of Lavas. 217 



Aet. XYL — On the Formation of Craters^ and the Kature of 

 the Liqxddity of Lavas :''^ by G. POULETT ScKOPE, Esq., M.P., 



F.RS., F.as. 



n. The nature of the liquidity of lavas, — So mucli for that 

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

 Avish now to ask attention to some circumstances respecting the 

 mode of emission and nature of tlie 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 viscosity of the heated matter at 

 the time of its efflux may occasion corresponding differences in 

 the character of the phenomena. Observation confirms this ex- 

 pectation ; 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 vapors evidently escaping with comparative 

 ease, while the more viscous and ultimately stony lavas, possess'- 

 ing a minor degree of liquidity, and consequently not allowing 

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

 escape from them, are protruded with fiercer explosive bursts, 

 and the ejection of far greater quantities of scoria and other frag- 

 mentary matters. 



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

 pT'esently advert, led me to an opinion expressed in the works 

 above referred to, that the ordinary crystalline or granular lavas 

 (making exception of the vitreous varieties), although at a white 

 beat at the moment of their emission from a volcanic vent, are 

 ^ot in a state of complete fusion ; that a large proportion, at least, 

 if not all, of the crystalline or granular particles of which, when 

 cooled and consolidated, they appear composed, are already 

 formed and solid, their mobility being aided by the intimate dis- 

 semination through the mass of a minute but appreciable quan- 

 tity of some fluid,— in all probability water, — which is prevented 

 from expanding whollv into vapor by the pressure to whicll it is 

 subjected while within the volcanic vent, or in the interior of 

 the current, until that pressure is sufficiently reduced to allow of 

 its expansion into bubbles, or its escape through pores or cracks, 

 by which it passes into the open air from the surHice of the intu- 



n^escent lava. 



. I was strengthened in this opinion by several concurrent con- 

 siderations :— 



1^ If all lavas are (as they are usually supposed to be) in a 

 state of complete fusion when they issue from a volcano, how is 



Concludetl from vol. xxiii, p. 359. 



SECOND SERIES, VOL. XXIV, NO. 71. — SEPT., 1857. 



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