Observations on Mount Vesuvius, Mount Etna, and Other Volcanos Part 5

35. Island of Procita.

36. A perfect cone and crater of a Volcano near Castiglione in the island of Ischia.

37. Lava that ran into the sea in the last eruption on this island, in the year 1301, or 1302: the place now called Le Cremate.

38. Town of Ischia and castle.

39. Lake of Licola.

40. Lake of Patria.

41. The river Volturnus.

42. Capua.

43. Caserta.

44. Aversa.

45. Mataloni.

46. Acerra.

47. Island of Ischia, anciently called naria, Inarime, and Pithecusa.

48. The mountain of St. Nicola, anciently called Mons Epomeus, supposed the remains of the princ.i.p.al Volcano of the island.

49. Castiglione, near which are the baths of Gurgitelli.

50. Lacco, near which is that very cold vapour called by the natives _ventarole_.

51. Ancient city of Pompeii, where his Sicilian Majesty's excavations are carrying on at present.

52. Rovigliano.

53. River of Sarno.

54. c.u.ma.

55. Hot sands and sudatory, called Nero's baths.

56. The Lucrine lake, supposed to have been here, and of which there is still some little remain.

57. Villa Angelica, Sir William Hamilton's villa, from whence he has made many of his observations upon Mount Vesuvius.

58. Cones formed by an ancient eruption called _viuli_; here are likewise cold vapours called _ventaroli_.

59. High grounds, probably sections of cones of ancient Volcanos, being all composed of _tufa_ and strata of loose pumice and burnt matter.

60. Plain of the Campagna Felice, four or five feet of excellent soil, under which are strata of burnt and erupted matter.

...... Marks the boundary of Sir William Hamilton's observations.

LETTER VI.[45]

To MATHEW MATY, M. D. Secretary to the Royal SOCIETY.

Naples, March 5, 1771.

Since I had the pleasure of sending you my letter, in which the nature of the soil of more than twenty miles round this capital is described; examining a deep hollow way cut by the rain waters into the outside cone of the Solfaterra, I discovered, that a great part of the cone of that ancient Volcano has been calcined by the hot vapours above described.

Pumice calcined seems to be the chief ingredient, of which several specimens of (as I suppose) variegated unformed marble are composed, and the beautiful variegations in them may have probably been occasioned by the mineral vapours. As these specimens are now sent to the Royal Society, you will see that these variegations are exactly of the same pattern and colours as are met in many marbles and flowered alabasters; and I cannot help thinking that they are marble or alabaster in its infant state. What a proof we have here of the great changes the earth we inhabit is subject to! What is now the Solfaterra, we have every reason to suppose to have been originally thrown up by a subterraneous explosion from the bottom of the sea. That it was long an existing Volcano, is plain, from the ancient currents of lava, that are still to be traced from its crater to the sea, from the strata of pumice and erupted matter, of which its cone, in common with those of other Volcanos, is composed, and from the testimony of many ancient authors.

Its cone in many parts has been calcined, and is still calcining, by the hot vapours that are continually issuing forth through its pores; and its nature is totally changed by this chemical process of Nature. In the hollow way, where I made these remarks, you see the different strata of erupted matter, that compose the cone, in some places perfectly calcined, in others not, according as the vapours have found means to insinuate themselves more or less.

A hollow way, cut by the rains on the back of the mountain on which part of Naples is situated, towards Capo di China, shews that the mountain is composed of strata of erupted matter, among which are large ma.s.ses of bitumen, in which its former state of fluidity is very visible. Here it was I discovered that pumice stone is produced from bitumen, which I believe has not yet been remarked. Some specimens shew evidently the gradual process from bitumen to pumice: and you will observe that the crystalline vitrifications, which are visible in the bitumen, suffer no alteration, but remain in the same state in the perfect pumice as in the bitumen.

In a piece of stratum, calcined from the outside of the Solfaterra, the form and texture of the pumice stones is very discernible. In several parts of the outside cone, this calcining operation is still carried on, by the exhalation of constant very hot and damp vapours, impregnated with salts, sulphur, alum, &c. Where the abovementioned vapours have not operated, the strata of pumice and erupted matter, that compose the cone of the Solfaterra, are like those of all the high grounds in its neighbourhood, which I suppose to have been thrown up likewise by explosion. I have seen here, half of a large piece of lava perfectly calcined, whilst the other half out of the reach of the vapours has been untouched; and in some pieces the centre seems to be already converted into true marble.

The variegated specimens then, above described, are nothing more than pumice and erupted matter, after having been acted upon in this manner by the hot vapours; and if you consider the process, as I have traced it, from bitumen to pumice, and from pumice to marble, you will think with me, that it is difficult to determine the primitive state of the many wonderful productions we see in Nature.

I found, in the _tufa_ of the mountain of Pausilipo, a fragment of lava: one side I polished, to shew it to be true lava; the other shews the signs of the _tufa_, with which it is incorporated. It has evidently been rounded by friction, and most probably by rolling in the sea. Is it not natural then to imagine that there must have been Volcanos near this spot, long before the formation of the mountain of Pausilipo? This little stone may perhaps raise in your mind such reflections as it did in mine, relative to the great changes our globe suffers, and the probability of its great antiquity.

FOOTNOTES:

[1] Having reflected since upon this circ.u.mstance, I rather believe that the weight of the atmosphere in bad weather, preventing the free dissipation of the smoke, and collecting it over the crater, gives it the appearance of being more considerable; whereas in fine weather the smoke is dispersed soon after its emission. It is, however, the common-received opinion at Naples (and from my own observation is, I believe, well founded), that when Vesuvius grumbles, bad weather is at hand. The sea of the Bay of Naples, being particularly agitated, and swelling some hours before the arrival of a storm, may very probably force itself into crevices, leading to the bowels of the Volcano, and, by causing a new fermentation, produce those explosions and grumblings.

[2] These ashes destroy the leaves and fruit, and are greatly detrimental to vegetation for a year or two; but are certainly of great service to the land in general, and are among the princ.i.p.al causes of that very great fertility which is remarkable in the neighbourhood of Volcano's.

[3] In the subsequent eruptions of Vesuvius, I have constantly remarked something of the same nature, as appears in my account of the great eruption of 1767. I have found the same remark in many accounts of former eruptions of Vesuvius: in the very curious one of the formation of a new mountain near Puzzole, in 1538, (as may be seen in my letter to Dr. Maty, Oct. 16, 1770[46],) the same observation is made. This phnomenon, is well worthy of a curious inquiry, which might give some light into the theory of the earth, of which, I believe, we are very ignorant.

[4] I am convinced, that it might be very practicable to divert the course of a lava when in this state, by preparing a new bed for it, as is practised with rivers. I was mentioning this idea at Catania in Sicily, when I was a.s.sured, that it had been done with success during the great eruption of Etna, in 1669; that the lava was directing its course towards the walls of Catania, and advancing slowly like the abovementioned, when they prepared a channel for it round the walls of the town, and turned it into the sea; that a succession of men, covered with sheep-skins wetted, were employed to cut through the tough flanks of the lava, till they made a pa.s.sage for that in the centre (which was in perfect fusion) to disgorge itself into the channel prepared for it.

A book I have since met with gives the same account of this curious operation; it is int.i.tuled, _Relatione del nuovo incendio fatto da Mongibello 1669. Messina, Giuseppe Bisagni, 1670_. His Sicilian Majesty's palace at Portici, and the valuable collection of antiquities that have been recovered from beneath the destructive lava's of Vesuvius, are in imminent danger of being overwhelmed again by the next that shall take its course that way; whereas, by taking a level, cutting away and raising ground, as occasion might require, the palace and museum would, in all probability, be insured, at least against one eruption; and, indeed, I once took the liberty of communicating this idea to the King of Naples, who seemed to approve of it.

[5] The late Lord Morton was pleased to give these specimens to Dr.

Morris, who has made several chemical experiments on them, the result of which will be communicated to the Royal Society.

[6] From what I have seen and read of eruptions of Vesuvius and Etna, I am convinced that Volcano's lie dormant for several years, nay even for centuries, as probably was the case of Vesuvius before its eruption in the reign of t.i.tus, and certainly was so before that of the year 1631.

When I arrived at Naples in 1764, Vesuvius was quiet, very seldom smoak was visible on its top; in the year 1766, it seemed to take fire, and has never since been three months without either throwing up red hot stones, or disgorging streams of lava, nor has its crater been ever free from smoak. At Naples, when a lava appears, and not till then, it is styled an eruption; whereas I look upon the five nominal eruptions I have been witness to, from March 1766 to May 1771, as, in effect, but one continued eruption.

[7] It is certain, that, by constant attention to the smoak that issues from the crater, a very good guess may be given as to the degree of fermentation within the Volcano. By this alone I foretold[47] the two last eruptions, and, by another very simple observation, I pointed out, some time before, the very spot from whence the lava has issued. When the cone of Vesuvius was covered with snow, I had remarked a spot on which it would not lie: concluding very naturally that this was the weakest part of the cone, and that the heat from within prevented the snow from lying; it was as natural to imagine that the lava, seeking a vent, would force this pa.s.sage sooner than another; and so indeed it came to pa.s.s.

[8] These are his words: "Nubes (incertum procul intuentibus ex quo monte Vesuvium fuisse postea cognitum est) oriebatur, cujus similitudinem & formam, non alia magis arbor, quam pinus expresserit.

Nam longissimo veluti trunco elata in altum, quibusdam ramis diffundebatur, credo quia recenti spiritu evecta, dein senescente eo dest.i.tuta, aut etiam pondere suo victa, in lat.i.tudinem evanescebat: candida interdum, interdum sordida & maculosa, prout terram cineremve sustulerat." Plin. lib. vi. ep. 16.

[9] The windows at Naples open like folding-doors.