330 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 23, 



cause which hut the other day was seen to excavate the new crater 

 of Vesuvius, through the heart of the same mountain, than to invent 

 for the former a different and fanciful process ? But if Somma be 

 admitted, notwithstanding its extent, to be a true crater of eruption, 

 the same origin cannot be denied to that of Palma, Santorini, or 

 others, on the ground of their size, which scarcely, if at all, exceeds 

 that of Somma. 



Sir Charles Lyell seems to doubt the Val di Bue being a true 

 crater of eruption upon two grounds. First, because the beds com- 

 posing the surrounding cliffs do not show the usual qua-qua-versal 

 dip, but generally slope towards the sea. This, however, is merely 

 the result of the eruption having broken out on one side of the 

 central axis of the mountain, — a circumstance of frequent occurrence ; 

 and naturally so, because the old central vent is apt to be sealed up 

 by the consolidated products of former eruptions, and the point of 

 least resistance to the subterranean eruptive force will often, there- 

 fore, be a little on one side, — probably on a fresh point of a fissure 

 broken through the flank of the mountain. 



In fact, there must be a contest between the resisting powers of 

 the sides of the mountain and of its upper part ; and the weakest 

 part, whichever it is, will give way, and be blown up. 



Sir Charles's second reason is, that a sufficient amount of conglo- 

 merates is not to be seen on the mountain-slopes around the Val di 

 Bue, to account for the vacuity. But, besides that he himself speaks 

 of " enormous masses of scoriae on the flanks of Etna," it should be 

 remembered that the aeriform explosions, when long continued, tri- 

 turate the ejected matters, owing to their repeated fall into and 

 rejection from the crater, to such a degree as to redace the greater 

 part at length to an impalpable powder, which is carried by the 

 winds to a distance, sometimes of hundreds of miles, and spread in 

 a thin layer over an enormous area of sea or land. And, moreover, 

 the larger the dimensions of any crater, the more powerful and en- 

 during will have been, in all probability, the explosions, and the 

 more thoroughly triturated, during the process of its gradual enlarge- 

 ment, would be the fragments thrown up by them. 



I remember being exceedingly surprised, after the termination of 

 the Vesuvian eruption .of 1822, forming a continual fountain of stones 

 and ashes some miles in height, lasting through twenty days, and in 

 the end completely gutting the mountain, to find that the prodigious 

 amount of fragmentary matter thrown out from the crater had coated 

 the outer slopes of the mountain only to an average thickness of a 

 foot or two at most. But then the ashes which day by day were 

 reduced to a finer and at length to an impalpable powder, so fine as 

 to penetrate the closest rooms in the houses of Naples, were borne 

 to vast distances by the winds. Much, too, was carried down into 

 the plain, or the sea below the mountain, by the torrents of rain 

 (producing lave di fango, or mud-lavas), such as overwhelmed Her- 

 culaneum, and which accompanied, as usual, the paroxysmal erup- 

 tion of 1822. 



Indeed, if we consider the statements adduced on good authority, 



