to 36 

 inches of salt water at the bottom. It failed to show anything but limestone 

 of a remarkably pure white color, much of it amorphous and not roughened 

 like the surface rock. The smaller shafts were likewise non-productive. I 

 therefore conclude that while phosphate may be found to extend below the 

 general depth of the field at some few individual points, such cases will be 

 exceptions to the rule, which limits the depth on either flat to about twenty 

 feet, varying from that figure to only four or five feet. 



Chemical Character of the Phosphates. 



Composition. — No very complete records of the chemical character of the 

 two varieties of phosphate found on the island seem to be available. The 

 composition varies considerably ; the gray perhaps averaging 65 to 70 per 

 cent, bone phosphate, and the red 50 to 65 per cent. 



A rather recently shipped sample of the gray variety showed, upon analysis, 

 the following result, which may be said to fairly represent the character of 

 the material which finds its way into commerce: 



Analysis of Gray Navassa Phosphate. 



Per cent. 



Water, at 100 C. 2.33 



Organic matter and water of combination 7.63 



Lime 34.22 



Magnesia .51 



Sesquioxide of iron and alumina 15.77 



Potash and soda .86 



Phosphoric acid ■ 31.34 



Sulphuric acid .28 



Chlorine .15 



Carbonic acid 1.84 



Silica 4.53 



Bone phosphate .• 68.46 



Bone phosphate (dry basis) 70.09 



