D. Forbes — On Volcanos, 317 



It having always been admitted that volcanos owed their origin to 

 forces operating from below, it was suggested by Von Buch, and 

 supported by Humboldt and others, that volcanic cones must be 

 formed by some portion of the surface of the earth weaker than the 

 rest being forced out, or as it were blown up like a soap-bubble, by 

 the pressure of the vapour and gases confined below, the strata being 

 thereby elevated, fractured, and tilted up on all sides, so as to pro- 

 duce a conical elevation, the central fissure in which became a crater 

 or vent for the escape and passage of the gaseous and liquid eman- 

 ations from below. 



This hypothesis, which accounted for the formation of volcanic 

 cones and craters by a process of upheaval, or as it was termed the 

 " Crater of Elevation," is here alluded to, only because it for a long 

 time was accepted by many eminent men of science, until the sub- 

 sequent researches, especially of Mr. Scrope and Sir Charles Lyell, 

 demonstrated conclusively that it is not confirmed when their 

 actual structure is studied in the field, and explained their true for- 

 mation by what is now termed the " Crater of Eruption" theory. 



If we imagine a volcanic cone cut through its centre, so as to pre- 

 sent us with a section of its entire mass, it will be seen that the 

 mineral matter of which it is composed possesses in itself a sort 

 of arrangement in layers, which at first sight somewhat resembles 

 beds of ordinary sedimentary origin broken through and tilted up 

 towards the centre ; a closer examination, however, shows that these 

 layers were never at any time horizontal, but that, on the contrary, 

 they had, from, the very first, been deposited in the same inclined 

 position in which they are now seen, and that they must have been 

 formed subsequently, not previous to the opening of the crater itself, 

 since they are entirely comjDosed of matter thrown up from its 

 orifice. Every step in the formation of such volcanic cones has now 

 been carefully studied and described ; and as I have myself had 

 good opportunities for watching their construction during most of 

 the stages, I shall attempt a short description of the general features 

 of a volcanic eruption. 



The commencement of an eruption is known in most cases by 

 certain preliminary symptoms indicative of great internal disturb- 

 ance, such as rumbling noises and sounds, as if of explosions below, 

 which have been likened to subterranean thunder ; the surface 

 waters, springs and wells in the vicinity, generally acquire an un- 

 usually high temperature, diminish in volume or disappear alto- 

 gether, and repeated earthquake shocks more or less severe are felt, 

 which eventually culminate in a grand convulsion, by which the 

 surface is rent asunder with fearfiil violence, allowing immense 

 volumes of previously pent-up vapour and gases to rush forth from 

 the fissure, with such impetuosity as to hurl high into the air huge 

 fragments of the shattered rocks, along with large quantities of 

 molten lava, in so liquid a condition, that during its ascent it is seen 

 to be splashed about in the air like water, and to become separated 

 into particles of all sizes. Vast quantities of these particles, to which 



-VOL. VII.— MO. LXXIII. 21 



