346 Analysis of Scientific Books and Memoirs. 



abundant as the lava cools, and the quantity of steam exhaled diminishes. 

 These substances are, by the author, supposed to proceed from the inter- 

 nal decomposition of some of the ingredients of the lava by its intense heat, 

 as the pressure created by the elasticity of the interstitial fluid diminishes, 

 owing to its expansion and partial escape through the pores and fissures of 

 the rock above. Specular iron is evidently a sublimation 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 internal 

 decomposition are taken up in solution by the steam, and deposited in 

 crystals or concretions, calcareous, siliceous, &c, in the vesicular cavities 

 of the rock. The cells of most araygdaloids are, however, supposed to 

 have been filled by subsequent filtration of water, carrying in solution mi- 

 neral particles from overlying rocks, and the author supposes the pressure 

 of a high column 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 composing the 

 sides and borders of these crevices, produces new decompositions and com- 

 binations. The sulphate of alumine of the Italian and Hungarian alum 

 works has this origin. These superficial 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 vapours escaping 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 confirmation of the 

 poetic fable respecting the Lake Avernus. 



The circumstances which determine the time occupied by lavas in cool- 

 ing, will depend on the figure of the mass, external circumstances, and the 

 structure and composition of the lava. Instances are quoted of currents 

 retaining a great heat for a considerable time. That of Jorullo, in Mexi- 

 co, is by no means cool yet, though produced in 1759. 



The next chapter treats of the divisionary structure assumed by lavas 

 on their consolidation. This process must be accompanied, at all times, 

 by a diminution of volume or contraction. Were it to commence at the cen- 

 tre of a mass, no separation of parts need take place ; but if at the surface, 

 different centres of contraction must establish themselves, and fissures of 

 retreat be formed between them. The figures these circumscribe, tend to 

 approximate to the hexagon. But since there is no opposition to the con- 

 tractile force, in a direction perpendicular to the surface, which subsides 



