part 1] PHOSPHATE DEPOSIT OF OCEAN ISLAND. 11 



VI. Method op Formation. 



Mr. Danvers Power considers that the present deposits were 

 formed, first of all, by the deposition of tricalcium phosphate in. 

 the cavities of the original coral. This form he named ' primary 

 rock-phosphate.' The coral-rock, with its phosphate- filled cavities,, 

 was then comminuted by marine action. The carbonate of lime 

 (being softer and more readily soluble than the phosphate) was 

 washed away, and left the waterworn particles of phosphate behind. 

 Such particles he termed ' primary alluvial,' and they correspond 

 to what I term ' incoherent phosphate rock.' When this material. 

 is cemented by more phosphate, Mr. Danvers Power calls it 

 ' secondary rock-phosphate.' To account for that which I term 

 ' fragmentary phosphate,' he assumes a second submergence of 

 the island and terms the material ' secondary alluvial phosphate.' 

 His theory, although ingenious, is not in consonance with the 

 information now available. Some of the divergences are stated 

 here. 



First, the dolomite, in the cavities of which Mr. Danvers Power's. 

 ' primary rock-phosphate ' occurs, is much harder than the general 

 mass of the phosphate. Even if it were not so, the complete absence 

 of unphosphatized coral fragments within the deposit would be 

 difficult to explain. Secondly, the theory in no way explains the 

 observed uniform variation in the percentage of tricalcium phos- 

 phate, according to the position of the sample taken. Thirdly,, 

 on the southern coast, where the phosphate reaches sea-level, the 

 veiy action which Mr. Power regards as having produced a material 

 containing 80 per cent, and more of tricalcium phosphate is in 

 actual operation at the present day, and the result is a coral-sand 

 containing usually less than 5 per cent, of tricalcium phosphate. 

 Lastly, what Mr. Power considers to be waterworn grains are 

 really pisolitic and oolitic particles, exhibiting, under the micro- 

 scope, an internal banded structure, the bands being conformable 

 with the outlines of the grain. 



My theory is that the guano was deposited on a rising mass of 

 coral, which had already suffered marine peneplanation, and had 

 been considerably eroded during previous periods of the history of 

 the island. As the island rose, the area on which the guano could 

 be deposited was, naturally, extended radially. Thus the guano 

 deposit would tend to be thick in the central parts of the island, 

 and thin near the coast. This explains the greater thickness and 

 the higher quality of the phosphate in the central parts of the 

 island as known at the present day, the higher percentage of 

 tricalcium phosphate being explained, of course, by the fact that 

 the solutions leached from the guano not only had a longer period 

 to act on the underlying coral of the central portions of the island, 

 but that they were concentrated by passing through a greater 

 thickness of guano before they reached the coral. 



The uniformity in the variation of the percentage of tricalcium 

 phosphate throughout the deposit points both to a uniform rate of 



