144 



Prof. J. Prestwich. 



Eigl 3. — Diagram section of Etna. 



rana and near Bronte, the line of water-level will rise in proportion as 

 the ground rises between a and b ; and in the same way in the great 

 central dome which rises rapidly above the lower slopes, the water- 

 line will rise more suddenly. It is therefore possible that in the 

 centre of the mountain the line of permanent saturation, I, may 

 occasionally stand higher, V, than the top of the Val del Bove (5292 

 French ft., Abich). It may lie too deep for springs to issue, but the 

 remarkable flood in that great depression described by Lyell,* and con- 

 nected with the eruption of the year 1755, seems explicable on the 

 supposition of a high central water-level more readily than on the 

 assumption of the sudden melting of a mass of ice in the interior of 

 the mountain. For although ice may be formed and retained under a 

 covering of lava in the manner described by Lyell near the summit of 

 Etna, the cold would not penetrate to a sufficient depth to allow of 

 the accumulation of a mass of ice of the dimensions required for so 

 great a flood. Any body of ice must be superficial, as the increase of 

 heat with the depth from the surface would be a bar to its existence 

 in the interior of the mountain, independently of the heat diffused at 

 all times from the central duct. Nor would the sudden melting of the 

 snow, which never lies very deep, and would frequently recur, explain 

 the sudden great and exceptional outburst of flood-waters described 

 by contemporary writers. f 



On the other hand, heavy rains or the prolonged repose of the 

 volcano, may have resulted in the exceptional rise to V of the level of the 

 underground water-line, I, so that if one of these radial fissures, so 

 frequently formed during the eruptions of Etna, suddenly opened in a 

 direction to traverse the water-logged strata, the effect would be to 

 tap and drain at once the whole of this subterranean reservoir to the 

 level of the point of escape — in the Val de Bove — a point still about 

 5000 feet below the summit of the mountain ; while the water, coming 

 as it would from the centre of the mountain, would also account for 

 its reported heat. 



* " Phil. Trans.," vol. 148 (1858), p. 68. 



f Canon Recupero, who reported on the catastrophe, came to the conclusion thrt 

 the water was vomited forth by the crater itself, and was driven out from some 

 reservoir in the interior of Etna (Lyell, op. cit.). 



