REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 41 



the absence of proofs to the contrary Darwin considered it as of subgerial origin. Like 

 most volcanic islands in the course of prevailing winds, Ascension has steep precipices 

 on the exposed side, where landing is very dangerous ; the west coast is less abrupt, and 

 there the British Eesidence is established. The influence of the prevailing winds appears 

 not only on the exposed coasts, but cinders and lapilli have been carried from the centre 

 of action in a westerly direction, and these have even been blown into the sea, where the 

 accumulation of incoherent volcanic products forms a bottom which affords good 

 anchorage. No traces are anywhere to be found of the island being at present in a state 

 of volcanic activity, but the cones of tufa are so little altered, their contours are so 

 sharp, and their brown and red colours so fresh, that they produce an irresistible 

 impression that the island has been quite recently formed by an accumulation of cinders 

 and scoriae, and that the fire still smoulders under the crust. The fundamental rock is 

 everywhere of a pale grey colour, and belongs to the trachytic series. These masses of 

 trachyte are best seen in the south-east part of the island. Almost the entire surface 

 is covered by streams of black scoriaceous lava of a basaltic nature. These beds 

 are dominated in certain places by hills, or isolated trachytic rocks. From the 

 Challenger's anchorage no trace of vegetation was visible except the light greenish 

 tint near the summit of Green Mountain, 6 miles from the coast ; all else was lava, 

 black and grey cinders, and volcanic peaks and cones. We might refer for geographical 

 details to Campbell's map, 1 in Darwin's Geological Observations, but it does not present 

 an exact and complete view of the island. It is now advantageously superseded by that 

 of C. A. Bedford, of H.M.S. "Raven," published by the Hydrographic Office in 1838, a 

 copy of which accompanies this Report (Map II.). Bedford's map shows the limits of 

 the scoriaceous rocks sufficiently clearly ; they stretch along almost the whole coast-line 

 on the north and south, dipping towards the sea, and are cut through by the channels 

 of the streams. The layers of scoriaceous lava are less apparent on the east and west ; 

 they only appear here and there, or form a belt along the shore. To the north of the 

 island these beds crop out again to a great extent, and send out branches which 

 surround the isolated hills of East Crater, Sister's Peak (1459 feet), and Bear's Back. 

 In the central and most disturbed part of the island lava is less common ; it is, projjerly 

 speaking, the region of trachytic rocks. In this central region, a little to the east, the 



with heaps of the fragments of black lava called ' clappers' by the English. . . . The shore is also composed of black, 

 trachytic, and porous lava, the surface being vesicular. . . . High sharp rocks shoot up from the sand. Elsewhere, at 

 the west point of Sandy Bay, the rocks are of black basalt, or covered with a thin greyish-white layer of obsidian like 

 a varnish." Lesson also notes calcareous deposits on the coast. We have cited this passage from the naturalist of the 

 " Coquille," because it is, we believe, the first work iu which the geology of the island is sketched. These few lines 

 give the gist of his description. We shall return farther on to some of the details he pointed out. We may refer for 

 the history of Ascension, and an account of its fauna and flora, to Sir Wyville Thomson's work, The Atlantic, vol. ii. 

 p. 262, etc. ; t > Moseley's Notes of a Naturalist on the Challenger, p. 561 ; and to the Narrative of the Cruise of 

 H.M.S. Challenger, vol. i. p. 927. 



1 A Plan of the Island of Ascension, by Lieut, Robert Campbell, 1819; frontispiece of Darwin's Geological 

 Observations. 



(PHYS. CHEM. CHALL. EXP. — PART VII.— 1889.) 6 



