The Bocks of Tristan cVAcunha. 13 



show, besides, sections of the same mineral, and of plagioclase of 

 smaller size, with clean-cut outlines, embedded in the glassy matrix, 

 and belonging to a secondary period of consideration. 



This tuff is overlain in turn by a rock of the same kind, but of 

 coarser grain. It consists of lapilli 2 to 3 centimetres in diameter, 

 and full of augite crystals visible to the naked eye. There also 

 occur in it fragmentary crystals of olivine, which show their clastic 

 origin very clearly under the microscope. The augite of the tuffs 

 has a great tendency to form twin-crystals, as polysynthetic as those 

 of some plagioclases. The crystals of plagioclase, augite, olivine, 

 and magnetite are often of somewhat large dimensions ; those of 

 augite and plagioclase are corroded, and show the action of the base 

 which surrounds them. 



Hornblende occurred only in one of the Challenger rocks, but the 

 specimen differed in no way from the other lavas except in the 

 presence of this one mineral. 



The dykes are very similar in composition. The minerals of the 

 first generation are magnetite, olivine, and plagioclase. The last- 

 named crystals are lamellar, and approach labradorite in composition. 

 The ground mass of the rock is almost entirely composed of augitic 

 microliths, which are grouped in rosettes or twinned crosswise, and 

 sometimes planted almost perpendicularly on the plagioclastic 

 lamellae, or between the small prisms of augite, forming a fibro- 

 radiating aggregate. Crystals of olivine with hexagonal or rhombic 

 contours are frequent, enclosing a nucleus of glassy substance. 

 Magnetite fills up the interstices between the various minerals that 

 constitute the matrix in the form of irregular grains.' 1 ' 



Inaccessible Island, from which some of our specimens come, is 

 very similar, and Sir Wyville Thomson was so struck with the 

 general resemblance in the physical geography of Tristan and Inac- 

 cessible, that he thought that these two eruptive masses, now 

 separated by twenty miles of water, had once been united. 



The island lies to the west of the other islands, and is a little 

 smaller than Tristan, from the summit of which its centre is about 

 twenty-three miles distant. Abrupt cliffs, fringed with a line of 

 breakers girdling the island, appear at first sight to make landing 

 impossible, but there is a narrow beach at the base of the vertical 

 rocks. Inaccessible Island is nearly quadrilateral in outline, the 

 angles being directed towards the cardinal points. The highest part 

 of the island is towards the west, where the cliffs rise to the height 

 of 1,840 feet above the sea-level, the average elevation of the rocky 

 wall being about 1,100 feet. A crag, 1,140 feet high, occupies the 



* A. Eenard, Phys. Chem. Chall. Exp. pt. vii. — 1889, section vi. p. 74. 



