is a stratum of good mould, of the thickness of about two feet and more
in some parts, in which vines flourish, except in some particular spots
of this vineyard, where they are subject to be blasted by a foul vapour,
or _mofete_, as it is called here, that rises from beneath the burnt
matter. The abovementioned shower of pumice stones, according to my
observations, extended beyond Castel-a-mare (near which spot the ancient
town of Stabia also lies buried under them) and covered a tract of
country not less than thirty miles in circumference. It was at Stabia
that Pliny the elder lost his life, and this shower of pumice stones is
well described in the younger Pliny's letter. Little of the matter that
has issued from Vesuvius since that time, has reached these parts: but I
must observe, that the pavement of the streets of Pompeii is of lava;
nay, under the foundation of the town, there is a deep stratum of lava
and burnt matter. These circumstances, with many others that will be
related hereafter, prove, beyond a doubt, that there have been eruptions
of Vesuvius previous to that of the year 79, which is the first recorded
by history.
The growth of soil by time is easily accounted for; and who, that has
visited ruins of ancient edifices, has not often seen a flourishing
shrub, in a good soil, upon the top of an old wall? I have remarked many
such on the most considerable ruins at Rome and elsewhere. But from the
soil which has grown over the barren pumice that covers Pompeii, I was
enabled to make a curious observation. Upon examining the cuts and
hollow ways made by currents of water in the neighbourhood of Vesuvius
and of other Volcanos, I had remarked that there lay frequently a
stratum of rich soil, of more or less depth, between the matter produced
by the explosion of succeeding eruptions[28]; and I was naturally led to
think, that such a stratum had grown in the same manner as the one
abovementioned over the pumice of Pompeii. Where the stratum of good
soil was thick, it was evident to me that many years had elapsed between
one eruption and that which succeeded it. I do not pretend to say, that
a just estimate can be formed of the great age of Volcanos from this
observation; but some sort of calculation might be made: for instance,
should an explosion of pumice cover again the spot under which Pompeii
is buried, the stratum of rich soil abovementioned would certainly lie
between two beds of pumice; and if a like accident had happened a
thousand years ago, the stratum of rich soil would as certainly have
wanted much of its present thickness, as the rotting of vegetables,
manure, &c. is ever increasing a cultivated soil. Whenever I find then a
succession of different strata of pumice and burnt matter, like that
which covers Pompeii, intermixed with strata of rich soil, of greater or
less depth, I hope I may be allowed reasonably to conclude, that the
whole has been the production of a long series of eruptions, occasioned
by subterraneous fire. By the size and weight of the pumice, and
fragments of burnt erupted matter in these strata, it is easy to trace
them up to their source, which I have done more than once in the
neighbourhood of Puzzole, where explosions have been frequent. The
gradual decrease in the size and quantity of the erupted matter in the
stratum abovementioned, from Pompeii to Castle-a-Mare, is very visible:
at Pompeii, as I said before, I have found them of eight pounds weight,
when at Castle-a-Mare the largest do not weigh an ounce.
The matter which covers the ancient town of Herculaneum is not the
produce of one eruption only; for there are evident marks that the
matter of six eruptions has taken its course over that which lies
immediately above the town, and was the cause of its destruction. These
strata are either of lava or burnt matter, with veins of good soil
between them. The stratum of erupted matter that immediately covers the
town, and with which the theatre and most of the houses were filled, is
not of that foul vitrified matter, called lava, but of a sort of soft
stone, composed of pumice, ashes, and burnt matter. It is exactly of
the same nature with what is called here the Naples stone; the Italians
distinguish it by the name of _tufa_, and it is in general use for
building. Its colour is usually that of our free stone, but sometimes
tinged with grey, green, and yellow; and the pumice stones, with which
it ever abounds, are sometimes large, and sometimes small: it varies
likewise in its degree of solidity.
The chief article in the composition of _tufa_ seems to me to be, that
fine burnt material, which is called _puzzolane_, whose binding quality
and utility by way of cement are mentioned by Vitruvius[29], and which
is to be met with only in countries that have been subject to
subterraneous fires. It is, I believe, a sort of lime prepared by
nature. This, mixed with water, great or small pumice stones, fragments
of lava, and burnt matter, may naturally be supposed to harden into a
stone of this kind[30]; and, as water frequently attends eruptions of
fire, as will be seen in the accounts I shall give of the formation of
the new mountain near Puzzole, I am convinced the first matter that
issued from Vesuvius, and covered Herculaneum, was in the state of
liquid mud. A circumstance strongly favouring my opinion is, that, about
two years ago, I saw the head of an antique statue dug out of this
matter within the theatre of Herculaneum; the impression of its face
remains to this day in the _tufa_, and might serve as a mould for a cast
in plaister of Paris, being as perfect as any mould I ever saw. As much
may be inferred from the exact resemblance of this matter, or _tufa_,
which immediately covers Herculaneum, to all the _tufas_ of which the
high grounds of Naples and its neighbourhood are composed. I detached a
piece of it sticking to, and incorporated with, the painted stucco of
the inside of the theatre of Herculaneum, and shall send it for your
inspection[31]. It is very different, as you will see, from the
vitrified matter called lava, by which it has been generally thought
that Herculaneum was destroyed. The village of Resina and some villas
stand at present above this unfortunate town.
To account for the very great difference of the matters that cover
Herculaneum and Pompeii, I have often thought that, in the eruption of
79, the mountain must have been open in more than one place. A passage
in Pliny's letter to Tacitus seems to say as much: "Interim e Vesuvio
monte pluribus locis latissimae flammae, atque incendia relucebant, quorum
fulgor et claritas tenebras noctis pellebat:" so that very probably the
matter that covers Pompeii proceeded from a mouth, or crater, much
nearer to it than is the great mouth of the Volcano, from whence came
the matter that covers Herculaneum. This matter might nevertheless be
said to have proceeded from Vesuvius, just as the eruption in the year
1760, which was quite independent of the great crater (being four miles
from it), is properly called an eruption of Vesuvius.
In the beginning of eruptions, Volcanos frequently throw up water mixed
with the ashes. Vesuvius did so in the eruption of 1631, according to
the testimony of many contemporary writers. The same circumstance
happened in 1669, according to the account of Ignazzio Sorrentino, who,
by his history of Mount Vesuvius, printed at Naples in 1734, has shewn
himself to have been a very accurate observer of the phaenomena of the
Volcano, for many years that he lived at Torre del Greco, situated at
the foot of it. At the beginning of the formation of the new mountain,
near Puzzole, water was mixed with the ashes thrown up, as will be seen
in two very curious and particular accounts of the formation of that
mountain, which I shall have the pleasure of communicating to you
presently; and in 1755, Etna threw up a quantity of water in the
beginning of an eruption, as is mentioned in the letter I sent you last
year upon the subject of that magnificent Volcano[32]. Ulloa likewise
mentions this circumstance of water attending the eruptions of Volcanos
in America. Whenever therefore I find a _tufa_ composed exactly like
that which immediately covers Herculaneum, and undoubtedly proceeded
from Vesuvius, I conclude such a _tufa_ to have been produced by water
mixing with the erupted matter at the time of an explosion occasioned by
subterraneous fire; and this observation, I believe, will be of more use
than any other, in pointing out those parts of the present _terra
firma_, that have been formed by explosion. I am convinced, it has often
happened that subterraneous fires and exhalations, after having been
pent up and confined for some time, and been the cause of earthquakes,
have forced their passage, and in venting themselves formed mountains of
the matter that confined them, as you will see was the case near
Puzzole in the year 1538, and by evident signs has been so before, in
many parts of the neighbourhood of Puzzole; without creating a regular
Volcano. The materials of such mountains will have but little appearance
of having been produced by fire, to any one unaccustomed to make
observations upon the different nature of Volcanos.
If it were allowed to make a comparison between the earth and a human
body, one might consider a country replete with combustibles occasioning
explosions (which is surely the case here) to be like a body full of
humours. When these humours concentre in one part, and form a great
tumour out of which they are discharged freely, the body is less
agitated; but when, by any accident, the humours are checked, and do not
find free passage through their usual channel, the body is agitated, and
tumours appear in other parts of that body, but soon after the humours
return again to their former channel. In a similar manner one may
conceive Vesuvius to be the present great channel, through which nature
discharges some of the foul humours of the earth: when these humours are
checked by any accident or stoppage in this channel for any considerable
time, earthquakes will be frequent in its neighbourhood, and explosions
may be apprehended even at some distance from it. This was the case in
the year 1538, Vesuvius having been quiet for near 400 years. There was
no eruption from its great crater, from the year 1139 to the great
eruption of 1631, and the top of the mountain began to lose all signs of
fire. As it is not foreign to my purpose, and will serve to shew how
greatly they are mistaken, who place the seat of the fire in the centre,
or towards the top, of a Volcano; I will give you a curious description
of the state of the crater of Vesuvius, after having been free from
eruption 492 years, as related by Bracini, who descended into it not
long before the eruption of 1631: "The crater was five miles in
circumference, and about a thousand paces deep; its sides were covered
with brush wood, and at the bottom there was a plain on which cattle
grazed. In the woody parts, boars frequently harboured; in the midst of
the plain, within the crater, was a narrow passage, through which, by a
winding path, you could descend about a mile amongst rocks and stones,
till you came to another more spacious plain covered with ashes: in this
plain were three little pools, placed in a triangular form, one towards
the East, of hot water, corrosive and bitter beyond measure; another
towards the West, of water salter than that of the sea; the third of hot
water, that had no particular taste."
The great increase of the cone of Vesuvius, from that time to this,
naturally induces one to conclude, that the whole of the cone was raised
in the like manner; and that the part of Vesuvius, called Somma, which
is now considered as a distinct mountain from it, was composed in the
same manner. This may plainly be perceived, by examining its interior
and exterior form, and the strata of lava and burnt matter of which it
is composed. The ancients, in describing Vesuvius, never mention two
mountains. Strabo, Dio, Vitruvius, all agree, that Vesuvius, in their
time, shewed signs of having formerly erupted[33], and the first
compares the crater on its top to an amphitheatre. The mountain now
called Somma was, I believe, that which the ancients called Vesuvius:
its outside form is conical; its inside, instead of an amphitheatre, is
now like a great theatre. I suppose the eruption in Pliny's time to have
thrown down that part of the cone next the sea, which would naturally
have left it in its present state; and that the conical mountain, or
existing Vesuvius, has been raised by the succeeding eruptions: all my
observations confirm this opinion. I have seen antient lavas in the
plain on the other side of Somma, which could never have proceeded from
the present Vesuvius. Serao, a celebrated physician now living at
Naples, in the introduction of his account of the eruption of Vesuvius
in 1737 (in which account many of the phaenomena of the Volcano are
recorded and very well accounted for), says, that at the convent of
Dominican Fryars, called the Madona del Arco, some years ago, in sinking
a well, at a hundred feet depth, a lava was discovered, and soon after
another; so that, in less than three hundred feet depth, the lavas of
four eruptions were found. From the situation of this convent, it is
clear beyond a doubt, that these lavas proceeded from the mountain
called Somma, as they are quite out of the reach of the existing
Volcano.
From these circumstances, and from repeated observations I have made in
the neighbourhood of Vesuvius, I am sure that no virgin soil is to be
found there, and that all is composed of different strata of erupted
matter, even to a great depth below the level of the sea. In short, I
have not any doubt in my own mind, but that this Volcano took its rise
from the bottom of the sea; and as the whole plain between Vesuvius and
the mountains behind Caserta, which is the best part of the Campagna
Felice, is (under its good soil) composed of burnt matter, I imagine the
sea to have washed the feet of those mountains, until the subterraneous
fires began to operate, at a period certainly of a most remote
antiquity.
The soil of the Campagna Felice is very fertile; I saw the earth opened
in many places last year in the midst of that plain, when they were
seeking for materials to mend the road from Naples to Caserta. The
stratum of good soil was in general four or five feet thick; under which
was a deep stratum of cinders, pumice, fragments of lava, and such burnt
matter as abounds near Vesuvius and all Volcanos. The mountains at the
back of Caserta are mostly of a sort of lime-stone, and very different
from those formed by fire; though Signior Van Vitelli, the celebrated
architect, has assured me, that, in the cutting of the famous aqueduct
of Caserta through these mountains, he met with some soils, that had
been evidently formed by subterraneous fire. The high grounds, which
extend from Castel-a-Mare, to the point of Minerva towards the island of
Caprea, and from the promontory that divides the bay of Naples from that
of Salerno, are of lime-stone. The plain of Sorrento, that is bounded by
these high grounds, beginning at the village of Vico, and ending at that
of Massa, is wholly composed of the same sort of _tufa_ as that about
Naples, except that the cinders or pumice stones intermixed in it are
larger than in the Naples _tufa_. I conceive then that there has been an
explosion in this spot from the bottom of the sea. This plain, as I have
remarked to be the case with all soils produced by subterraneous fire,
is extremely fertile; whilst the ground about it, being of another
nature, is not so. The island of Caprea does not shew any signs of
having been formed by subterraneous fire; but is of the same nature as
the high grounds last mentioned, from which it has been probably
detached by earthquakes, or the violence of the waves. Rovigliano, an
island, or rather a rock, in the bay of Castel-a-Mare, is likewise of
lime-stone, and seems to have belonged to the original mountains in its
neighbourhood: in some of these mountains there are also petrified fish
and fossil shells, which I never have found in the mountains which I
suppose to have been formed by explosion[34].
You have now, Sir, before you the nature of the soil, from Caprea to
Naples. The soil on which this great metropolis stands has been
evidently produced by explosions, some of which seem to have been upon
the very spot on which this city is built; all the high grounds round
Naples, Pausilipo, Puzzole, Baia, Misenum, the islands of Procita and
Ischia, appear to have been raised by explosion. You can trace still in
many of these heights the conical shape that was naturally given them at
first, and even the craters out of which the matter issued, though to be
sure others of these heights have suffered such changes by the hand of
time, that you can only conjecture that they were raised in the like
manner, by their composition being exactly the same as that of those
mountains which still retain their conical form and craters entire. A
_tufa_, exactly resembling the specimen I took from the inside of the
theatre of Herculaneum, layers of pumice intermixed with layers of good
soil, just like those over Pompeii, and lavas like those of Vesuvius,
compose the whole soil of the country that remains to be described.
The famous grotto anciently cut through the mountain of Pausilipo, to
make a road from Naples to Puzzole, gives you an opportunity of seeing
that the whole of that mountain is _tufa_. The first evident crater you
meet with, after you have passed the grotto of Pausilipo, is now the
lake of Agnano; a small remain of the subterraneous fire (which must
probably have made the bason for the lake, and raised the high grounds
which form a sort of amphitheatre round it) serves to heat rooms, which
the Neapolitans make great use of in summer, for carrying off diverse
disorders, by a strong perspiration. This place is called the Sudatorio
di San Germano; near the present bagnios, which are but poor little
hovels, there are the ruins of a magnificent ancient bath. About an
hundred paces from hence is the Grotto del Cane; I shall only mention,
as a further proof of the probability that the lake of Agnano was a
Volcano, that vapours of a pernicious quality, as that in the Grotto
del Cane, are frequently met with in the neighbourhood of Etna and
Vesuvius, particularly at the time of, before, and after, great
eruptions. The noxious vapour having continued in the same force
constantly so many ages, as it has done in the Grotto del Cane (for
Pliny mentions this Grotto[35]), is indeed a circumstance in which it
differs from the vapours near Vesuvius and Etna, which are not constant.
The cone forming the outside of this supposed Volcano is still perfect
in many parts.
Opposite to the Grotto del Cane, and immediately joining to the lake,
rises the mountain called Astruni, which, having, as I imagine, been
thrown up by an explosion of a much later date, retains the conical
shape and every symptom of a Volcano in much greater perfection than
that I have been describing. The crater of Astruni is surrounded with a
wall, to confine boars and deers (this Volcano having been for many
years converted to a royal chace). It may be about six miles or more in
circumference: in the plain at the bottom of the crater are two lakes;
and in some books there is mention made of a hot spring, which I never
have been able to find. There are many huge rocks of lava within the
crater of Astruni, and some I have met with also in that of Agnano; the
cones of both these supposed Volcanos are composed of _tufa_ and strata
of loose pumice, fragments of lava and other burnt matter, exactly
resembling the strata of Vesuvius. Bartholomeus Fatius, who wrote of the
actions of King Alphonso the First (before the new mountain had been
formed near Puzzole), conjectured that Astruni had been a Volcano.
These are his words: "Locus Neapoli quatuor millia passuum proximus,
quem vulgo Listrones vocant, nos unum e Phlegraeis Campis ab ardore
nuncupandum putamus." There is no entrance into the crater of either
Astruni or Agnano, except one, evidently made by art, and they both
exactly correspond with Strabo's description of Avernus; the same may be
said of the Solfaterra and the Monte Gauro, or Barbaro as it is
sometimes called, which I shall describe presently.
Near Astruni and towards the sea rises the Solfaterra, which not only
retains its cone and crater, but much of its former heat. In the plain
within the crater, smoak issues from many parts, as also from its sides;
here, by means of stones and tiles heaped over the crevices through
which the smoak passes, they collect in an aukward manner what they call
_sale armoniaco_; and from the sand of the plain they extract sulphur
and alum. This spot, well attended to, might certainly produce a good
revenue, whereas I doubt if they have hitherto ever cleared 200_l._ a
year by it. The hollow sound produced by throwing a heavy stone on the
plain of the crater of the Solfaterra seems to indicate, that it is
supported by a sort of arched natural vault; and one is induced to think
that there is a pool of water beneath this vault (which boils by the
heat of a subterraneous fire still deeper), by the very moist steam that
issues from the cracks in the plain of the Solfaterra, which, like that
of boiling water, runs off a sword or knife, presented to it, in great
drops. On the outside, and at the foot of the cone of the Solfaterra,
towards the lake of Agnano, water rushes out of the rocks, so hot, as to
raise the quicksilver in Fahrenheit's thermometer to the degree of
boiling water[36], a fact of which I was myself an eye-witness. This
place, well worthy the observation of the curious, has been taken little
notice of; it is called the _Pisciarelli_. The common people of Naples
have great faith in the efficacy of this water; and make much use of it
in all cutaneous disorders, as well as for another disorder that
prevails here. It seems to be impregnated chiefly with sulphur and alum.
When you approach your ear to the rocks of the Pisciarelli, from whence
this water ouzes, you hear a horrid boiling noise, which seems to
proceed from the huge cauldron, that may be supposed to be under the
plain of the Solfaterra. On the other side of the Solfaterra, next the
sea, there is a rock, which has communicated with the sea, till part of
it was cut away to make the road to Puzzole; this was undoubtedly a
considerable lava, that ran from the Solfaterra when it was an active
Volcano. Under this rock of lava, which is more than seventy feet high,
there is a stratum of pumice and ashes. This ancient lava is about a
quarter of a mile broad; you meet with it abruptly before you come in
sight of Puzzole, and it finishes as abruptly within about an hundred
paces of the town. I have often thought that many quarries of stone,
upon examination, would be found to owe their origin to the same cause,
though time may have effaced all signs of the Volcano from whence they
proceeded. Except this rock, which is evidently lava and full of
vitrifications like that of Vesuvius, all the rocks upon the coast of
Baia are of _tufa_.
I have observed in the lava of Vesuvius and Etna, as in this, that the
bottom, as well as the surface of it, was rough and porous, like the
cinders or scoriae from an iron foundery; and that for about a foot from
the surface and from the bottom, they were not near so solid and
compact as towards the centre; which must undoubtedly proceed from the
impression of the air upon the vitrified matter whilst in fusion. I
mention this circumstance, as it may serve to point out true lavas with
more certainty. The ancient name of the Solfaterra was, _Forum Vulcani_;
a strong proof of its origin from subterraneous fire. The degree of
heat, that the Solfaterra has preserved for so many ages, seems to have
calcined the stones upon its cone, and in its crater, as they are very
white, and crumble easily in the hottest parts.
We come next to the new mountain near Puzzole, which, being of so very
late a formation, preserves its conical shape entire, and produces as
yet but a very slender vegetation. It has a crater almost as deep as the
cone is high, which may be near a quarter of a mile perpendicular, and
is in shape a regular inverted cone. At the basis of this new mountain
(which is more than three miles in circumference), the sand upon the
sea shore, and even that which is washed by the sea itself, is burning
hot for above the space of an hundred yards; if you take up a handful of
the sand below water, you are obliged to get rid of it directly, on
account of its intense heat.
I had been long very desirous of meeting with a good account of the
formation of this new mountain, because, proving this mountain to have
been raised by mere explosion in a plain, would prove at the same time,
that all the neighbouring mountains, which are composed of the same
materials, and have exactly or in part the same form, were raised in the
like manner; and that the seat of fire, the cause of these explosions,
lies deep; which I have every reason to think.