1849.] LYELL ON THE STRUCTURE OF VOLCANOS. 227 



upon the whole corroborates the doctrine of its having been formed 

 by eruption, in proof of which the following passage is cited from Por- 

 zio's description of the event : '^ Verum quod omnem superat admi- 

 rationem, mons circum cam voraginem ex pumicibus et cinere plus- 

 quam mille passuum altitudine una nocte congestus aspicitur." Sig- 

 nor Scacchi also adds, that the ancient temple of Apollo, which is now 

 at the foot of Monte Nuovo, and the walls of which still retain their 

 perfect perpendicularity, could not possibly have maintained that po- 

 sition had the cone of Monte Nuovo really been formed by upheaval. 



Speaking of the fossil marine shells found in the tuff of Monte 

 Nuovo, the same geologist observes, that as the tuff of the new vol- 

 cano was formed in great part out of fragments of the ancient marine, 

 shell-bearing tuff, the appearance of such fossils is easily explained. 



In one part of the circuit of Astroni he alludes to beds of ejected 

 materials which for a short space are inclined at an angle of 40°, and 

 which he therefore imagines may have been partially dislocated, 

 although the materials of the rest of the same cone remain in their 

 original position. Here I may point to the fact mentioned by Mr. 

 Dana in his account of the Sandwich Islands, that strata of ejected 

 substances have sometimes an original inclination of 40° in the '' cin- 

 der cones," although in the "tufa cones" formed near the sea, the 

 slope of the beds does not exceed an angle of 30°, 



Etna. 



The great valley on the east side of Etna, called the Val del Bove, 

 which forms a grand amphitheatre between four and five miles in dia- 

 meter, is surrounded for more than three parts of its circuit by nearly 

 vertical precipices which vary from 1000 to nearly 3000 feet in height. 

 As this hollow is not in the centre but on the flanks of a great coni- 

 cal mountain, the precipices at the upper end of the valley are the 

 loftiest, and they diminish gradually in height towards the lower 

 side. The original form of the lower boundary of this enormous 

 cavity is somewhat obscured by deluges of modern lava which have 

 passed over it ; but there can be scarcely a doubt, that were these re- 

 moved, the nearly circular escarpment surrounding the vast cavity 

 would be complete, although of slight elevation on the lower or east- 

 ern side where the lavas have poured over the edge of the rampart, 

 seeming to have scaled it, just as they passed over the walls of Cata- 

 nia in 1669. There seems however to have been always one point, 

 where there was a breach in the boundary cliffs of the Val del Bove. 

 This was situated at the south-eastern end of the valley and is called 

 the Valley of Calanna, a narrow ravine, on one side of which perpen- 

 dicular precipices 400 and 500 feet high display a succession of vol- 

 canic strata intersected by a few dikes. The Valle di San Giacomo 

 is the continuation of the Val di Calanna, and I conceive them to 

 stand in the same relation to the Val del Bove, which the fossa grande 

 probably held to the Atrio del Cavallo, or which the Baranco de las 

 angustias holds to the caldera of Palma. 



After my visit to Etna in 1828, I suggested that the Val del Bove 

 may have been produced by engulfment, an opinion which M. de 



