September 1, 1SS6.] 



♦ KNOWLEDGE ♦ 



325 



Aviclth of the Val del Bove, and flowed no less than four 

 miles in the first two days. This torrent of lava was soon 

 after enlarged by the accession of enormous streams of 

 burning matter flowing from the three caverns which 

 had formed in the first instance. The river of lava at 

 length reached the head of the Colonna Valley, where 

 there is a vast and almost vertical precipice, over which 

 the lava stre;\med in a cataract of fire. But there was 

 a peculiarity about the falling lava which gave to the scene 

 a stn\nge and awful character. As the burning cascade 

 rushed down, it became hardened through the cooling 

 effects due to its contact with the rocky face of the precipice. 

 Thus, the matter which had flowed over the head of the 

 valley like a river of fire fell at the foot of the precipice in 

 the form of solid masses of rock. The crash with which the 

 falling crags struck the bottom of the valley is described as 

 inconceivably awful. At first, indeed, the Catanians feai-ed 

 that a new eruption had burst out in that part of the 

 mountain, since the air was filled with clouds of dust, 

 produced by the abrasion of the face of the precipice as 

 the hardened masses swept over it. 



The length of time during which the lava of 1819 con- 

 tinued to flow down the slopes of the great valleys is well 

 worth noticing. Mr. Scrope saw the current advancing at 

 the rate of a yard per hour nine months after the occurrence 

 of the eruption. The mode of its advance was remarkable. 

 As the mass slowly pushed its way onward, the lower 

 portions were arrested by the resistance of the ground, 

 and thus the upper part would first protrude itself, and 

 then, being unsupported, would fiill over. The fallen mass 

 would then in its turn be covered by a mass of more liquid 

 lava, which poured over it from above. And thus " the 

 current had all the appearance of a huge heap of rough 

 and large cmdei-s rolling over and over upon itself by the 

 eflfect of an extremely slow propulsion from behind. The 

 contraction of the crust as it solidified, and the friction of 

 the scoriform cakes against one another, produced a crackling 

 sound. Within the crevices a dull red heat might be seen 

 by night, and vapour issuing in considerable quantity was 

 visible by day." 



The circumstance that Etna upreai-s its head high above 

 the limit of pei'j)etual snow has a remarkable bearing on the 

 characteristics of this volcano. The peculiarit}' is touched 

 on by Pindar in the words already quoted, in which he 

 speaks of Etna as " the nurse of everlasting frost concealing 

 within deep caverns the fountains of unapproachable fire." 

 It will be i-eadily conceived that the action of molten lava 

 upon the enormous masses of snow which lie upon the 

 upper part of the mountain must be calculated to produce 

 — under special circumstances — the most remarkable and, 

 unfortunately, the most disastrous eflfects. It does not 

 always happen that fire and ice are thus brought into 

 dangerous contact. But records are not wanting of cata- 

 strophes produced in this way. In 1755, for example, a 

 tremendous flood was occasioned by the flow of the two 

 streams of lava from the highest crater. The whole moun- 

 tain was at the time (March 2) covered with snow, and the 

 torrent of lava formed by the union of the two streams was 

 no less than three miles in width. The flow of such a 

 mass of molten fire as this over the accumulated snows 

 of the pa.st winter led to most disastrous consequences. 

 "A frightful inundation resulted," says Sir Charles Lyell, 

 " which devastated the sides of the mountain for eight miles 

 in length, and afterwards covered the lower flanks of Etna 

 (whei-e they were less steep), together with the plains 

 near the sea, with great deposits of smd, scoria?, and blocks 

 of lava." 



In connection with this part of the subject may be men- 

 tioned the singular and apparently paradoxical circumstance 



that in 1828 a large mass of ice was found, which had been 

 preserved for many years from melting by the fact that a 

 current of red-hot lava had flowed over it. "We might 

 doubt the occurrence cf so strange an event, were it 

 not that the fact is vouched for by Sir Charles Lyell, who 

 visited the spot where the ice had been discovered. He thus 

 relates the circumstances of the discovery : — " The extra- 

 ordinary heat experienced in the south of Europe, duiing the 

 summer and autumn in 1828, caused the supplies of snow 

 and ice which liad been preserved in the spring of that year 

 for the use of Catania, and the adjoining parts of Sicily, and 

 the island of Malta, to fail entirely. Great distress was 

 consequently felt for want of a commodity regarded in those 

 countries as one of the necessiries of life rather than an article 

 of luxur}', and the abundance of which contributes in some 

 of the larger cities to the salubrity of the water and 

 tlie general health of the community. The magistrates of 

 Catania applied to .Siguor Gemmelaro, in the hope that his 

 local knowledge of Etna might enable him to point out some 

 crevice or natural grotto on the mountain where drift snow 

 was still preserved. Nor were they disappointed, for he 

 had long suspected that a small mass of perennial ice at the 

 foot of the highest cone was part of a large continuous 

 glacier covered by a lava current. Having procured a large 

 body of workmen, he quan-ied into this ice, and proved the 

 superposition of the lava for several hundred yards, so as 

 completely to satisfy himself that nothing but the subsequent 

 flowing of the lava over the ice could account for the position 

 of the glacier " (in other words, the ice had not accumulated 

 in a cavern of moderate extent accidentally formed beneath 

 overhanging lava masses). " Unfortunately for the geologist," 

 adds Lyell, " the ice was so extremely hard, and the exca- 

 vation so expensive, that there is no probability of the 

 operations being renewed." 



This strange phenomenon is explained, in all likelihood, 

 by the fact that the drift of snow over which the lava flowed 

 had become covered with a layer of volcanic sand before the 

 descent of the molten matter. The effect of sand in resist- 

 ing the passage of heat is well known. Nasmyth, the 

 inventor of the steam-hammer, illustrated this property in a 

 remarkable manner, by pouring eight tons of molten iron 

 into a cauldron one-fourth of an inch thick, lined with a 

 laver of sand and clay somewhat more than half an inch 

 thick. When the fused metal had been twenty minutes in 

 the cauldron the outside was still so cool that the palm of 

 the hand could be applied to it without inconvenience. And 

 lava consolidates so quickly that there must soon have 

 been formed over the snow a solid covering, strong enough 

 to resist the effects of the fresh molten matter which was 

 continually .streaming over it. In this way we may readily 

 conceive, as Sir Charles Lyell has remarked, that a glacier 

 10,000 feet above the sea-level would endure as long as the 

 snows of Mont Blanc, unless heated by volcanic heat from 

 below. 



It is worthv of notice that in the Antarctic seas there is 

 an island called Deception Island, which is almost entirely 

 composed, according to the authority of Lieut. Kendall, of 

 alternate layers of ice and volcanic ashes. 



One of the most perplexing subjects to geologists is the 

 existence of so remarkable a valley as the Val del Bove, 

 breaking the contour of the dome of p]tna nearly to the 

 summit. It must lie remembered that there are lew sub- 

 jects which have been more carefully examined than the 

 question of the formation of valleys and ravines. The 

 primary agent recognised by geologi.-ts is the action of 

 subterranean forces in upheaving and depressing the land. 

 In this way, doubtless, all the principal valleys have been 

 foi'med. But fluviatile influences have also to be con- 

 sidered ; and a valley which exists upon the flank of a 



