TWO PRINCIPAL VARIETIES OF PHOSPHATE. 79 



earth, without cover, in the condition most favorable for its economical ex- 

 traction. 



The yellow plum, india-rubber tree, iron-wood, cactus and century plants 

 (the latter 15 to 20 feet high) and a profusion of scrub palms and a rank 

 grass make up the principal growth ; but nowhere do any of the trees grow 

 to sufficient size to furnish merchantable timber, nor in sufficient abundance 

 to at all present the appearance of tropical growth. 



Character, Variety and Methods of Occurrence of the 



Phosphates. 



Principal Varieties. — There are two principal varieties of phosphate mined 

 and shipped from the island : 1. A gray phosphate, confined to the lower 

 flat, from 30 to 300 yards wide and from 10 to 70 feet above sea level ; 2. A 

 red phosphate, occupying the oval flat on the top of the island, an area about 

 two miles long and varying in width from a half mile near the center of the 

 island to about 2,000 feet at the southeastern end and 500 feet at the north- 

 western end. The upper flat is further subdivided by barren strips of lime- 

 stone, locally called " white rock reefs," into seven districts or " fields " and 

 as many smaller distinct pockets, in addition to a great number of individual 

 areas not sufficiently large or prolific to merit special distinction but which 

 taken collectively will furnish a large amount of phosphate. The lower flat, 

 on the contrary, occurs as one distinct deposit completely encircling the 

 island except at its extreme northwestern point. 



To the eye the only distinction between the two varieties is the difference 

 in color ; but the gray phosphate is the better grade, containing from 10 to 

 15 per cent, more of phosphate of lime and a comparatively low percentage 

 of sesquioxide of iron and alumina, of which this gray variety may, however, 

 contain anywhere from 5 to 15 per cent. 



Perhaps 85 per cent, of both varieties occurs as soft earth, made up of 

 rounded oolitic grains, forming, when dried, a coarse granular powder. The 

 balance, or 15 per cent., occurs as hard rock, generally attached to limestone 

 walls of the individual pockets, but occasionally found as undecomposed 

 lumps within the body of the softer earthy phosphate. Under a lens these 

 lumps show a dense mass of closely cemented oolitic grains, frequently so 

 coarse and so loosely held together as to plainly show the granulated struc- 

 ture of the rock to the naked eye. In digging this rock variety, striking bars 

 and dynamite must be used, and it is often hard to separate from the at- 

 tached limestone ; but the softer material yields readily to scrapers, picks 

 and shovels. 



Systemic Character. — In Bulletin 48 of the United States Geological Sur- 

 vey, " The Nature and Origin of Deposits of Phosphate of Lime,'' by K. A. F. 



