130 



Prof. J. Prestwich. 



mountain. Scrope* mentions as examples of such paroxysmal erup- 

 tions, — 13 eruptions of Vesuvius, 8 of Etna, 2 of Teneriffe, 1 of San 

 Georgis in the Azores, 3 of Palma, and 1 of Lancerote (Canary 

 Islands) , and all the recorded eruptions of Iceland. 



" Sometimes in these eruptions no absolute escape of lava takes 

 place, scorias alone being projected. In all cases when lava is emitted 

 its protrusion marks the crisis of the eruption, which usually attains a 

 maximum of its violence a day or two after its commencement. The 

 stopping of the lava in the same manner indicates the termination of 

 the crisis, but not of the eruption, for the gaseous explosions continue 

 often for some time with immense and scarcely diminished energy, "f 



It seems to me therefore evident from these and such other cases, 

 that there is no definite relation between the quantity of explosive 

 gases and vapours and the quantity of lava discharged from the 

 volcanic foci. It is conceivable that the enormous force of some of 

 the explosions may, in the paroxysmal outbursts, shatter and blow to 

 fragments all the lava as it rises in the crater, but this seems hardly 

 sufficient to account for the proportionally large quantity which 

 should accompany such vast volumes of vapours, were those vapours 

 the cause of the extrusion of the lava. It is still more difficult to 

 conceive on this hypothesis the excessive discharge of lava in tranquil 

 eruptions without a greater escape of vapour. 



If the escape of lava depended altogether on the escape of the im- 

 prisoned vapours, it is not easy to see how the constant supply, whethei 

 of the lava or of the steam, is maintained. The rise and escape out- 

 wardly of the lava in a volcanic vent has been likened to the boiling up 

 and over of any other thick and viscid matter exposed to heat from 

 beneath in a narrow-mouthed vessel, J and Constant Prevost compared 

 it to the overflow caused during fermentation by the evolution of car- 

 bonic acid gas. But the cases are not analogous. In the one the 

 aeriform fluid is part of the substance of the vaporisable matter, which 

 is not the case with the lava where the substance causing ejection is 

 foreign to it. In the first case the elastic fluids are generated by a 

 molecular change of the heated substance itself, and the supply is 

 therefore, so long as that lasts, unlimited; whereas in the case of lava, 

 which cannot undergo such changes, it is only the supposed occluded 

 vapour in it, that, with the relief of pressure would be subject to 

 expand and escaj)e, and thereby displace and expel a proportionate 

 quantity of the lava. Besides, could that result take place before 

 the lava began to rise ? If not, there must be an independent cause 

 to originate the rise. We might also ask whether that very rise of 



* " Volcanos," p. 25. 

 + Ibid., p. 23. 



% Scrope's " Volcanos," p. 40, and Lyell's " Principles," toI. ii, p. 221. 



