~ Natural History of Volcanos 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 voleanic power, and that the beds of salt are a conse- 
quence of voleanic 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 voleanic phenomena in. intense chemi- 
cal action as untenable. 
Il. The hypothesis which supposes the temperature of the earth 
Sradually to increase towards the centre, to a red and white 
heat, explains in a satisfactory manner (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. 
hese 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 indéed are not wanting in any of them. So that, in gen- 
eral, the more abundantly alkaline minerals, as felspar, mica, leu- 
tite, &e., are contained in volcanic masses, the more readily will 
they fuse.* 
‘Sir James Hall+ has endeavored to ascertain the degree of fusi- 
bility of various lavas and other volcanic rocks. Lava from Ve- 
Ropar hol nea ae ASA eG SO ne eet 
: According to Von Buch, (Abhandlungen d. Kénigl. Acad. d. Wissenschaften 
zu Berlin, 1818-1819, p. 62,) it may be taken as a general rule, that all real lavas, 
Whi h flow in streams deen the sides of volcanos, contain glassy felspar. Vesuvius 
ng the only exception out of so many is not worth mentioning. 
Fansact. of the Roy. Soc. of Edinburgh. Vol. v, &c. 
