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



1840 feet above the sea level, the average elevation of the rocky wall being about 

 1100 feet. A crag 1140 feet high occupies the southern angle, and a conical mound of 

 700 feet rises on the south-west, the two heights being separated by a V-shaped ravine, 

 probably produced by atmospheric erosion. 



The geological structure of Inaccessible is identical with that of Tristan, and the 

 appearance of the two islands is consequently similar. The vertical cbffs present a 

 series of good sections, which show the island to be built up of successive horizontal 

 beds of eruptive rocks, traversed by oblique or vertical dykes. As at Tristan, the coast 

 cliffs terminate in a plateau. Boulders, broken off by the waterfalls from the lava- 

 beds and dykes, have collected at the base of the rocks, passing on the seaward side 

 into a belt of rounded basaltic pebbles. The rocks dip almost vertically into the 

 sea, and there are very few places where they can be climbed in order to reach the 

 central plateau. Soundings of from fifty to ninety fathoms occur a few yards from 

 the cliffs. 



Sir Wyville Thomson was so struck by similarities in the physical geography of 

 Tristan and Inaccessible as to hazard the opinion that these eruptive masses, now 

 separated by twenty miles of water, had once been united. According to the descrip- 

 tion of the naturalists of the Challenger, the rocks of Inaccessible very closely resemble 

 those of Tristan, and they have the same arrangement. We will first describe the 

 rocks forming the lava sheets and the tufa. 



Almost all the specimens from Inaccessible are felspathic basalts ; the differences 

 between them are chiefly in texture, and sometimes in the development of a vitreous 

 base. A porphyritic basalt, which appears to take an important place in the structure 

 of the island, has given rise by decomposition to a yellowish earthy substance, to be 

 described further on. This basalt is a black scoriaceous rock containing many crystals 

 of augite, sometimes a centimetre in length, olivine, and felspar. Felspar is the least 

 abundant constituent, and its crystals are the smallest. Microscopic preparations show 

 that the ground-mass in which these porphyritic crystals are embedded is formed by 

 a yellowish or altered base, which penetrates all the fissures of the larger minerals. 

 This ground-mass contains small augite sections, some of them star-shaped, showing 

 penetration twins ; these microliths are associated with minute plagioclase sections 

 and with magnetite. The large porphyritic crystals of augite are zonary, and have a 

 somewhat pale pink colour ; the regular sections of olivine are a little smaller, and 

 have been slightly altered at the edges ; a yellowish zone surrounding this mineral 

 shows that it is being decomposed into hematite. It contains numerous inclusions of 

 magnetite, and shows traces of twinning. If there were no small crystals of plagioclase 

 in the base this rock would be classed with limburgite, which it resembles rnacro- 

 scopically in several ways. 



This basalt decomposes into a yellowish earthy substance, from which crystals of 



