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



summits are of a yellowish colour. The whole ridge is excessively rugged. 1 They were 

 visited during the cruise of the " Beagle" in 1832, and again by Sir James Ross during the 

 cruise of the " Erebus " in 1839 ; and later, on the 23rd August 1873, by the Challenger 

 Expedition. Their greatest length, from one end of the group to the other, is under a 

 quarter of a mile. Their diminutiveness is one of the features that most impress 

 those who visit the island. Their greatest altitude does not exceed 60 feet. Darwin 

 says : 2 " They rise abruptly out of the ocean, and, except on the western side, 

 soundings were not obtained, even at the short distance of a quarter of a mile from 

 the shore." 



The barrenness of these rocks is remarkable, as neither plants nor even lichens grow 

 on them. I may here mention that this barrenness is one of the characteristics of all 

 olivine and serpentine rock-masses. The rocks of St. Paul are almost destitute both of 

 flora and fauna, as observed by Darwin and Sir Wyville Thomson, who paid particular 

 attention to the natural history of this island. 



Darwin, speaking of the lithological characters of this group of rocks, describes it as 

 follows : 3 " It is composed of rocks unlike any which I ever met with, and I cannot 

 characterise them by any name. One of the most abundant kinds is a very compact, 

 heavy, greenish-black rock, having an angular, irregular fracture, with some points just 

 hard enough to scratch glass. Specimens are found of a less deep green, but whose crystal- 

 line structure is more marked, that are translucent on the edges, and fusible to a green 

 enamel." He considered the northern rock of the group to be formed of a sort of " harsh 

 stone," which breaks up into fragments so regular as to be mistaken for blocks of altered 

 orthoclase. Darwin, moreover, saw what he considered to be veins of serpentine running 

 through the whole mass. The observers of the Challenger Expedition have, like Darwin, 

 classed these rocks of St. Paul as serpentine, and we shall soon see that in doing so they 

 have placed them very nearly in the class they should occupy in the lithological series. 

 Sir Wyville Thomson 4 insists on the resemblance between these rocks and the serpentines 

 of Cornwall and Ayrshire, while he admits that they differ very much from them in 

 certain marked characteristics. They do not appear to him, however, to bear any 

 resemblance to modern volcanic rocks. Mr. Buchanan 5 ascertained during the voyage of 

 the Challenger that the rock contained magnesia, alumina, and peroxide of iron, and that 

 many specimens gave out water in the closed tube. The naturalists who visited the 

 island have drawn attention to the fact that the rocks to the south are covered over with 

 a substance that gives them at a distance a dazzling white appearance. This is due in 



1 Sir Wyville Thomson, Voyage of the Challenger, vol. ii. p. 106 ; Moseley, Notes of a Naturalist on the Chal- 

 lenger, chap. iii. 



2 Darwin, Volcanic Islands, p. 32. 



3 Ibid. 



4 Sir Wyville Thomson, Voyage of the Challenger, vol. ii. p. 106. 



5 J. Y. Buchanan, Proe. Roy. Soc., cvii. 1876, p. 613. 



