The Bocks of Tristan d'Acunha. 29 



brought there by some gigantic iceberg that had floated thither in 

 olden days. Mellard Eeade regards it as evidence of the far- 

 reaching effect of sub-oceanic currents? but I think it is more likely 

 to be a quiet volcanic eruption of the type Chaper refers to, and the 

 quartz sand to be a part of the old sedimentary beds forming the 

 ocean bed blown to fragments and strewn over the ocean floor. 



Branco * has called attention to the fact that the inclusions in the 

 tuff-necks in Swabia are only in rare cases affected by heat, and 

 the same has been observed in the granite and gneiss blocks in the 

 Weinf elder Maar in the Eifel, as described by von Dechen.f Sir 

 A. Geikie J in the rocks included in the necks along the Firth of 

 Forth says the same thing ; in one instance, in the vent at Elie Neck, 

 he says : — " Among the non- volcanic contents of the agglomerate, 

 special reference may be made to the numerous fragments of 

 crinoidal limestone in certain layers. These show no trace of 

 metamorphism, their crowded organisms being as clearly recognis- 

 able as in pieces of limestone from a quarry." In the Schleursbach 

 volcano in Swabia, the marly middle Lias blocks, as well as the 

 rock in the circumference of the pipe, are burnt black, while the 

 included Belemnites are altered to white marble. Branco says the 

 alteration has been caused by the heat of the tuff, and not by the 

 action of hot gases, as Deffner § maintained, but this heat has been 

 so little that the fossils have not been obliterated. Branco deplores 

 that no diamonds have been found in the German pipes. None have 

 been found in the Scotch ones. 



In Borneo || diamonds occur in a peridotite like the Kimberley 

 rock, but there is no mention of a pipe. In Australia, however, the 

 Bingara pipe is in most respects similar to the South African 

 diamond pipes, and diamonds have been found in it. IT The very 

 large number of similar pipes that were found by Mr. Eogers in 

 Sutherland, ** leads one to suspect that the Indian and Brazilian 

 diamonds may eventually be traced to a breccia neck like those in 

 South Africa have been, and that new areas in which these cold 

 volcanoes are found may be added to those already known. 



It is interesting to note that one can trace a temperature gradation 

 in these breccias from entirely cold ones like the rock in the Elie 



* " Swabiens 125 Vulkan Embryonen," Stuttgart, 1894, p. 761. 

 f " Geogn. Fiihrer zu der Vulkanriehe der Vordereifel," Bonn, 1861, p. 254. 

 \ Geology of Eastern Fife, Mem. Geol. Survey, Scotland, Glasgow, 1902, p. 241. 

 § " Swabiens Yulkan Embryonen," p. 546. 



|| Knop, Sep. Abdruk a.d. Bericht. 23 Versamml. des oberrhein geol. Vereins. 

 IT Stonier Occurrence of Diamonds at Bingara, Records Geol. Survey, N. S. 

 Wales, 1894, vol. iv., pt. 2, p. 51 ; see also Card, ib., vol. vii., pt. 2, p. 29. 

 ** Ann. Kept. Geol. Comm., 1903, Cape Town, 1904. 



