324 
THE GEOLOGIST. 
" Such openings on the steep parts of a cone might easily become 
water-courses, and give passage to floods during the winter's rain 
and the mehing of the snow, and these might gradually deepen and 
widen such fissures." Paroxysmal explosions like 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 wovild at certain seasons sweep down into it, and might 
increase its dimensions. " To account for the position of so gTeat 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, lil^e 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 Finochio 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 eveiy part of the escarjiments, 
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 woidd 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 
