l.VKI,:, - ON t'KATKK'S l)]' K l,K V ATln.N . 
323 
other by ribs or ridgos of volcanic rocks, compai-ed by .lunglmlin lo 
the spokes of .an umbrelUi. All these furrows grow narrower and 
shallower when traced upwards, and come to an end befoi-e they reach 
the rim of the crater ; whereas, in such volcanic cones as have been 
truncated by explosions and subsidences, after considerable aqueous 
erosion, the rim is invariably indented. On applying this rule it was 
found that the crest of the soiithern escarpment of the Val del Bove, 
between Montagnuola and Zoccolaro, was very entii'e and iinbroken ; 
" but that there were notches, or deep depressions, several hundred 
feet deep, precisely at the two points wliere the upper ends of the 
valleys, called the Val dei Zappini and the Valle del Tripodo, joined 
the crest. Hence, it is natural to conchide that the valleys in ques- 
tion are of older date than the Val del Bove, and that their higher 
extremities were once prolonged towards the upper region of the 
cone, and were cut off when the Caldera was formed. Such an ex- 
planation of the facts would, however, be fatal to any theory which 
I'efers to a single catastrophe, or to any one mode of operation, 
whether slow or sudden, the upheaval of Etna, the tilting of the in- 
clined beds, and the opening of the great cavity called the Val del 
Bove." 
The erosion of the Valle del Tripodo is stated to be stdl going on, 
and a small inclined delta at its mouth furnishes the means of learn- 
ing how much matter has been brought down in a given time, or 
during the sixty-six years which have elapsed since 1722, when a 
powerful flow of lava crossed the lower extremity of a narrow valley, 
and suddenly put a stop to the transportation of allu\'ium to lower 
levels. " The waters of the torrent, even when most swollen, no 
sooner arrive at the margin of the lava, than they are absorbed by its 
spongy, scoriaceous crust, and by the superficial rents and grottos in 
which it abounds. The engulfed waters continue their course under- 
ground ; but the mud, sand, and boulders are all left behind and form 
a deposit, already several hundred feet long and thirty or forty deep, 
which " proves, on the one hand, how much erosion has gone on in 
little more than half a century ; and, on the other, how entirely all 
aqueous erosion ceases in areas once covered with fresh lava, and 
where a superficial drainage is turned into a subterranean one." 
It is not, however, attempted to attribute the origin of the Val del 
Bove exclusively to the action of running water ; and it is presumed 
local catastrophes of paroxysmal intensity may have given rise to the 
first breaches which ended in the production of this enormous cavity. 
The Cisterna, an elliptical hollow, now about 120 feet deep, was pro- 
duced in 1792 on the platform of the Piano del Lago by the sinking 
of the ground, and deepened again by subsidence in 1832. On a still 
higher level near the Philosopher's Tower, is a fosse-like depi-ession 
known to have originated during the same eruption of 1832. The great 
rent of Mascalucia, a mile in length and 30 feet deep, formed in 1381, 
is still open ; and another fissure, 6 feet broad and of unknown 
depth, was formed in the plain of San Lio in 1669, and is said to 
have been twelve miles long, reaching to near the summit of Etna. 
