The Bocks of Tristan d'Acunha. 23 



of felspar, quartz, and small dark patches of a decayed mineral, one 

 particle of which, he was able to ascertain by its cleavage, to be 

 hornblende. Thirdly, a mass of confusedly crystallised white felspar, 

 with little nests of a dark-coloured mineral, often carious externally 

 and rounded, having a glassy fracture, but no distinct cleavage ; 

 from comparison with the second specimen he had no doubt that it 

 was fused hornblende. One other large fragment was a conglomerate, 

 containing small fragments of granitic, cellular and jaspery rocks, 

 and of hornstone porphyries embedded in a base of wacke, threaded 

 with numerous layers of a concretionary pitchstone passing into 

 sideromelan. 



In the Challenger Eeports there is confirmatory evidence of 

 Darwin's observations ; Professor Eenard records from Ascension, 

 amphibolic granite, granitite, diabase and gabbro, torn up from the 

 depths by eruptions of basalt or trachyte.* Doelter, also, has 

 recorded that in the island of Mayo, in the Cape Verde Islands, 

 there occur the very rocks that Sir John Murray says are not to be 

 found on oceanic islands, for instance, compact limestones and 

 crystalline schists, t The Challenger specimens, however, do not 

 confirm Doelter's observations. Gneiss also occurs in the Canary 

 Islands. 



In the far south there is another volcanic island, the Gaussberg, 

 which is an isolated basalt cone rising from the sea at the edge 

 of the inland ice of the Antarctic. There are no tuffs, but em- 

 bedded in the lava there are large boulders of granite and gneiss. 

 The dark constituents, biotite and probably hornblende, are altogether 

 melted away, the lighter materials, quartz and felspar, are also 

 affected and altered by heat but still remain in situ. Into the 

 cavities which the melting of the dark minerals has left, the glassy 

 lava has penetrated and occupies the original spaces of the crystals 

 by a sort of pseudomorphism. Granite and gneiss boulders lay 

 plentifully about the summit of the mountain, showing that the 

 inland ice once covered it. It is hence not quite clear whether the 

 blocks in the lava have been brought up from the throat of the 

 volcano or are simply surface-blocks entangled in the molten mass ; 

 the intensity of the alteration by heat, however, suggests the former 

 explanation.;|: 



* Rept. Results Challenger Expecl., " Physics and Chemistry," II., pt. vii. 

 "Report on the Petrology of Oceanic Islands," p. 62. 



f " Spuren eines alten Festlandes auf den Cape verdischen Inseln," Verh. k.k. 

 Geol. Reichanstalt, Wien, 1881, p. 16. 



I E. Philippi, " Veroffntiichungen des Inst, fur Meereskunde," Heft 5, 1903, 

 p. 126. 



