Part 7

Simultaneously with the grand fissure of the cone, two large craters opened at the summit, discharging with a dreadful noise, audible at a great distance, an immense cloud of smoke and ashes with bombs and flakes, rising to the height of 1300 metres[C] above the brim of lava (_sull' orlo de essi_). The white ashes, before described, although they did not fall beyond the Crocella, were carried by the wind as far as Cosenza, from whence they were sent to me by Dr. Conti. These ejections were followed by dark sand, with lapilli and small fragments of scoriæ of the same colour. The smoke, driven up with violence, assumed the usual aspect of a pine tree, of so sad a colour that it reminded us of the shadowy elm of Virgil's dreams ("_ulmus opaca ingens_"). From the trunk and branches of the pine-tree cloud fell a rain of incandescent material, which frequently covered all the cone. The lapilli and the ashes were carried to greater distances.

The victims of the morning of the 26th, the torrents of fire which threatened Resina, Bosco and Torre Annunziata, and which devastated the fertile country of the Novelle, of Massa, St. Sebastiano and Cercola, the two partially buried villages, the continual and threatening growlings of the craters, caused such terror that numbers fled from their dwellings near the mountain into Naples, and several in Naples went to Rome or to other places. Very many delayed from the knowledge that I was in the Observatory, and held themselves in readiness for flight whenever I should abandon it.

The rapidity with which the vast torrent of fire assailed the houses (_i.e._, in these villages), and the great heat which spread to a distance, scarcely allowed the fugitives to carry away any of their belongings; many were completely destitute. The authorities vied with each other in zealous efforts to relieve the distress, and the municipality of Naples sheltered and fed the wretched beings for many days.

The igneous period of the eruption was short, for on the morning of the 27th the lava stream, bearing down upon Resina, having covered a few cultivated fields, stopped; the lava descending from the summit of the mountain towards the Camaldoli also stopped; and the great lava torrent, which passed the shoulders of the Observatory through the Fossa della Vetrana, lowered the level of its surface below those of its two sides, which appeared like two parallel ramparts above it.

If these streams had continued on the 27th, flowing in the same manner as they did on the night of the 26th, they would have reached the sea, bringing destruction to the very walls of Naples.

But before leaving the subject of these lavas I must narrate an important fact to which I was witness, and which was thrice repeated, near the banks of the great river of fire that ran close to the Observatory. At three several points, and at different times, I observed great balls of black smoke issue from the lava, driven up with continued violence, as if from a crater; through the smoke I frequently observed numerous projectiles thrown up into the air, but I could not say whether with noise or in silence, for the noise of the central crater was deafening. Each of these little eruptions, which I may call _external eruptions_, lasted from fifteen to twenty minutes. The first took place at the most elevated point of the Fossa della Vetrana, on the right bank of the torrent; the second, under the hill of Apicella, where the lava divided into the two branches, before described; and the third near to the Observatory on the left bank of the lava stream. These singular explosions terminated without leaving little cones or craters, the lava in its impetuosity carrying every trace away. These eruptions were seen from Naples, and the Observatory was justly believed to be in danger. One has been clearly photographed, the one which was the best seen from Naples, being the nearest and the least darkened by the smoke of the lava. (Plate 4.) Is this the first time that the phenomenon has been remarked? I believe that it is at least the first time it has been authenticated. The authority of Julius Schmidt, quoted by Scrope, has no weight with me, for I was also a witness of what happened at Vesuvius in 1855; and, although these cones were in the midst of the lava in the Atria del Cavallo, they originated, according to the opinion of everyone, from the fissure from which the other and much larger cones proceeded. The same phenomenon was observed in the Atria del Cavallo in 1858, when I caused two of the little cones to be brought to the Observatory; but these also might belong to the fissure along which the other cones were arranged. The same may be said of the little craters observed, after they had been exhausted, by Professor Scacchi in 1850. But the discharging mouths now observed in the Fossa della Vetrana, which existed for twenty minutes and then disappeared, and which were not at all in a continuous line, and could not be supposed to correspond with any fissure beneath, constitute a circumstance which, if not new, is evident for the first time, and cause the recognition of a power in the lava itself to form eruptive fumaroles.[2]

The igneous period of the eruption having terminated on the evening of the 27th, the ashes, lapilli, and projectiles became a little more abundant, whilst the roaring noises of the craters apparently became greater. The pine-tree cloud was of a darker colour, and was furrowed by continual lightning, visible by daylight from the Observatory. Many writers on the subject of Vesuvius affirm that the flashes which appear through the smoke cloud were lightning unaccompanied with thunder, but they studied the phenomena from Naples, or some place more or less distant from the crater, where the report of the thunder was inaudible, or could not be distinguished from the bellowing and detonation of the mountain. The fact is that these flashes were constantly followed by thunder, after an interval of about seven seconds.[D] When the flash was very short, a simple noise like the report of a gun was heard, but if it were long, a protracted sound like that from torn paper ensued.

On the 28th the ashes and lapilli, continuing to fall abundantly, darkened the air, yet without diminishing the terrible noise; at Resina, Portici, St. Giorgio a Cremano, Naples, etc., terror was universal.

On the 29th, with a strong wind blowing from the east, scoriæ of such a size fell at the Observatory, that the glass of the windows unprotected by external blinds was broken. The noise from the crater continued, but the projectiles rose to a less height, indicating a diminution in the dynamic power of the eruption. Towards midnight the noise of the craters was no longer continuous, and recurred with less force and for shorter intervals. Almost at the same hour a tempest burst over the Campania with loud thunder and a little rain. The grass, the seeds, the vine tendrils, the leaves and tops of the trees dried up immediately, and the country was changed from spring to winter. The storm, although repeated on the following days, passed away by degrees, and thus the floods, which I strongly feared, did not occur. Almost always after great eruptions of Vesuvius, storms of heavy rain have followed, and the ground being covered with ashes, the water could not filtrate through into the soil, but descended in muddy torrents over the adjacent country, occasioning as much damage as the fire itself.

On the 30th, the detonations were very few, and the smoke issued only at intervals, and by the 1st May the eruption was completely over.

When the smoke had cleared off the figure of the cone was seen to be changed. (_Vide_ Plate 5a.)

The ground was perpetually disturbed whilst the Volcano raged, so that the Observatory oscillated continually. Some shocks were felt not only in the adjacent territory, but at a greater distance, at Montovi and elsewhere. The oscillations at the Observatory were chiefly undulatory, from N.E. to S.W. They were observed for some days after the termination of the eruption, but not continuously, although they maintained some intensity.

If we refer to January, 1871, we shall find that that eruption was preceded by several earthquakes, among which were those of the months of October, November and December, in the previous year, that wrought such destruction in Calabria, and especially in the province of Cosenza; if we consider that as only the last phase, we shall find that it was preceded by great shocks of earthquake that devastated some regions of Greece.[3]

The great quantity of lapilli which fell buried the scoriæ with which the Vesuvius cone was covered, so that it became somewhat more difficult to ascend to the summit, and much less difficult to descend. Having reached the top of the mountain, I found a large crater divided into two parts by what seemed a cyclopean wall. The two abysses had vertical sides, and revealed the internal structure of the cone. Their vertical depth was 250 metres; and beyond that I observed a sort of tunnel perforated in the rock, with a covering arch raised above the bottom of the eastern abyss about 12 metres, judging by the eye. The interior walls of the crater showed neither the usual stalactitic scoriæ nor sublimations, nor fumaroles, but alternate beds of scoriæ and of compact lava. The fumaroles and sublimations abounded, only about the brims of the craters. Hydrochloric and sulphuric acid and sometimes sulphuretted hydrogen affected respiration, and the temperature rose sometimes to 150 degrees. Various fissures about the brim of the double crater indicated prolongations downwards, which allowed me to descend with a rope, in order to examine the interior of the tunnel to which I have just alluded. The highest brim of the crater was fissured for a distance of 80 metres, and the greatest depth of fissure was at that place.

By measurement with the barometer, we ascertained approximately (for only one barometer was used) that the height of the Vesuvian cone was somewhat diminished.

Not only the Vesuvian cone, but the whole adjacent country appeared white for many days, as if covered with snow, when exposed to sunlight. This was due to the sea-salt contained in the ashes with which the surface was strewn.

A great quantity of coleoptera assembled on the flat roof of the Observatory, where the ashes and lapilli were heaped up two decimetres in height. I found the same species on the cone, where many insects were observed on other occasions, such as the _Cuccinella septempunctata_; the crysomela populi, etc., were wanting. This phenomenon of the extraordinary concourse of insects on the top of Vesuvius, in order to die in some of the fumaroles, especially noted previous to and after great eruptions, is a circumstance for which I cannot account.[4] The whole of the lava emitted in this eruption occupies a surface of about five square kilometres; allowing an average thickness of four metres, we obtain a mass of twenty millions of cubic metres. About three-fifths of this lava did no injury, being deposited upon other pre-existing lava. However, the lava in the Novelle, which was deposited upon the lava of 1858, covered quarries of the best stone which had been worked at the time, covered many paths that had been cleared, and buried the new Church of St. Michele, with some houses that surrounded it, which had been rebuilt on the site of the former church, which was covered by the lava of 1868. The destruction of land in occupation, of buildings and of crops, exceeded three million francs in value. Many proposals for relieving the sufferers have been received. Wishing to aid in this benevolent work, I gave a public lecture, admission for each person being one franc; and this lecture, from notes badly taken, was printed by private speculation, and I was compelled to repudiate the report of it through the public papers.

The evolutions of carbonic acid (_mofette_), which usually appear at the end of great Vesuvian eruptions at low-situated spots or hollows, with very rare exceptions, were observed on this occasion a few days after the eruption had completely ceased. They appeared in the direction of Resina. I found the most elevated at Tironi, and the most numerous between La Favorita and the Bosco Reale di Portici.

The water in wells was on this occasion neither deficient nor scarce previous to the eruption, but was very acid after the appearance of the carbonic acid evolutions in those neighbourhoods in which they abounded. Having stated that the disastrous conflagration of the 26th April ought, in my opinion, to be regarded as the last phase of a long period of eruption, which commenced at the beginning of 1871, I consider it right to discuss the question at somewhat greater length.

Not only from twenty years' personal observations, but from the attentive study of accounts of previous eruptions, I have found that when the central crater awakens with small eruptions after a certain time of previous repose, these almost always have a long duration, and, after various phases of increase and decrease, terminate in a great eccentric eruption, that is to say, with the production of an aperture from which a copious lava stream issues. The eruptions of 1858, 1861, 1868 and 1872, furnish the most recent examples of what I affirm. I might cite many others of earlier date, but I shall content myself with recording the greatest conflagration of this century, that of October, 1822.

Before the erection of the Vesuvian Observatory, it was impossible to obtain a consecutive account of all the phases which the Volcano presented; but we generally obtained the description of the more splendid phases of the eruption which arrested the attention of everyone. Hence, notices of the small phenomena which preceded a great eruption are frequently wanting. We cannot always ascertain whether the fumaroles of the craters became active and at what periods, what was their temperature and what the diverse nature of their emanations, etc.: whether and when any change in the crater with slight eruptive manifestations occurred; discharges which sometimes commenced in the bottom of a crater becoming active, and so are invisible at Naples.

But it may be asked whether the inverse proposition be equally true, that is, whether all the great eruptions of our Volcano were preceded by small fiery manifestations of long duration? There have undoubtedly been great eruptions not preceded by small central eruptions, but these also had their period of preparation or precursory signs. After the great eruption of 1850, Vesuvius remained in apparent repose until the end of May, 1855, when there was an eccentric eruption and a great flow of lava lasting twenty-seven days. But for a year before the fumaroles on the top of the mountain had acquired great activity, their temperature increased, and hydrochloric and sulphuric acid became more abundant, and generated the usual coloured products on the adjacent scoriæ. Finally, in the month of January, a crater was formed by falling in of the ground, and although it did not discharge fire, yet it poured forth dense smoke. This was the beginning of the fissure manifested four months afterwards.

Ignazio Sorrentino, who spent a long life in the study of Vesuvius, and frequently ascended it, considered the increase of those yellow products--which are chiefly chlorides of iron, but were, at that time, mistaken for sulphur--as the sign of an approaching eruption.

The only grave objection that can be alleged is that of the memorable eruption of 1631, which surprised the neighbouring population so suddenly that many perished miserably, surrounded or covered with lava. But that terrible conflagration occurred after centuries of repose, so that trees had grown in the interior of the crater. No one suspected the possibility of danger. It took place, too, at the end of autumn, when the cone is usually covered with clouds, and, therefore, no one had an opportunity of observing any precursory phenomena.

When the Observatory was established, I was able--in the first instance, at my own expense, and afterwards with some slight assistance from Government--to undertake studies more assiduous than any previously made. I had two instruments adjusted to indicate the internal efforts of the Volcano, viz., M. Lamond's apparatus of variations, which, by means of finely-balanced needles and methods of amplification proposed by Gauss, indicates the slightest trepidation of the ground, and my own electro-magnetic seismograph, a self-registering instrument of exquisite delicacy. These instruments, when attentively observed, give the most valuable information with respect to the activity of the adjacent Volcano.

If the very slightest eruption occurs, these instruments manifest slight perturbation, increasing with the activity of the mountain. When the Volcano attains a certain degree of activity, and the instruments are proportionately disturbed, it is impossible to foresee a new phase of increase without constantly watching the changes in the intensity of the perturbations; and to effect this it is requisite to have upon the spot a staff of assistants sufficiently numerous, scientific and intelligent. If, therefore, on the night preceding the 26th of April the instruments had been properly watched, they would have undoubtedly indicated the great increase in the activity of the Volcano. The perturbations on the 23rd were steadily increasing, and on the evening of the 25th they were much stronger than on the 24th, but on the morning of the 26th they had become extraordinarily strong; they must, therefore, have increased considerably during the night.

II.

NATURE OF THE LAVAS.

When the observer is near the source of the lava, he sees matter in a state of fusion, which, like a torrent of liquid fire, runs along, with more or less impetuosity, between two banks formed by itself. But as soon as the surface of the torrent cools to the point of congelation, it loses the splendour of its first incandescence. The part which begins to harden breaks readily in some lavas into fragments which float on the viscous fluid beneath; these, increasing in number with distance from the source, conceal the molten matter beneath and retard its progress, and at last nothing is seen but the more or less red-hot scoriæ moving along. These lavas I shall call "_Lavas with fragmentary scoriæ_."

On other occasions, a skin forms on the surface of the lava, which, gradually thickening, keeps flexible for some time, and then wrinkles or swells or extends and breaks to give egress to the hot fluid within, which, in its turn, skins over and repeats the same phenomena. This I shall call "_Lavas with a united surface_."

These, in their course, discharge less smoke than the first, draw out more easily into threads, and, when cold, have a dark colour, something like bitumen or pitch. _The lava with fragmentary scoriæ_, when stretched, breaks easily, discharges smoke copiously, and, when hardened, has a more bluish tint, like clods of upturned earth (_formato di zolle_). It is noisy in its course, because the incoherent scoriæ that it carries along strike and crunch against each other; the other lava flows silently, except for a sort of crackling arising from the actual fracturing up of the solid skin by distension from the liquid matter within. If required to give the mineralogical characteristics of this lava, I would say that it was rich in leucite and contained little or no pyroxene; the fragmentary lava, on the contrary, is poor in leucite and rich in pyroxene. The lavas of 1871 were of the "united surface" character; those of 1872 were "fragmentary," with some characteristics which I shall describe:

1. They were of the clearest tint I have ever seen, when regarded superficially, but, when broken, the fracture was darker than any other lava.

2. They had very little leucite and abounded in pyroxene and olivine, and sometimes contained a few crystals of amphibole.

3. Their specific gravity varied with their porosity; the most compact attained 2·75.

4. These lavas carried along in their course a quantity of scoriæ which had long been subjected to the action of the acids of the fumaroles close to the craters, and also a great many bombs (_bombe_)--that is, round masses similar to those ejected from craters. These varied in size, some having a diameter of four to five meters. They frequently contained a large nucleus of very leucitic lava, like that of 1871, with a larger or smaller quantity of feroligiste (peroxide of iron). Others contained lavas changed by the action of the acid vapours near the craters. These bombs must have flowed out with the lava, for they are found through its whole course, and they were certainly not ejected from the crater; for not only are they found on the lava exclusively, but masses so enormous were not thrown up from the craters during the eruption; those lying on the cone near the craters seldom exceed a decimetre in diameter.

As to the qualitative chemical analysis of the lavas, it always presents the same elements, with the exception of small quantities of some metals, lead for example, which have escaped the researches of good chemists, but which I have constantly found in the sublimations of the fumaroles of the lava. With respect to the quantitative analysis, two specimens of the same lava appear indeed to have their constituents in different proportions. To arrive at any conclusion a long and patient investigation, requiring means and assistance which the Observatory does not possess, would be necessary.

Professor Fuchs, of Heidelberg, has devoted himself to this work for years past, and if he continue it with well-selected and sufficiently large specimens we may hope some day to obtain satisfactory results.

5. Every specimen of lava which I examined with a very sensitive magnetoscope improved by myself, was invariably magneto-polar, not excepting the pieces of the bombs, whether rejected from the crater or carried along with the lava.

III.

FUMAROLES OF THE LAVAS.

Smoke generally issues from all lava when it cools down to a certain degree, hence it is more abundant at the edges of the fiery torrent, or is liberated from the scoriæ that form on its surface. But when the lava stops, the smoke issues only from certain vent-holes, through which we can still see the fire, and at the edge of which different amorphous or crystallized matters collect by sublimation. These centres of heat, of more or less duration, are the fumaroles of the lavas. I believe I have on other occasions shown that a fumarole is nothing but a communication between the more or less cooled and hardened surface of the lava and the interior, which is still incandescent. Some fumaroles last but a day, others preserve their activity for weeks, months or years, according to the depth of lava through which they penetrate; and when they cease to be active, that is, when the sublimations are formed, or smoke or other æriform matters issue from them, they still retain a rather elevated temperature. In the lavas of 1858, in a place where they had a transverse width of 150 metres, a vent-hole may still be found where the thermometer registers 60° and the scoriæ are warm. Sometimes, while the lava is in process of cooling, new fumaroles appear, in which the fire is visible. This phenomenon, which appeared marvellous and inexplicable when I first observed it in 1855, is now very easily understood; the cooled and hardened crust of the lava fractures with noise and suddenly, and so a new communication is opened with the incandescent lava below, thus creating a new fumarole.