212 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DcC. 19, 



It will be seen therefore that to account for the excavation of certain 

 large crateriform cavities in some of these same islands, I am merely 

 introducing a force, which is already acknowledged to have been most 

 energetically exerted in destroying extensive masses of rock formerly 

 emdroning the spaces called elevation-craters. 



Having said thus much of the denuding or remo^dng power, I shall 

 next offer a few prefatory remarks on the mode of origin of the 

 dome-shaped volcanic masses, of which I consider the boundary rocks 

 of every denudation-crater to be the basal remains. Mr. Scrope, 

 writing in 1827, attributed the formation of a volcanic cone chiefly to 

 matter ejected from a central orifice, but partly to the injection of 

 lava into dikes, and to that force of gaseous expansion, the intensity 

 of which in the central parts of the cone is attested, he said, by the 

 local earthquakes which often accompany eruptions*. But it was 

 reserved for M. E. de Beaumont, seven years later, to point out that 

 the extent, uniform thickness, and compact structure of many sheets 

 of basaltic lava, which constitute the flanks of many volcanic cones, 

 such as Etna and Somma, leave very little doubt that they were ori- 

 ginally poured out on a surface, much less inchned to the horizon 

 than the angle at which they now slope. To the same observer we 

 are indebted for most valuable researches into the laws governing the 

 flow of lava streams, the result of which he published after his visit 

 to Etna in 1834. In his memoir on that mountain he endeavoured 

 to prove that the numerous up-filled fissures or dikes are the evi- 

 dence and measure of the elevation of the distended volcanic mass, 

 consisting of sheets of lava and alternating conglomerates, and that 

 the whole mountain is probably undergoing upheaval bodily from time 

 to time, as often as it is traversed by star-shaped cracks, such as oc- 

 curred during the eruption of 1832. 



In the later edition of my ^Principlesf I referred to the labours of 

 M. de Beaumont, and admitted that a greater part of the beds ex- 

 posed in the precipices of the Val del Bove were " originally less in- 

 clined, some of them perhaps much less so than now." At the same 

 time I attributed the change of position to the " successive fracturing, 

 distension and upheaval of the cone," not to a sudden upthrow. 

 Whether I still underrated the amount of unequal elevation by which 

 certain beds are believed to have been tilted and changed from their 

 pristine horizontality, I know not, but I feel as convinced as ever that 

 I was right in continuing to reject the hypothesis of elevation-craters, 

 of which MM. de Beaumont and Dufresnoy have been the able and 

 strenuous advocates. When repeating in my different publications 

 the objections previously urged by myself and others to the theory of 

 Von Buch, I always cited the argument so strongly insisted upon by 

 M. Constant Prevost, that if beds of non-elastic materials had yielded 

 suddenly to a violent pressure directed from below upwards, we 

 should not find a circular cavity with an even and unbroken rim, but 

 an irregular opening where many rents converged, and these rents 



* Geological Transactions, 2nd Series, vol. ii. p. 341. 

 t See edition of 1847, p. 401. 



