The Bocks of Tristan d'Acunha. 33 



throat of the volcano, we get an almost identical rock to the Cave 

 Sandstone. To take the resemblances point by point : — 



Both contain cavities filled in with zeolites ; in the Cave Sand- 

 stone agates replace the calcite amygdules, a natural consequence 

 on the difference in basicity. 



Both contain patches of lighter and darker material, which 

 character is also present in Darwin's white rock from Ascension. 



Both contain fossils such as might be entombed in a liquid 

 mud. 



Both are made of minute fragments of non-volcanic rocks ; in 

 the peperino only to a small extent, in the Cave Sandstone generally 

 to the exclusion of volcanic particles, but patches do occur in which 

 there are volcanic fragments. 



Both are volcanic products, and both attain a maximum thickness 

 of 800 ft., but the continuous area of the Cave Sandstone is greater 

 than that of the Italian rock. 



Both are nearly always solid, imbedded masses, but both occasion- 

 ally are separated by ash beds. 



Both are the result of the activity of a great number of vents. 



The peperino is supposed to have poured out as a mud. Pongi * 

 maintains that it came up in the throat as a mud, like in the case 

 of the mud volcanoes of Java and South America, but other authors 

 hold that it is only a volcanic tuff which has obtained its water from 

 the atmosphere either from rain or from masses of snow.t The 

 teaching of the Kimberley pipes seems to point to the fact that 

 Pongi was right, but the matter is very hard to adjudicate on. 



In the Cave Sandstone we have many peculiar features that could 

 be explained by the supposition that it flowed out from the crater 

 mouths as a mud. For instance, it is hard otherwise to account for 

 the immense thickness of the embedded mass ; it is hard to explain 

 the sudden change of great thicknesses of the white rock to a red 

 clayey material ; and still more mysterious is the pseudo-bedding 

 that one can see at N'quatsha's Nek, where the stratification is just 

 such as would be produced had the whole been stirred round in 

 a gigantic pot like a pudding. The relation of the present river 

 drainage to the chain of volcanoes shows that the whole area must 

 have been dry land, and the sub-aqueous origin of the Cave 

 Sandstone must, therefore, be considered very doubtful. Leopold 

 Von Buchj. thought the Italian peperino had been deposited in the 

 sea, but it can be proved that this rock, at any rate, was never 



* Reale Accad. dei Luicei, Rome, 1879-80, Memorie. 

 f Branco, " Swabens 125 Vulkan Embryonen," p. 699. 



J " Geognostiscbe Beobacbtungen auf Reisen," Teil II., Berlin, 1809, p. 70. 



3 



