74 THE VOYAGE OF H.M.S. CHALLENGER. 



apparently the same nature. These masses when broken had an earthy texture, but 

 on their outsides, and especially at their extremities, they were formed of a pearly 

 substance, generally in little globules, like the enamel of teeth, but more translucent, 

 and so hard as just to scratch plate-glass. This substance slightly blackens under the 

 blowpipe, emits a bad smell, then becomes quite white, swelling a little, and fuses into 

 a dull white enamel ; it does not become alkaline ; nor does it effervesce in acids. The 

 whole mass had a collapsed appearance, as if in the formation of the hard glossy crust, 

 the whole had shrunk much." * Darwin states in a note that when he described this 

 substance in his Journal he viewed it as an impure calcium phosphate. 2 We have tested 

 some small fragments of the incrustation collected at Ascension ; there remains no 

 doubt as to this being the true interpretation. The coating gives the reactions of 

 phosphoric and sulphuric acids, and the microscopical characters resemble those of the 

 incrustations on St. Paul's Rocks. 3 It may therefore be admitted that it was formed, 

 like the latter, by the decomposition of the excrement of birds. In his description of 

 Ascension, Lesson was the first to lay stress on the accumulation of birds' droppings 

 which covered the rocks of the island. The insoluble residue exposed to the rays of the 

 sun and the action of waves has hardened, and forms the coating which clothes the 

 rocks of the coast. 4 



VL— NOTES ON THE ROCKS OF THE TRISTAN DA CUNHA GROUP OF 



ISLANDS. 



Until the Challenger Expedition explored these islands, we had only very uncertain 

 notions of the nature of the rocks that constitute the group of Tristan da Cunha. We 

 have borrowed from the Narrative, vol. i., and the works of Sir Wyville Thomson 5 and 

 Moseley, 6 and especially from Buchanan's report, 7 the local details that accompany these 

 lithological researches. The following observations do not form a complete geological 

 monograph of the Tristan da Cunha group ; in general, they have reference only to the 



1 Darwin, Geol. Obs., pp. 32, 33. 2 Ibid., p. 33. 



3 See A. Renard, Report on the Petrology of the Rocks of St. Paul, p. 18 {Nan: Chall. Exp. vol. ii. Appendix B). 

 We give there a micrographic description and analysis of these layers and veinules of calcium phosphate. The incrusta- 

 tion Darwin saw at St. Paul's, which he compares to that at Ascension, is described on p. 21 of our memoir. On 

 analysing a specimen we found phosphoric acid (P2O5), 33 -61, and lime (CaO), 5051, besides traces of iron, manganese, 

 and sulphuric acid. This incrustation can thus be viewed as tribasic calcium phosphate with calcium sulphate, and 

 perhaps carbonates of lime, magnesia, and iron (see Darwin, Voyage of the Beagle, chap. i. p. 8 ; Buchanan in Thomson, 

 The Atlantic, vol. ii. pp. 107, 108.) For phosphates very like those we describe, see also Phipson, Amcr. Jorum. Sci. 

 vol. xxxvi. p. 423 ; Julien, ib. p. 242 ; Piggott, ib. 2nd Ser. 1856, No. 22. 



4 Lesson bad observed this shining layer, but mistook its nature ; he says, " a grey enamel-like obsidian clothes 

 the rocks of the coast," he. cit. p. 492. 



5 Wyville Thomson, The Atlantic, vol. ii. p. 152. 



c Moseley, Notes of a Naturalist on the Challenger, p. 108. 

 7 Buchanan, Proc. Hoy. Soc, vol. xxiv. p. 593. 



