52G 



KNOWLEDGE ♦ 



[September 1, 1886. 



mountain may, in nearly every instance, be ascribed to the 

 action of running water. 



In the case of the Val del Bove, however, we are forced 

 to come to a diflereut conclusion. If this valley had been 

 formed Ijy the action of running water in some long-past era 

 of the mountain's history, the chasm would have deepened 

 as it approached the base. On the contrary, the precipices 

 . which bound the Val del Bove are loftiest at the upper ex- 

 tremity, and gradually diminish in height as we approach 

 the lower regions of the mountain. 



Nor can we imagine that the valley has been formed by 

 a landslip. The dimensions of the depression are altogether 

 too great for such an explanation to be available. And, 

 passing over this circumstance, we are met by the considera- 

 tion tliat, if the land which once filled this valley had 

 "slipped" (in the ordinary sense of the term), we should 

 see the traces of the movement, and be able to detect the 

 existence of the removed mass. Not only is there no 

 evidence of a motion of this sort, but the slightest examina- 

 tion of the valley at once disposes of the supposition that 

 such a motion can at any time have taken place. 



It remains only that we suppose the valley to have been 

 caused by the bodily subsidence of the whole mass which 

 had formerly filled up what is now wanting to the dome- 

 shaped figure of the mountain. And the subsidence must 

 have taken place in a sudden manner — not necessarily in a 

 single shock, but certainly not by a slow process of sinking. 

 For the mass which has sunk is sharply separated from the 

 rest, so that the precipitous walls of the valley exhibit the 

 structure of the mountain's frame to a depth of from 3,000 

 to 4,000 feet below the summit of the cone. In other 

 words, a portion of the crust has been separated from the 

 rest, and has then sunk bodily down, leaving the remainder 

 unchanged. 



When we consider the dimensions of the valley, such an 

 event becomes very startling. " The Yal del Bove," says 

 Lyell, " is a vast amphitheatre, four or five miles in diameter, 

 surrounded by nearly vertical precipices." One might almost 

 be prepared to doubt that such a valley as this could be 

 formed in the manner described, were it not that within 

 recent times we have had evidence of the occurrence of 

 similar events. During a violent earthquake and volcanic 

 eruption which took place in .Java in 1822, the face of the 

 mountain Galongoon was totally changed, " its summits 

 broken down, and one side, which had been covered with 

 trees, became an enormous gulf in the form of a semicircle. 

 This cavity was about midway between the summit and the 

 plain, and surrounded by steep rocks." Yet more remark- 

 able was the great subsidence w-hich took place in the year 

 1772 on Papendayang, the largest volcano in the island of 

 Java. On that occasion, "an extent of ground fifteen miles 

 in length and six in breadth, covered by no less than forty 

 villages, was engulfed, and the cone of the mountain lost 

 4,000 feet of its height." 



There is nothing unreasonable, therefore, in supposing 

 that some such event may have resulted in the formation 

 of the strange valley which mars the dome-shaped figure of 

 Mount Etna, although no such events have been witnessed 

 in the neighbourhood in recent times. 



One singular feature of the valley remains to be men- 

 tioned. The vertical face of the precipices which bound it 

 are broken by what, at a distant view, appear to be dark 

 buttresses, strangely diversified in figure, and of tremendous 

 altitude. On a closer inspection, however, these strange 

 objects are seen to be composed of lava jutting out through 

 the face of the cliffs. Being composed of harder materials 

 than the cliifs, they waste away less i-apidly, and thus it is 

 that they are seen to stand out like buttresses. Now, we 

 would invite the close attention of the reader to this part 



of our subject, because, as it seems to us, it illustrates in a 

 singularly interesting manner the mode in which volcanic 

 cones are aflfected during eruption. 



We have seen that in the eruption of 1811 there was 

 evidence of a perpendicular rent having taken place in the 

 internal framework of Etna, and in 1669 a fissure was formed 

 which extended right thiough the outer crust. In one case 

 lava was forced through the rent, and burst out at the side 

 of the mountain. In the other, the brilliant light which was 

 emitted indicated the presence of molten lava deep down in 

 the fissure. Now, when we combine these circumstances 

 with the dijhes seen in the Val del Bove, and with the similar 

 appearances seen round the ancient crater of Vesuvius, we 

 can come, as it appears to me, to but one concLision. Before 

 and during an eruption, the lava which is seeking for exit 

 must be forced with such tremendous energy against the 

 internal framework of the mountain's dome as to fracture 

 and rend the crust, either in one or two enormous fissures 

 or in a multitude of smaller ones. It does not follow that 

 all or any of the fissures would be visible, because the outer 

 surfaces of the crust may not be rent. Into the fissures thus 

 formed the lava is forced by the pressure from below, and, 

 there solidifying, the crust of the dome remains as strong, 

 after the liquid lava has sunk to its usual level, as it was 

 before the eruption. When we see dykes situated as in the 

 Val del Bove, we learn that the fissures caused by the 

 pressure of the lava extend far down the flanks of a volcanic 

 mountain. That they are numerous is evidenced by the 

 fact that those seen in the Val del Bove amount, according 

 to Sir Charles Lyell, to •' thousands in number." 



And perhaps we may understand from such considera- 

 tions as these the manner in which the Val del Bove itself 

 was formed. For a wide strip of country between two great 

 fissures might be so waved and shaken by the action of the 

 sea of molten lava beneath as to be fractured crosswise ; 

 and then, on the subsidence of the lava, the whole mass 

 below the fracture would sink down bodily. We gain an 

 extended conception of the energy of the forces which 

 are at work during volcanic eruptions, when we see that 

 they thus have power to rend the whole framewoi-k of a 

 mountain. 



Among recent eruptions of Mount Etna, one of the 

 most singular was that of the year 1852, which began so 

 suddenly that a party of Englishmen, who were ascending 

 the mountain, and had nearly reached the foot of the highest 

 cone, were only able to escape with great difficulty. The 

 eruption which had commenced so abruptly did not cease 

 with corresponding rapidity, but continued with but few 

 slight intermissions for fully nine months. 



In the last week of May, 1879, a fissure opened on the 

 north side of the mountain, and volumes of smoke and 

 flame were seen to issue from it. From the crater itself a 

 great cloud of black ashes was poured forth, " rendering the 

 mountain invisible," said one writer, " and obscuring the 

 rays of the sun " (by which the writer presumably meant 

 obstructing their passage), "even at a distance of many 

 miles. These ashes were carried far and wide, and even 

 covered the ground as far away as Eeggio, on the adjacent 

 coast of Calabria. Three new craters opened in the direc- 

 tion of Eandazzo, on the north side of the mountain, and 

 the lava ran rapidly towards the town of Fi'ancavilla, where 

 great alarm was felt, though that town is situated beyond 

 the river Alcantara, and on the very outskirts of the region 

 usually threatened by eruptions. On the opposite side of 

 the mountain, Palermo and the adjacent villa of Santa 

 Maria di Licodia were also greatly alarmed." The new 

 craters, and the fissure with which the eruption began, lay 

 all on the northern ."-ide of the mountain. The stream of 

 lava, which was estimated to be 70 metres (about 75 yards) 



