The Bocks of Tristan d'Acunha. 31 



and the sandstones in them are composed of the detritus of a 

 granitic region. The origin of this latter rock is to be sought for in 

 the existence in those times of a prolongation of the Madagascar 

 ridge southwards, till it touched the present eastern coast of the 

 colony ; * the material must have been derived from the south, for 

 to the north hundreds of miles separate the Matatiele district from 

 the granitic regions of the Transvaal, and the intervening rocks are 

 all sedimentary, or, when igneous, of a basic type. 



The break in the deposition of the Molteno beds marked the dis- 

 appearance of the source of supply ; and the great difference between 

 the nature of the glittering sandstones of the Molteno beds, which 

 with all their high content of unaltered felspar, are interbedded in 

 clays containing fossil forces, and are, therefore, truly sedimentary, 

 with the Cave Sandstone, in which the fossils are large animals and 

 bits of tree trunks and other plant stems such as might have been 

 entombed in a mud, all point to the volcanic origin of the Cave Sand- 

 stone. The discovery by Mr. du Toit of pond deposits intercalated 

 in the Cave Sandstone full of phyllopods, remains of cockroaches 

 and other land insects, is further evidence that the Cave Sandstone 

 was not formed under water, f 



At Ascension Darwin noticed a white earthy stone forming in 

 places isolated hills, at others associated with columnar trachyte (?) 

 He was struck with its resemblance to a sedimentary tuff, but he 

 ruled out the supposition owing to his inability to explain the 

 presence of crystals of felspar, black microscopical specks and small 

 stains of a dark colour occurring in proportional numbers in an 

 aqueous deposit ; he imagined, therefore, that the rock was a mass 

 of trachyte that had been weathered completely through and through. 

 I have no access to a more recent or better description of the rock, 

 for the Challenger Reports contain only guesses as to what Darwin's 

 rock may have been. EhrenbergJ has described a white rock occur- 

 ring at the Devil's Riding School as a pyrobiolith, that is a tuff 

 containing minute organic particles, and in the Challenger Reports§ 

 this same rock is said to be made up almost entirely of the siliceous 

 particles of grasses, and not of diatoms. Prestwich thought that the 

 diatoms lived in underground waters which became caught up in the 

 ascending lava.|| May this rock not be a mass of volcanic ash mixed 



* See "Volcanoes of Griqualand East," Trans. Phil. Soc, vol. xiv. p. 98, Cape 

 Town, 1903. 



f Ann. Rep. Geol. Comm. 1904. 



I Ueber einen bedeutenden Inf usorien haltenden Vulkanischen Ashen Tuff (Pyro- 

 biolith) auf der Insel Ascension, Berichte d.k. Akad. d. Wiss, Berlin, 1845, p. 140. 



§ " Chemistry and Physics," II., VII., p. 42. 



|| Proo. Roy. Soc, 1886, No. 246, p. 156. 



