i2G Volcanos. 



strata, which are' elevated together with them. Such a circum- 

 stance would be in complete conformity with all the laws of sub- 

 terranean effervescence, and in the granitic axes of most mountain; 

 chains, we recognize facts which can only be accounted for by 

 such a mode of production. 



" The aqueous vapours that escape from most lavas, as they 

 are consolidated, are accompanied by mineral substances, which 

 become more and more abundant as the lava cools, and the quan- 

 tity of steam exhaled diminishes. These substances are, by the 

 author, supposed to proceed from the internal decomposition of 

 some of the ingredients of the lava by its intense heat, as the 

 pressure created by the elasticity of the interstitial fluid diminish- 

 es, owing to its expansion and partial escape through the pores 

 and fissures of the rock above. Specular iron is evidently a sub- 

 limation produced in this manner, as well as the delicate crystals 

 of hornblende, augite, melilite, and other minerals which occur 

 in the cellular cavities and fissures of some lava rocks. Sulphur 

 is similarly sublimed, and the same origin must be allowed to the 

 sulphates of lime and ammonia, the muriates of soda and ammonia, 

 &c, which are deposited often in great abundance at the sides and 

 edges of the fumarole. Other minerals resulting from this inter- 

 nal decomposition are taken up in solution by the steam, and de- 

 posited in crystals of concretions, calcareous, siliceous, &c. in the 

 vesicular cavities of the rock. The cells of most amygdaloids 

 are, however, supposed to have been filled by subsequent filtra- 

 tion of water, carrying in solution mineral particles from over- 

 lying rocks, and the author supposes the pressure of a high col- 

 umn of water, (as in lavas of submarine origin,) necessary to effect 

 its penetration through the minute-pores of the lava rock. 



" The sulphuric and muriatic acids are also often met with among 

 the emanations of the fumarole, and their action on the lava com- 

 posing the sides and borders of these crevices, produces new de- 

 compositions and combinations. The sulphate of alumine of the 

 Italian and Hungarian alum works has this origin. These super- 

 ficial alterations of lava have acquired for the spots where they 

 occur, and which are always within the craters of some volcano, 

 the name of solfatara or souffriere. 



" Thermal springs, and the generality of mineral sources, are 

 attributed by Mr. Poulett Scrope to the condensed vapors es- 

 caping from a subterranean mass of lava. Some are intermittent 

 like the Geisers, and the cause of this phenomenon, is dwelt on, 

 and explained. The permanent gases evolved from lavas are 

 next treated of; and an instance related by M. Bory de St. Vin- 

 cent, of seven or eight birds being seen to drop suddenly, while 

 flying over the volcano of Bourbon, is supposed to offer some con- 

 firmation of the poetic fable respecting the Lake Avernus. 



