Volcanos and Earthquakes. 35 



II. The hypothesis ichkh supposes the temperature of the earth (jradu- 

 allf/ to increase towards the centre, to a red and white heat, explains 

 in a satisfactory manner (according to the prresent state of science) 

 volcanic phenomena as well as eao'thquakes. 



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, 

 consequently, 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 hypo- 

 thesis, be looked upon as the seat of volcanic action. The crys- 

 talline 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 general, the more abundantly alkaline mine- 

 rals, as felspar, mica, leucite, &c., are contained in volcanic 

 masses, the more readily will they fuse.* 



Sir James Hall-f- has endeavoured to ascertain the degree of 

 fusibility of various lavas and other volcanic rocks. Lava from 

 Vesuvius of the year 1785, melted at 18° of Wedge wood's py- 

 rometer, lava from Torre del Greco not till 40°. But their fu- 

 sibility varied very considerably, according as the melted lava 

 liad been cooled rapidly to a glass, or more slowly to a stony 

 crystalline mass. Thus, for example, those two lavas, when in 

 the form of a glass, both melted at the same degree (18°), whilst 

 the lava of 1785 was less fusible than that of Torre del GrecOy 

 when of a stony nature. :[ I*rom other appearances it may, 

 in general, be concluded, that the fusibility of lavas is between 



* According to Von Buch (Abhaudlungen d. Kbnigl. Acad. d. Wissen- 

 schaften 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 volcanoes, eontaia 

 gliissy felspar. Vemtius being the only exception out of so many is not 

 Avorth mentioning. 



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



X Glass is well known to be acted upon in a similaa* manner. When c«)Ji- 

 vorted, by being melted and slowly cooled again, into Reaumur's porcelain, 

 it comes less fusible. 



