1 24 Volcanos. 



group of brilliant rays resembling the thunder bolts placed 

 by the ancient artists, in the hands of the clould-compelling 

 Jove. 



" The consolidation of lavas is next treated of. This is effect- 

 ed equally by the condensation or escape of its fluid vehicle. 

 Its condensation takes place either by increased pressure or di- 

 minished temperature. This mode of consolidation is supposed 

 peculiarly favourable to the reunion of many of the disintegrated 

 crystals, the gradual diminution of the vapour bringing the par- 

 ticles by slow degrees within the sphere of their reciprocal at- 

 tractive forces, while the remaining elasticity leaves a sufficient 

 mobility to permit of the reversion of their poles in obedience 

 to these forces ; and thus a partial recrystallization may be ex- 

 pected to take place. Such crystals, it is shown, will have their 

 longest dimensions perpendicular to the pressure upon that part 

 of the lava. 



" But that portion only of the elastic fluids will be condensed, 

 which cannot effect its direct escape. This is completely pre- 

 vented in some cases, as in dikes, &c. But where the lava is ex- 

 posed to contact with air or water, this escape takes place to a 

 greater or less degree, in one or both of two modes : viz. 1. By 

 ascent in bubbles through the liquid lava. The more fine-grained 

 the lava, the more spherical the bubbles, from the equalization of 

 the pressure on all sides. These vesicles are often elongated as 

 the lava moves onwards ; their size will be proportioned to the 

 specific gravity and liquidity, in other words, to the fluidity of 

 the lava, and the same circumstances determine the proportion 

 of vapour which escapes in bubbles, to that which remains be- 

 hind. Of the latter, a part escapes in the 2d mode, viz. by per- 

 colation through the pores and crevices of the already solid ex- 

 terior. This process advances from the surface inwardly, with 

 a rapidity proportioned to the porosity of the resulting rock, 

 which will vary directly with the average size and irregular ar- 

 rangement of its crystalline particles. 



" From these considerations, the author deduces the following 

 propositions, as to the conduct of differeut varieties of lava, when 

 protuded upon the surface of the earth. 



"1. If of extremely fine grain, and low specific gravity, the 

 superficial congelation of the mass will be rapid, that of the inte- 

 rior slow : its fluidity considerable ; air-bubbles spherical, some- 

 times elongated horizontally or vertically ; the scorias of such 

 lavas is pumice. Owing to the extreme slowness of the consoli- 

 dation of the interior, and its great mobility of parts, a more or 

 less perfect recrystallization, or concretionary process, will take 

 place. Pearlstones, radiated or not, or variolites, will be pro- 



