Natural History of Volcatios and Earthquakes. 239 



rock-salt are of volcanic origin. But this proves nothing more 

 than that rock-salt may have been raised from the interior of the 

 earth by volcanic power, and that the beds of salt are a conse- 

 quence of volcanic action, but not conversely, that chlorides and 

 the disengagement of muriatic acid are the cause of that phe- 

 nomenon. 



Now, since neither any process of oxidation, nor processes in 

 which chlorides take an active part, are capable of affording a 

 satisfactory explanation of volcanic phenomena, we can scarcely 

 conceive any other powerful chemical process, which could alone 

 give rise to them. We may, therefore, look upon the hypothesis 

 which seeks the cause of volcanic phenomena in intense chemi- 

 cal action as untenable. 



11. The hypothesis which supposes the temperature of the earth 

 gradually to increase toioards the centre, to a red and white 

 heat, explains in a satisfactory man7ier {according to the pres- 

 ent state of science) volcanic phenomena as well as earthquakes. 



If the heat of the earth continually increases with the depth, 

 the rocks must at a certain depth be in a state of fusion. But 

 since they possess such various degrees of fusibility, the more 

 fusible rocks must be in a liquid state, at depths in which the less 

 fusible ones are still solid. At certain depths there must, conse- 

 quently, be masses of melted rocks, enclosed in the solid rock, in 

 the same manner as iron ores are melted and reduced in the less 

 fusible masses of which blast furnaces or crucibles are composed. 

 These depths must, according to the above hypothesis, be looked 

 upon as the seat of volcanic action. The crystalline rocks are 

 the most easy of fusion on account of their containing alkalies, 

 which indeed are not wanting in any of them. So that, in gen- 

 eral, the more abundantly alkaline minerals, as felspar, mica, leu- 

 cite, &c., are contained in volcanic masses, the more readily will 

 they fuse.* 



Sir James Hallf has endeavored to ascertain the degree of fusi- 

 bility of various lavas and other volcanic rocks. Lava from Fe- 



* According to Von Buch, (Abhandlungen d. Konigl. Acad. d. Wissenschaften 

 zu Berlin, 1818-1819, p. 62,) it may be taken as a general rule, that all real lavas, 

 which flow in streams down the sides of volcanos, contain glassy felspar. Vestivius 

 being the only exception out of so many is not worth mentioning. 



t Transact, of the Roy. Soc. of Edinburgh, Vol. v, &c. 



