1859.] SCEOPE CONES AND CKATEKS. 547 



element in the theory of volcanic action above sketched — that the 

 central parts of a volcanic mountain will from time to time suffer a 

 certain amount of derangement, and even of absolute elevation, in 

 the course of its progressive accumulation, — an amount correspond- 

 ing (as I said in mj work on Yolcanos of 1825, and again in 1827) 

 to the number and bulk of the injected dykes that penetrate its beds 

 being formed by the filling up with intruded lava of fissures broken 

 through its framework by the expansive throes or local earthquakes 

 that more or less accompany each eruption. But this "inward 

 growth or distension " will be trifling in comparison with the in- 

 crease of the mountain by outward accretion or accumulation ; and 

 that it has always been so may be seen from the proportionate bulk 

 of the beds of lava and conglomerate, and of the dykes that traverse 

 them, wherever the interior of a volcanic mountain has been ex- 

 posed *. Above all, it must have been a slow and gradual process, 

 accompanying, throughout, the gradual accumulation, in very much 

 larger masses, of sloping beds, both solid and fragmentary, formed 

 from the external dejections of the volcano ; and therefore in no 

 degree justifies the theory of the single, sudden, and simultaneous 

 upheaval of such a mountain by " a bubble-shaped swelling-up 

 of the ground " — enounced by MM. de Humboldt, de Buch, de Beau- 

 mont, and Dufrenoy, as the normal mode of formation of a volcanic 

 mountain. 



Still less is any confirmation of their doctrine to be found in 

 the amount of elevation en masse of large superficial areas, upon 

 which the products of volcanic eruptions have been previously ac- 

 cumulated. This no one doubts to have occurred in many instances — 

 as, for example, the entire western coast of Italy, the submarine base 

 of Etna, of Teneriffe f? of Madeira, and of numerous other volcanic 

 islands or districts. It is, however, a remarkable fact, that in the 

 interior of continents, where the greatest amount of elevation en 

 masse has taken place, few or no volcanic eruptions have occurred ; 

 and contrariwise, the greater number of (indeed nearly all) volcanic 

 districts are found in islands, or along coast-lines more or less 

 removed from, though exhibiting remarkable parallelism to, the 

 great neighbouring continental mountain-ranges or elevated areas — 

 a disposition favourable to the generally received notion that volcanos 

 act as safety-valves, letting off the local excess of subterranean 

 expansive force or caloric, which, where such escape is denied, dis- 

 tends, dislocates, and upheaves the solid superficial rocks in masses 

 of a much more extended area. This remark, made by me in 1825, 

 was illustrated by a rude map of the parallel Volcanic and Plu- 

 tonic ranges of the globe t. It has obtained the sanction of many 

 geologists, even of MM. de Humboldt and de Buch. But I submit 



* See Trans. GTeol. Soc. Lond. 2nd ser. vol. ii. p. 341, 1827. 



t This was, of course, the cause of the position of the marine shells found 

 in the Isle of Teneriffe at insignificant heights above the sea-level, which are 

 brought forward by Mr. Piazzi Smyth as a proof of the upheaval of the whole 

 mountain. See Sir C. Lyell's notice in Phil. Mag. July 1859. 



J Considerations on Volcanos, 1825-6. 



