536 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY, [Feb. 2, 



We do not hear in such cases (or indeed in any) of one single 

 explosion Hke that of a mine, or the bursting of a bubble, followed 

 by the engulfment of the shattered rocks, and immediate quiescence. 

 I know of no reliable report from any quarter, or at any period, of 

 an eruption of such an ephemeral character*. The explosive dis- 

 charges of vapour having once begun, are always continuous, for days, 

 weeks, months, or even occasionally for years, evidently proceeding 

 from a mass of subterranean lava in a state of ebullition, which, 

 having once forced a communication with the open air, at the weakest 

 point of some fissure broken by its expansive efforts through the 

 overlying rocks, blows itself out through this opening by degrees, 

 although with terrific violence, — just as would the boiler of a high- 

 pressure steam-engine, of enormous dimensions and infinite lateral 

 strength, when the valve of the steam-pipe, or an accidental crack 

 were opened — and not after the manner of a boiler bursting, and 

 discharging all its steam at once, or of an exploding mine of gun- 

 powder. 



It is not by one explosion, but by the continued repetition of mul- 

 tiplied explosions or eructations, caused by the successive upward 

 rush and discharge of innumerable vapour-bubbles of prodigious 

 elastic tension, that an eruption is characterized, and it is by their 

 continued action that the solid mass of rock overlying and obstruct- 

 ing the vent is — not at one shock, but by degrees — broken up and 

 ejected; many of the fragments falling back repeatedly into the 

 cavity, and being re-ejected, until they are for the most part ground 

 by friction into lapilli (i. e. small globular or bouldered scoriae), or 

 even to the finest powder, which the winds carry to vast distances. 

 This process it is that, as it were, eviscerates the moimtain, leaving 

 at the close of the eruption, when the ebuUition has spent its force, 

 that circular or oval chasm surrounded by a ring of steep sloping 

 sides, or precipitous cHffs, which is the well-known usual form of 

 the larger volcanic craters, and which will be generally of a size 

 proportioned to the violence and duration of the eruption — certainly 

 to the amount of fragmentary matter thrown out by it and spread 

 over the surrounding slopes, or the adjoining surfaces of sea or land. 



I may perhaps, myself, entertain a more distinct conception than 

 many others, of the mode of formation of a great volcanic crater, 

 from having enjoyed the good fortune of witnessing, indeed of closely 

 observing, throughout its progress, the most violent eruption that 

 has occurred in Europe within the memory of living men — I mean 

 that of Yesuvius in October 1822, when continuous and rapid ex- 

 plosions (too rapid to be counted), and throwing up a column of 



* The examples usually quoted of such a supposed phenomenon are those of 

 Carguairazo in 1698, Papandayang in 1772, and Galongoon in 1822. In each 

 of these cases, no doubt, there is ample testimony to the fact that the top of the 

 mountain was destroyed by the eruption, and a vast chasm produced in its place. 

 But the explosions which effected this were not single, like that of a mine, and 

 consequently followed by subsidence of the mountain-top into some subterranean 

 hollow, but lasted for months, and the materials of the mountain-top were blown 

 outwards, and spread over the adjoining regions in prodigious quantities and to 

 a vast distance. 



