338 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [iVpiil 23, 



produced in the same manner as the internal, and similarly consti- 

 tuted ones ? And therefore those who refuse to believe the former to 

 be of eruptive origin must be prepared to extend their incredulity to 

 the latter. Indeed the elevation-crater theorists usually do not 

 shrink from this consequence. With them the cone of Vesuvius, 

 and that of Monte Nuovo itself, v^ere not the products of eruption, 

 but of elevatory expansion by a single shock. Obviously, it ought 

 to follow, that no volcanic mountain was ever in eruption at all, that 

 the whole is an ocular illusion ; at least, that the lava-streams we see 

 pouring for weeks and months from the summit of a cone and hard- 

 ening there, and the enormous showers of fragmentary matter which, 

 during equally long periods, we see thrown up from the crater and 

 falling on the surface of the cone, do not, even in the lapse of ages, 

 add to its bulk or tend by their frequent repetition to compose the 

 substance of a volcanic mountain, but, by some unaccountable pro- 

 cess, disappear without leaving a trace behind. I own that, to my 

 mind, such an hypothesis is wholly unintelligible. I see in the 

 ordinary phaenomena of a volcanic mountain, such as I have described 

 them in the brief reiiiord of the principal eruptions of Vesuvius 

 during the last century, a simple and natural process by which such 

 a mountain is gradually built up ; and, having observed this mode of 

 formation going on in some instances before my eyes, I think it 

 reasonable to apply it to explain the mode of formation of other 

 mountains of the same class, with their cones and craters, old and 

 new, central and lateral, or parasitic ; and making allowance, as I 

 said at first, for a certain amount of internal accretion and elevation, 

 by means of intrusive dykes and earthquake shocks, I know nothing 

 in the appearance, figure, or structure of any volcanic mountain yet 

 discovered, which such an ordinary and observed mode of formation 

 will not account for. 



II. The nature of the liquidity of lavas. — So much for that 

 branch of my subject, — the formation of cones and craters. I wish 

 now to ask attention to some circumstances respecting the mode of 

 emission and nature of the lavas that proceed from them. I have 

 already spoken of the comparatively tranquil manner in which some 

 lava-streams are seen to well out from the flank of a volcano, or its 

 summit, and the probability that differences in the liquidity or vis- 

 cosity of the heated matter at the time of its efflux may occasion 

 corresponding differences in the character of the phsenomena. Ob- 

 servation confirms this expectation ; and it has been remarked, that 

 the very liquid and vitrified lavas, such as those of Kilauea and 

 Bourbon, are poured out more or less tranquilly without any very 

 violent explosions, their imprisoned vapours evidently escaping with 

 comparative ease, while the more viscous and ultimately stony lavas, 

 possessing a minor degree of liquidity, and consequently not allowing 

 so easy a passage to the vapours that rise through, and struggle to 

 escape from them, are protruded with fiercer explosive bursts, and 

 the ejection of far greater quantities of scoriae and other fragmentary 

 matters. 



This observation, coupled with other reasons to which I shall pre- 



