LYELL OX CRATERS OF ELEVATIOX. 



323 



other by ribs or ridges of volcanic rocks, compared bv Jiingbnlm to' 

 the spokes of an nmbrella. All these furrows grow narrower and 

 shallower when traced upwards, and come to an end before they reach 

 the rim of the crater ; whereas, in such volcanic cones as have been 

 ti"micated by explosions and subsidences, after considerable aqueous 

 erosion, the lim is invaiiably indented. On appUniig this rale it was 

 found that the crest of the southern escarpment of the Yal del Bove, 

 between Montagnuola and Zoccolaro, was very entire and unbroken; 



but that there were notches, or deep depressions, several hundred 

 feet deep, precisely at the t^'o points where the r.pper ends of the 

 valleys, called the Val dei Zappini and the Talle del Tripo do. joined 

 the crest. Hence, it is natural to conclude that the valleys in ques- 

 tion are of older date than the Yal 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 foi-med. Such an ex- 

 planation of the facts would, however, be fatal to any theory Avhich 

 refers 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 gTeat cavity called the Yal del 

 Bove." 



The erosion of the Yalle del Tripodo is stated to be still going on, 



and a small inclined delta at its mouth furnishes the means of leam- 



inof how much matter has been brouo-ht down in a eiven 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 alluvium to lower 

 levels. " The waters of the toiTent. even when most swollen, no 

 sooner arrive at the margin of the lava, than they are absorbed by its 

 spongy, scoriaceous crast, 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 orig-in of the Yal del 

 Bove exclusively to the action of ranning water ; and it is presumed 

 local catasti'ophes of parox;\'smal 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 depression 

 kno^vn to have originated during the same eraption 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. 



