508 PKOCEEDINGS OP THE GEOLOGICAL SOCIETT. [Feb. 2, 



tion of volcanic mountains (which I need hardly say I myself 

 adopted and enforced in my ' Considerations on Yolcanos/ published 

 in 1825) has been substituted by certain later geologists, chiefl)^ of the 

 Continental schools, one which ascribes the production of all, or nearly 

 all, volcanic mountains to the sudden ' upheaval,' at one shock, of a 

 tract of pre-existing horizontal strata of lava and volcanic conglome- 

 rate, into the shape of a hollow cone or dome, inflated like a bladder 

 by the sudden expansion of a great volume of vapour beneath; 

 which bubble or bladder is further supposed to have burst at the 

 top whenever a crater is found there. In order to avoid misrepre- 

 sentation, I will quote the definition given by Baron Humboldt, the 

 latest exponent (as well as, I believe, the original inventor) of this 

 theory. In the recently published volume of his ' Kosmos*,' he 

 says : — 



" In regard to volcanos, the form-giving, or shaping, activity is 

 exerted by the upheaval of the ground ; not (as was formerly and 

 almost exclusively believed) in building tip hy successive accumulation 

 of scoriae and strata of lava deposited over one another. The re- 

 sistance which the fiery-fluid masses, pressed in too great abundance 

 (from below) against the siu-face, flnd at the sjiot which is to be the 

 channel of eruption, occasions the augmentation of the upheaving 

 force. There arises a ^ bubble-shajped pusliing-u]) of the ground,'' as 

 is indicated by the reg-ular outward slope of the ujpheaved strata. A 

 mine-like explosion, the bursting of the central and highest portion 

 of this convex swelling of the ground, sometimes produces what 

 Leopold von Buch has termed a ^ crater of elevation,' — and, when 

 the structure of a permanent volcano is to be completed, a dome- 

 shaped or conical mount likewise, in the middle of the * crater of 

 elevation,' which inner mount is also, in the greater number of cases, 

 open at its summit," etc. 



The first suggestion of this theory is, I believe, to be found in 

 M. de Humboldt's own description of the Mexican volcano of JoruUo, 

 in his great work on New Spain f. In his 'Atlas Geographique,' 

 ed. 1814, he gives also views and j)lans of the j)roducts of the great 

 eruption of 1759 at that locality, which, according to his notion, 

 occasioned the sudden swelling up of the surface of a previously 

 flat plain into an immense hollow convexity in the shape of a blister 

 or bubble, " en forme de vessie ;" from the midst of which convexity 

 or "j)laine bombee,"" locally called the " Malj)ais," arose six great 

 conical hills, covered with volcanic ashes, the largest of which — the 

 mountain of Jorullo proper — has a crater, and a massive promontory 

 of basaltic lava attached to it, and appearing to cascade, as it were, 

 from the crater at a point high up on the flank of the cone. The 

 surface of the convex plain was also studded with thousands of small 

 hillocks under ten feet high, which, at the time of M. de Humboldt's 

 visit in 1780, twenty years after the eruption, were still smoking, 

 and thence called " hornitos," or ovens, by the natives of the country; 



« Part I. p. 224, Sabine's Translation, 1858. 



t Essai Politique sur la Nouvelle Espagne, ed. 1811, vol. i. p. 251. 



