EUROPEAN EXTINCT OR DORMANT VOLCANOES.
We are naturally led on from a consideration of the active volcanoes of
Europe to that of volcanoes which are either dormant or extinct in the
same region. Such are to be found in Italy, Central France, both banks
of the Rhine and Moselle, the Westerwald, Vogelsgebirge, and other
districts of Germany; in Hungary, Styria, and the borders of the Grecian
archipelago. But the subject is too large to be treated here in detail;
and I propose to confine my observations to some selected cases which
are to be found in Southern Italy, Central France, and the Rhenish
districts, where the volcanic features are of so recent an age as to
preserve their outward form and structure almost intact.
(_a._) _Southern Italy._--Extinct volcanoes and volcanic rocks occupy
considerable tracts between the western flanks of the Apennines and the
Mediterranean coast in the Neapolitan and Roman States, forming the
remarkable group of the Phlegraean fields (Campi Phlegraei), with the
adjoining islands of Ischia, Procida, Nisida, Vandolena, Ponza, and
Palmarola; at Melfi and Avellino. All the region around Rome extending
along the western slopes of the Apennines from Velletri to Orvieto,
together with Mount Annato in Tuscany, is formed of volcanic material,
and the same may be said of a large part of the island of Sardinia. From
these districts I shall select some points which seem to be of special
interest.
_Monte Nuovo and the Phlegraean Fields._--The tract of which this
celebrated district forms a part lies as it were in a bay of the
Apennine limestone of Jurassic age. The floor of this bay is composed of
puzzolana, a name given to beds of volcanic tuff of great thickness, and
rising into considerable hills in the vicinity of the city of Naples,
such as that of St. Elmo. Its composition is peculiar, as it is chiefly
formed of small pieces of pumice, obsidian, and trachyte, in beds
alternating with loam, ferriferous sand, and fragments of limestone. It
is evidently of marine formation, as Sir William Hamilton, Professor
Pilla, and others have detected sea-shells therein, of the genera
_Ostraea_, _Cardium_, _Pecten_ and _Pectunculus_, _Buccinum_, etc. It is
generally of a greyish colour, and sometimes sufficiently firm to be
used as a building stone. The Roman Campagna is largely formed of
similar materials, which were deposited at a time when the districts in
question were submerged, and matter was being erupted from volcanic
vents at various points around, and spread over the sea-bed.
Such is the character of the general floor on which the more recent
crater-cones of this district have been built. These are numerous, and
all extinct with the exception of the Solfatara, near Puzzuoli, from
which gases mixed with aqueous vapour are continually being exhaled. The
gases consist of sulphuretted hydrogen mixed with a minute quantity of
muriatic acid.[1] This district is also remarkable for containing
several lakes occupying the interiors of extinct craters; amongst
others, Lake Avernus, which, owing to its surface having been darkened
by forests, and in consequence of the effluvia arising from its stagnant
waters, has had imparted to it a character of gloom and terror, so that
Homer in the _Odyssey_ makes it the entrance to hell, and describes the
visit of Ulysses to it. Virgil follows in his steps. Another lake of
similar origin is Lake Agnano. Here also is the Grotto del Cane, a
cavern from which are constantly issuing volumes of carbonic acid gas
combined with much aqueous vapour, which is condensed by the coldness of
the external air, thus proving the high temperature of the ground from
which the gaseous vapour issues. This whole volcanic region, so replete
with objects of interest,[2] may be considered, as regards its volcanic
character, in a moribund condition; but that it is still capable of
spasmodic movement is evinced by the origin of Monte Nuovo, the most
recent of the crater-cones of the district. This mountain, rising from
the shore of the Bay of Baiae, was suddenly formed in September 29th,
1538, and rises to a height of 440 feet above the sea-level. It is a
crater-cone, and the depth of the crater has been determined by the
Italian mineralogist Pini to be 421 English feet; its bottom is thus
only 19 feet above the sea-level. A portion of the base of the cone is
considered partly to occupy the site of the Lucrine Lake, which was
itself nothing more than the crater of a pre-existent volcano, and was
almost entirely filled up during the explosion of 1538. Monte Nuovo is
composed of ashes, lapilli, and pumice-stones; and its sudden formation,
heralded by earthquakes, and accompanied by the ejection of volcanic
matter mixed with fire and water, is recorded by Falconi, who vividly
depicts the terror and consternation of the inhabitants of the
surrounding country produced by this sudden and terrible outburst of
volcanic forces.[3]
(_b._) _Central Italy and the Roman States._--The tract bordering the
western slopes of the Apennines northward from Naples into Tuscany, and
including the Roman States, is characterised by volcanic rocks and
physical features of remarkable interest and variety. These occur in the
form of extinct craters, sometimes filled with water, and thus converted
into circular lakes; or of extensive sheets and conical hills of tuff;
or, finally, of old necks and masses of trachyte and basalt, sometimes
exhibiting the columnar structure. The Eternal City itself is built on
hills of volcanic material which some observers have supposed to be the
crater of a great volcano; but Ponzi, Brocchi, and Daubeny all concur in
the opinion that this is not the case, as will clearly appear from the
following account.
The geological structure of the valley of the Tiber at Rome is very
clearly described by Professor Ponzi in a memoir published in 1850, from
which the accompanying section is taken.[4] (Fig. 16.) From this it will
be seen that "the Seven-hilled City" is built upon promontories of
stratified volcanic tuff, of which the Campagna is formed, breaking off
along the banks of the Tiber, the hills being the result of the erosion,
or denudation, of the strata along the side of the river valley. As the
strata dip from west to east across the course of the river, it follows
that those on the western banks are below those on the opposite side;
and thus the marine sands and marls which underlie the volcanic tuff,
and are concealed by it along the eastern side of the valley, emerge on
the west, and form the range of hills on that side. Such being the
structure of the formations under Rome, it is evident that it is not
"built on a volcano."
[Illustration: Fig. 16.--Geological Section across the Valley of the
Tiber at Rome. 1. Alluvium of the Tiber; 2. Diluvium; 3. Volcanic tuff
(recent deposits); 4. Sands, etc.; 5. Blue marl (sub-Apennine
deposits).]
The tuff contains fragments of lava and pebbles of Apennine limestone,
and was deposited under the waters of an extensive lake at a time when
volcanic action was rife amidst the Alban Hills. This lacustrine
formation rests in turn on deposits of marine origin, containing
oysters, patellae, and other sea-shells, of which the chain of hills on
the right bank of the Tiber is chiefly formed.
The district around Albano lying to the south of Rome is of peculiar
interest from the assemblage of old crater-lakes which it contains; as,
for instance, those of Albano, Vallariccia, Nemi, Juturna, and the lake
of Gabii. The lake of Albano, one of the most beautiful sheets of water
in the world, is about six miles in circumference, and surrounded by
beds of peperino, a variety of tuff presenting a bright, undecomposed
aspect when newly broken. The level of this lake was lowered by the
Romans during the siege of Veii by means of a tunnel, so that the waters
are 200 feet lower than the level at which they originally stood. In the
same district is the lake of Nemi, very regular in its circular outline;
that of Juturna lying near the foot of the Alban Hills, and that of
Ariccia lying in a deep hollow eight miles in circumference;--all may be
supposed to have been the craters of extinct volcanoes, both by reason
of their shape and of the materials of which they are formed. All these
old craters are, however, according to Daubeny, "only the dependencies
and offshoots, as it were, of the great extinct volcano, the traces of
which still remain upon the summit of the Alban Hills, and which is
comparable in its form to that of Vesuvius, as it is surrounded by an
outer circle of volcanic rock comparable to that of Somma."[5]
To the north of the city of Rome are several crateriform lakes, some of
which are of great size, such as that of Bolsena, over twenty miles in
circumference, and the Lago di Bracciano, almost as large, and lying
about twelve miles from the city. These extensive sheets of water are
surrounded by banks of tuff and volcanic sand, in which fragments of
augite, leucite, and crystals of titanite are distributed. The town of
Viterbo is built up at the foot of a steep hill called Monte Cimini, the
lower part of which is composed of trachyte; this is surmounted by
tuff, which appears to have been ejected from an extinct crater
occupying the summit of the mountain, and now converted into a lake
called the Lake of Vico. This crater is perfectly circular, and from its
centre rises a little conical hill covered by trees.
(_c._) _Physical History._--Space does not permit of a fuller
description of the remarkable volcanic features of the tract lying along
the western slope of the Apennines; but from what has been stated it
will be clear that volcanic forces have been in operation at one time on
a grand scale in the Roman States and the South of Tuscany, over a tract
extending from Mount Annato to Velletri and Segni.
This tract was separated from that of the Neapolitan volcanic region by
a range of limestone hills of Jurassic age between Segni and Gaeta, a
protrusion of the Alban Hills westward; but the general structure and
physical history of both regions are probably very similar, with the
exception that the igneous forces still retain their vitality in the
more southerly region. In the case of the Roman volcanic district, a bay
seems to have been formed about the close of the Miocene period, bounded
on all sides but the west by hills of limestone, over whose bed strata
of marl, sandstone, and conglomerate were deposited. This tract was
converted by subsequent movements into a fresh-water lake, and
contemporaneously volcanic operations commenced over the whole region,
and beds of tuff, often containing blocks of rock ejected from
neighbouring craters, were deposited over those of marine origin.
Meanwhile numerous crater-cones were thrown up; and, as the land
gradually rose, the waters of the lake were drained off, leaving dry
the Campagna and plain of the Tiber. Ultimately the volcanic fires
smouldered down and died out, whether within the historic epoch or not
is uncertain; lakes were formed within the now dormant craters, and the
face of nature gradually assumed a more placid and less forbidding
aspect over this memorable region, destined to be the site of Rome, the
Mistress of the World.
[1] As determined by Daubeny in 1825.
[2] Including the ruins of the Temple of Serapis, whose pillars are
perforated by marine boring shells up to a height of about 16 feet from
their base; indicating that the land had sunk down beneath the sea, and
afterwards been elevated to its present level.
[3] The account of Falconi, and another by Pietro Giacomo di Toledo, are
given by Sir W. Hamilton, _op. cit._, p. 198, and also reproduced by Sir
C. Lyell, _Principles_, vol. i. p. 608.
[4] Guiseppe Ponzi, "Sulla storia fisica del Bacino di Roma," _Annali di
Scienze Fisiche_ (Roma, 1850).
[5] Daubeny, _Volcanoes_, p. 171.