84 ERNEST H. L. SCHWARZ 



rock — serpentine, peridotite, or sherzotite, whatever we choose 

 to call it — was a metamorphic and not an eruptive rock.' 



Evidence from Tristan d'Acunha. — In the Tristan d'Acunha 

 group one block of gneiss was obtained by Mr. Hammond Tooke 

 on a voyage thither in H. M. S. "Odin." Further specimens from 

 Nightingale Island showed fragments of an earlier consolidated 

 rock imbedded in a glassy matrix; the fragments were very minute 

 and had the appearance of having been derived from a felsite or 

 microgranite. ^ Both these cases would have been too small evi- 

 dence on which to build any theory of the rock substructure of 

 the islands, did not the description of Ascension Island support 

 them. In Carmichael's account of the ascent of the peak on 

 Tristan d'Acunha we read that the plain from which the central 

 cone rises "is encumbered with large detached masses of porphy- 

 ritic stone, and with others, inclosing crystals of sulphur or augite, 

 which seem to have been ejected in their present state from the 

 interior of the mountain. "^ Latter-day petrologists may smile 

 at a man who cannot distinguish between crystals of sulphur and 

 augite, but Carmichael was sound on general geological questions, 

 and his description of Cape Town, with the sandstone resting on 

 slate and granite with dolerite dykes, ^ is far better than many of 

 the later accounts. 



Volcanic ejectanienta as indices of deep-seated formations. — It 

 is a remarkable fact that we can thus sample the deep-seated rocks, 

 far below the zone of direct observation, by the ejectamenta of 

 volcanoes. In South Africa I have made use of them in the Dra- 

 kensberg, where many thousands of feet of Permian, Triassic, and 

 perhaps Jurassic strata lie upon an Archaean base,^ and in the 

 central Karroo portions of the Palaeozoic floor are brought to the 

 surface in this manner. Often enough, in South Africa especially, 



' Erdgeschichte (Leipzig, 1890), p. 199. 



2 Transactions of the South African Philosophical Society, Vol. XVI (Cape Town, 



i905)> P- 49- 



3 Transactions of the Linnean Society, Vol. XII (London, 1817), p. 491. 



4 Biographical sketch, Hooker's Botanical Miscellanies. Vol. II (London, 1831), 

 PP- i-59> 259-89. 



s"The Volcanoes of Matatiele," Annual Report of the Geological Commission 

 (Cape Town, 1902), p. 39. 



