34-4 Analysis of Scientific Books and Memoirs. 



obstacle, such as a wall, which it cascades over without touching it, is then 

 noticed and explained; as also the arched gutters and caverns often 

 formed from its subsidence ; and its effect on grass, trees, and fragments of 

 other rocks ; on marshy ground, and when it enters the sea or any body 

 of water. Its progress below the water is shown to be similar to that on 

 dry land, though slower, with the same degree of fluidity. The water is 

 heated and discoloured by it, and fish often killed in numbers. The fos- 

 sils of Monte Bolca are attributed by our author to such a catastrophe, 

 since the beds in which they occur are topped by basalt and volcanic cal- 

 careous conglomerate. 



The consolidation of lavas is next treated of. This is effected equally 

 by the condensation or escape of its fluid vehicle. Its condensation takes 

 place either by increased pressure or diminished temperature. This mode 

 of consolidation is supposed peculiarly favourable to the reunion of many 

 of the disintegrated crystals, the gradual diminution of the vapour bring- 

 ing the particles 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 expected to take place. Such crys- 

 tals, 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 prevented in some cases, 

 as in dikes, &c. But where the lava is exposed 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 elongat- 

 ed as the lava moves onwards ; their size will be proportioned to the speci- 

 fic 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 behind. Of the latter, a part escapes in 

 the 2d mode, viz., by percolation through the pores and crevices of the al- 

 ready solid exterior. 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 arrangement of its 

 crystalline particles. 



From these considerations, the author deduces the following propositions, 

 as to the conduct of different varieties of lava, when protruded 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 interior slow ; its fluidi- 

 ty considerable ; air-bubbles spherical, sometimes elongated horizontally 

 or vertically ; the scoriffi of such lavas is pumice. Owing to the extreme 

 slowness of the consolidation of the interior, and its great mobility of parts, 

 a more or lessperfect recrystallization, or concretionary process, will take 

 place. Pearlstones, radiated or not, or variolites, will be produced ; and 



