64 Mr Scrope's Observationc on 



or bubble, (of four square miles in extent !) and covered by an 

 effort of the same extraordinary and incomprehensible nature 

 with thousand of small basaltic cones, each owing to a similar 

 process, has been generally received by geologists as an as- 

 certained fact, and made the basis of further and still more 

 strange hypotheses for the purpose of explaining the origin of 

 the dome-shaped mountains of frequent occurrence in Tra- 

 chytic countries, and the still more common conical peaks of 



basalt. 



If, from the reasons adduced above, it appears most proba- 

 ble that the convexity of the plain of Malpais is simply owing 

 to its forming the surface of a massive subjacent bed of lava, 

 emitted contemporaneously by the six volcanic cones which 

 rise from its surface, it will be obviously impossible to draw 

 any argument from the formation of the Hornitos, none of 

 which exceed nine feet in height, as to that of mountains like 

 the Puy de Dome, Chimboraco or Pichinca, the two latter of 

 which are from 15 to 18,000 feet in height. The theory, 

 therefore, built on the supposed example of Jorullo, must fall 

 to the ground. * 



It has been seen, in another part of this Essay, that the pe- 

 culiarity of figure, assumed occasionally by masses of trachyte 

 and basalt, is easily to be accounted for without having re- 

 course to the agency of unknown and imaginary forces, or, 

 indeed, any other than those with the operation of which we 

 are thoroughly conversant, and which are fully equal to the 

 purpose. 



I trust to be forgiven the apparent presumption of thus 

 calling in question opinions formed by an observer of such 

 acknowledged sagacity and experience as M. de Humboldt, 

 upon facts to which, except through his accounts, I am neces- 



* The other example adduced by Humboldt for the same purpose, viz 

 the supposed intumescence of the plains that form themselves on the sum- 

 mit of volcanic cones in place of their craters, is equally inadmissible. It 

 has been already shown, in the body of this work, that the craters are 

 filled by a general law, from the accumulation of fragmentary ejections, 

 and of lava swelling up from below. This will necessarily tend to produce 

 a final convexity of surface ; but it would be manifestly absurd to argue, 

 from this form, the existence of a vaulted cavity below. 



