20 



THE VOYAGE OF H.M.S. CHALLENGER. 



the appearance of lava, but it was in reality nothing but a block of honeycombed 

 mammillated phosphate. This specimen is covered over with limonite ; the few sections 

 of altered olivine rock coated with phosphate are so thoroughly decomposed that the 

 finger-nail will scratch them. 



VI. (B). 



100-00 



We have now to consider the composition of the white enamel that gives the South 

 Rock of the group of St. Paul the dazzling appearance described by Darwin. He 

 attributed the white colour partly to the excrements of the sea-fowls that inhabit the 

 rock, and partly to a hard and brilliant enamel with which its surface is covered. 

 According to him, specimens examined with the lens show that they are composed of 

 exceedingly thin layers, the entire thickness not exceeding the tenth of an inch. This 

 substance, which contains a very large proportion of animal matter, appeared to him to 

 be phosphate of lime. He mentions at the same time that he found in the Isle of 

 Acension, and in the Abrolhoos, certain substances, ramified in form, that must evidently 

 have been produced in the same fashion. He lays great stress on the resemblance 

 between these ramified forms and nullipores. 1 Darwin has shown specimens of this 

 incrustation to geologists, and all have admitted that it was due to some volcanic or 

 igneous action. Without enumerating the detailed opinions that have been entertained 

 with regard to this crust, or discussing the similarity of appearance and of physical and 

 chemical properties existing between certain mineral substances and the harder parts of 

 organisms, I may remark that Darwin and Mr. Buchanan 2 regard this white coating as due 

 to the accumulation of excrement of sea fowl, the insoluble residue of which has been 

 exposed during very long periods of time to the action of the sun's rays and of the waves 

 of the ocean. Subsequently, concretions have been formed, and thus the entire exposure 

 has become coated with this kind of enamel. I am led to adopt this explanation as the 

 true one, and I consider it applicable, not only to the substance in question, which the 

 chemical analysis shows, beyond a doubt, to be a tribasic phosphate of lime, but also to 



1 Darwin, Voyage of the " Beagle," chap. i. p. 8. 



1 Sir AVyville Thomson, Voyage of the Challenger, vol. ii. pp. 107, 108. 



