224 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 19, 



as in modern times. It explains, in short, why Somma, like YesuTius, 

 towers aboTe all the other volcanic eminences. 



The summit of the submarine dome may probably have had a 

 gentle slope on all sides, not exceeding perhaps 8° or 10°, although 

 we have yet to learn in regard to subaqueous lavas, whether, moving 

 through a denser and more resistmg medium than the atmosphere, 

 they may not spread in wider sheets and assume a compact texture, 

 or an inclination exceeding that required to produce the same effects 

 when their course is subaerial. The more general absence of ravines 

 and valleys in the bed of the sea, where volcanic eruptions occur, would 

 promote the spreading out of the melted matter in an even sheet, and 

 the pressure of the incumbent water would check the expansion of 

 the gases and prevent the mass from acquiring a more open and cel- 

 lular texture. AATien M. Pilla had attentively observed in 1837 and 

 aftei-wards in 1845 the similarity of the disposition of the beds in 

 Somma and the modern Vesuvius, he could not resist the conclusion 

 that both were formed in an analogous manner, and he rejected the 

 theory of elevation-craters as applied whether to the one or the 

 other*. At a later period however jSI. Pilla admitted, that a sheet of 

 basalt 1000 metres above the sea in Somma, inclined at an angle of 

 24° and very compact, proved that there had been an upheaval of the 

 massf ; an opinion which is compatible with the views embraced in 

 this paper respecting the gradual increase of a cone by internal and 

 external additions. That a vast number of eruptions were concen- 

 trated within a narrow space is assumed by M. Dufresnoy himself, who 

 considers the dikes of Somma as having been the feeders of successive 

 beds or sheets of lava. It was not overlooked that a long series of 

 eruptions occurring within very confined limits must in the coui'se of 

 time have given rise to a conical mass composed in this instance of 

 superimposed fragmentary and porphyritic beds. Such a result how- 

 ever was opposed to Yon Buch's hypothesis, and in order to escape 

 from it in this and other analogous cases, a Yevj arbitrary hypothesis 

 was resorted to ; — a depression in the bed of the sea was assumed to 

 have pre-existed, in which the beds of lava and scoriae accumulated 

 in horizontal masses, and the position of the mass thus formed was 

 finally inverted, the convex side being made to project upwards in- 

 stead of downwards. 



Somma is the remains of a crater about three miles in diameter, the 

 walls of which we may infer, from a passage in Plutarch, were before 

 the great eruption of Vesuvius in the year 79, veiy perfect and entire, 

 except on one side, where there was a single breach. Dr. Daubeny has 

 shown in his comments on the passage, that when Spartacus encamped 

 his gladiators in the crater in the year 72, Clodiusthe Praetor besieged 

 him there, and keeping this single entrance carefuUy guarded, let down 

 his soldiers by scaling ladders over the steep precipices which sur- 

 rounded the cavity, now called the Atrio del Cavallo, where the insur- 

 gents were encamped;}:. Originally therefore Somma had the usual 

 forai of craters of denudation, a single ravine interrupting the circuit' 



* Pilla, cited by Arcliiac, Hist, des Progres de la Geol. torn. i. p. 538. 

 t Arcliiac, ibid. p. 518. % See Daubeny's Volcanos, p. 216. 



