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



Among the specimens of rock from the Island of Tristan, there is a vitreous frag- 

 ment, very compact, and with a slight reddish reflection, which the inhabitants use 

 for striking fire. This rock when examined under the microscope is seen to have a 

 very dark vitreous matrix ; in some parts it is slightly transparent and brown. The 

 minerals developed in it are augite and plagioclase. This latter mineral is present in 

 lamellar sections, somewhat large at times, and sometimes riddled with vitreous in- 

 clusions ; the large plagioclase crystals are even visible with the lens ; we observe also 

 much smaller lamellae of triclinic felspar, scattered sporadically in the ground-mass. 

 The dimensions of the crystals of augite with magnetite inclusions are the same as those 

 of the large plagioclase crystals ; their forms are well marked, and a certain number 

 among them are twinned like those previously described in the lavas of the island. 

 There are also to be seen in the base a great many small sections of augite, as well as 

 some microscopical sections of olivine. This rock, which one would at first sight place 

 alongside of obsidian, ought to be referred to the felspathic basalts ; it constitutes a 

 very vitreous variety of that type. 



The soundings of the Challenger around the Island of Tristan brought up 

 samples of the sediments that are deposited near the island. The mineral particles 

 that occur in these deposits are exclusively of volcanic origin. The fragments that 

 enter into their composition are microscopical fragments of the rocks which we have 

 just described, or of the minerals that form these rocks. One dredging (18th 

 October 1873) brought up a fragment of hard, black, massive rock, weathered on the 

 surface ; microscopical examination showed that it belonged to the basalts ; it greatly 

 resembled the rocks forming the dykes in Tristan. We find in it microliths of plagio- 

 clase elongated in a direction parallel to P\M, and giving extinctions of about 30° ; 

 and, still further, small sections of olivine and apatite. The black pigment of the 

 ground-mass is concentrated at certain points. All the characters of this rock go to 

 show that it came originally from the Island of Tristan. The same statement does 

 not hold good of the fragments of pumice collected in the same dredging. The 

 ubiquity of pumice in pelagic deposits is a well-known fact, and Mr. Murray has 

 shown how these volcanic products may come to be deposited at points far removed 

 from their place of origin. We are thus led to regard these fragments of pumice as 

 in no way appertaining to the rocks of Tristan. Macroscopic examination shows the 

 presence of sanidine in this pumice ; under the microscope the same mineral is seen in 

 splintered crystals, without either regular outlines or hemitropic striae. Plagioclase, 

 with the twinnings of albite and pericline, is also present. 



B. — Rocks of Inaccessible Island. 



Inaccessible belongs to the same group as Tristan da Cunha. It lies to the west of 

 the other islands, and is a little smaller than Tristan, from the summit of which its 



