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



Darwin, in his work on Volcanic Islands, 1 reports that he visited Fernando 

 Noronha during the voyage of the " Beagle," but his stay there was of short duration. 

 He states that these islands are of volcanic origin, but that he did not observe any 

 crater. According to Darwin, one of the most salient features of the topography is a 

 hill about 1000 feet high, forming an escarpment, and crowned by a summit, 400 feet 

 high, of a phonolithic rock ; this rock contains, he says, numerous crystals of felspar 

 and some prisms of hornblende. From the highest point of this hill he was able to 

 observe that the other islands of the group had conical summits of the same nature. 

 He recalls the fact that at St. Helena, also, great phonolithic masses occur, rising 

 vertically to 1000 feet; these have evidently been injected into crevices while fluid. 

 If this hill of Fernando Noronha, he adds, owes its origin to the same cause, as seems 

 probable on other accounts, we are forced to admit that denudation has occurred 

 here on a great scale. Near the base of this eminence Darwin observed some 

 beds of whitish tufa, traversed by numerous dykes, some of amygdaloidal basalt, 

 others of trachyte. He noticed, also, some beds of fissile phonolite, in which the planes 

 of schistosity ran N.W. and S.E. Certain parts of this rock, where the crystals are less 

 numerous, resemble slate altered by contact with a trap dyke. The lamination of the 

 rock, which at first had incontestably been in a state of igneous fusion, seemed to him 

 an important subject for investigation. Darwin concludes his brief description by 

 adding that he found on the shore numerous fragments of compact basalt ; they 

 appear to come from a columnar rock which is seen in the neighbourhood. 



The craggy phonolithic mass, to which Darwin alludes, is St. Michael's Mount. 

 Mr. Buchanan a remarks that at the foot of the eminence the rock is columnar, while 

 towards the summit it assumes a massive structure. On the west side of Fernando 

 Noronha the columns are inclined at an angle of about 30° to the horizon. Their 

 section is almost square, but the angles are greatly rounded off, and the columns are 

 not very thick. He adds that the rock is greenish, and that crystals of sanidine occur 

 in it, lying with their broad faces in a plane perpendicular to the length of the columns. 

 The slopes of St. Michael's Mount are covered with blocks of massive phonolite, often 

 decomposed, and thus exhibiting the sanidine crystals in relief. This rock possesses 

 the characteristic properties of the phonolites : it rings under the hammer. The 

 specimens which we have examined are less schistose or fissile than many of the rocks 

 of the same type, but both the naked-eye and the microscopical characters confirm 

 the determination of Darwin and of Buchanan. 



One specimen, taken from a columnar block, appears compact to the eye, is 

 greenish grey in colour, dotted with white, has an irregular scaly fracture, and a 



1 Darwin, Geological Observations on Volcanic Islands, p. 23. See also Wyville Thomson, The Voyage of the 

 Challenger, The Atlantic, vol. ii. p. 109, London, 1877 ; Moseley, Notes of a Naturalist on the Challenger, London, 1879. 

 3 J. Y. Buchanan, Proc. Roy. Soc, vol. xxiv. p. 613, 1876. 



