part 1] PHOSPHATE DEPOSIT OE OCEAN ISLAND. 13. 



Per cent. 



Ca 3 P o s 87-5 



CaC0 3 3-5 



Organic matter 2'5 



CaO combined with the above . . . l'O 



CaP 2 3-0 



A1 2 0.„ Fe.,0 3 0-7 



Mg-0 0-3 



CaS0 4 0-5 



Si0 2 1-0 



Total 100-0 



In general, the tricalcium phosphate, calcium carbonate, and 

 organic matter make up about 93 5 per cent, of the material 

 (whatever its grade), the other components showing but little 

 variation, except in the subvitreous varieties which contain a 

 smaller percentage of impurities. 



The excess of lime shown in the analyses has given much trouble 

 to Dr. Carl Elschner, who has invented various compounds to 

 account for it. There is, however, no reasonable doubt that it is 

 simply combined with the organic matter. Elschner states, with- 

 out bringing forward any evidence, that the calcium fluoride is 

 combined as apatite. The physical and chemical properties of the 

 phosphate render it unlikely that such a resistant substance as 

 apatite is present in any quantity, and it is my opinion that the 

 fluorine oecm's combined chiefly as silico-fluoride. 



Mr. Dan vers Power believes that the central portion of the 

 deposit occupies the site of an old lagoon ; but, so far as they have 

 been revealed, the contours of the underlying coral-mass do not . 

 lend themselves to this view. The highest point of the island is, 

 in fact, an outstanding and completely phosphatized coral-pinnacle, 

 the roots of which extend, at a small depth from the surface, over 

 a considerable area. I have every reason to believe that the flat 

 top of the island represents the remains of an uppermost marine 

 platform of coral. 



VII. Conclusions. 



(1) The base of Ocean Island consists of a typical fossil coral- 

 reef, much altered by marine and subaerial denudation. Before 

 the deposition of the guano occurred, this reef was partly dolomi- 

 tized and suffered considerable erosion, the hollows in the eroded 

 surface being afterwards filled in by detrital and oolitic limestones 

 formed by marine action. The evidence available makes it probable 

 that the reef has suffered more than one submergence and emer- 

 gence previous to the deposition of the guano. 



Practically all the fossil remains found in the deposit are so 

 altered as to be indeterminable, the only specimen in good pre- 

 servation being a single tooth of Carcharodon inegalodon,, x which 

 would indicate that the age of the deposit is post-Miocene. 



1 Teste Dr. A. Morley Davies, in litt. 



