324 



THE GEOLOGIST. 



" Sucli openings on the steep parts of a cone miglit easily become 

 water-courses, and give passage to floods during the winter's rain 

 and the melting of the snow, and these might gi-adually deepen and 

 widen such fissures." Paroxysmal explosions lil^e that of Vesuvius 

 in the year 79 might also be powerful agents ; and, " if a great ex- 

 plosion happened to be lateral instead of central, the new chasm 

 being commanded by higher ground, or by the region of snow, floods 

 of water would at certain seasons sweep do^vn into it, and might 

 increase its dimensions. " To account for the position of so great a 

 cavity on one side only of a cone, we may, in the case of Etna, 

 imagine a connection between the Val del Bove and the old axis of 

 Trifoglietto. The ancient habitual duct or chimney may, like that of 

 the ancient Vesuvius, after being plugged up for ages, have again 

 given passage to vast volumes of pent-up gases or steam, blowing up 

 the incumbent lavas of Mongibello, which had filled the crater and 

 overtopped the secondary cone. Moreover, the accumulated snow 

 and ice, and consequently the action of running water, may at some 

 earlier period have been greater in the higher region, when the cone 

 of Mongibello was larger and loftier, before its truncation, especially 

 if the first excavation of the Val del Bove dates as far back as the 

 close of the glacial period, or when the Alpine glaciers reached the 

 plains of the Po ; for at that time the climate of a Sicilian winter 

 could hardly fail to be colder than now." 



Isolated outliers of ancient rock, such as Pinochio and Musara, are 

 striking monuments of waste, helping to prove the former continuity 

 of the northern escarpment of the Val del Bove in a southerly 

 direction; and the multitude of dikes projecting from ten to fifty feet 

 above the general level of the ground in every part of the escarpments, 

 shows clearly to what an extent the softer and more destructible 

 beds have been wasted away by atmospheric aud torrential action. 

 Such dikes are records of the former existence of masses of rocks 

 now no more, though we can still trace the exact shape of the fissures 

 by which they were at one period traversed. The lateral ravines also 

 before mentioned bear testimony to the removing power of running 

 water since the Val del Bove was bounded by lofty precipices." 



The obliteration of the river Amenano by the lava of 1669 is given 

 as an example of the antagonism of aqueous erosion and volcanic 

 activity ; and in like manner it is suggested that " at some former 

 period there may have existed many rivers in the Val del Bove like 

 those now draining the calderas of Palma and Tiraxana in the 

 Canaries ; and, like them, they may, after uniting, have issued by one 

 principal gorge ; yet they would inevitably be all effaced from the 

 map, and the gorge filled up with stony matter whenever the time 

 arrived, during a new phase of eruption, for fresh floods of lava to 

 traverse the Caldera." Sir Charles then brings forward the great 

 flood of 1755, the only authenticated instance of a great body of 

 water having passed from the higher region of Etna through the 

 Val del Bove to the sea. " An eruption had taken place at the sum- 

 mit of the volcano in the month of March, a season when the top 



