614 Mr. J. Y. Buchanan on Geological Work 



tube it gives off alkaline-reacting and empyreumatic- smelling water. It 

 was found to consist of phosphate of alumina and iron, with some silicate 

 and sulphate of lime. 



Besides St. Michael's Mount, Bat Island and Platform Island were 

 landed upon. Bat Island is the largest of the secondary islands, and 

 the one most distant from the main island. It is composed on the western 

 side of massive basaltic rock, and on the eastern of sandstone. This 

 latter probably overlies the basalt, as in its structure it bears the marks 

 of having been deposited in drifts, and the sand is calcareous, consisting 

 of shell debris. On our way to and from Bat Island we had to pass all 

 along the western side of Booby Island. The wave-worn cliffs showed 

 that the island was entirely formed of the above-mentioned calcareous 

 sandstone. No igneous rock was visible at all ; and as the peculiar wind- 

 blown stratification marks are continued below the level of the sea, it is 

 probable that the land here is sinking, or at least has sunk. 



Platform Island consists of a mass of perfect basaltic columns rising 

 out of the water, and supporting a covering of massive basalt, on which 

 is spread out the platform of calcareous rock, on which are the ruins of 

 an old fort, and from which the island doubtless takes its name. 



A day was spent on each of the islands of the Tristan d'Acunha 

 group. It consists of three : — Tristan, the largest, Inaccessible, and 

 Nightingale. The last is more properly a group of islands studded all 

 round with isolated rocks. The difficult nature of the ground in Inac- 

 cessible and Nightingale, and that combined with the uncertain state of 

 the weather when at Tristan, rendered excursions to a distance from the 

 landing-place impossible. The rock specimens, therefore, which have 

 been collected are all from the neighbourhood of the landing-places. 

 Each island has its distinctive features. Inaccessible is a raised plateau, 

 bounded on all sides by cliffs over 1000 feet high, which plunge directly 

 into the sea. Looked at from the sea, there is no appearance of a crater 

 on the top, the only inequalities in the surface being apparently due to 

 erosion. The waterfalls, which tumble from the cliffs, afford most 

 picturesque proof of the abundance of water above. Tristan Island, like 

 Inaccessible, consists of a very similar plateau, bounded by cliffs of about 

 the same height and almost equally impracticable. This plateau, how- 

 ever, is surmounted by a lofty peak rising to the height of 8300 feet 

 above the sea. Nightingale Island differs from the other two as much 

 in geological structure as in outward appearance. The principal island 

 rises from the sea in low cliffs, nowhere on the eastern side exceeding 

 30 or 40 feet, and generally lower. At the south end the ground rises 

 from these cliffs by several steep ascents to the peak 1105 feet high, 

 which on its northern side presents a nearly perpendicular face of about 

 half its height from the sea. The rest of the island is more undulating, 

 the light grey rocks appearing only here and there through the openings 

 in the long tussac grass. Of recent volcanic action there appears to 



