preserve throughout Italy the same twofold character, with the same
types of sediment and of organic remains, the observer who tries to
follow their development in the Campagna is soon puzzled by the way
in which they there suddenly disappear and allow their place to be
taken by later deposits of volcanic origin, which are known by the
general designation of Tuff. The most astonishing example of this
local peculiarity is to be found at the Monte Verde, south of the
Janiculan ridge. The clays and sands which rise in horizontal layers
almost to the top of Monte Mario are there entirely cut out, and the
tuff, which lies as a mere thin capping on the crest of that hill,
suddenly descends across the truncated edges of the Pliocene strata to
the alluvial plain of the Tiber. On the opposite side of the river, the
upper sands have been almost entirely removed and the tuff is found
lying almost immediately on the lower clay. It is clear that there
must have been an extensive, though no doubt local, erosion of these
marine strata, before the main body of the tuff was laid down. By what
agency this erosion was effected is not quite clear. Not improbably a
gap occurs here in the record, representing an interval of considerable
duration of which no chronicle has survived.
Passing over this hiatus, we still encounter marine deposits, but these
are of volcanic origin, and leave us meanwhile to speculate in the dark
as to whether the denudation was the work of the sea or of terrestrial
waters. Owing to the thick covering of tuff which has overspread the
Campagna and concealed all that lies below, it has become difficult
to obtain adequate data for the discussion of this question, which
is of considerable interest in the history of the Campagna. The most
reliable evidence would be supplied by a series of borings across the
district in different directions. Such a series may perhaps hereafter
be undertaken for the purpose of obtaining water for the farms and
homesteads, which sanguine patriots foresee taking in the future the
place of the present solitude, and in that event, the geologists of
Rome will no doubt be on the watch for all the information that can be
gathered from this source as to the nature of the rocks underneath, and
their relations to each other.
In the meantime, much might be done in this attractive department of
local geology by a far more detailed study than has yet been attempted
of the surroundings of the Campagna. In particular the recognisable
stratigraphical horizons among the Pliocene strata should be definitely
traced and mapped in detail, where they emerge from under the volcanic
tuff. It would then be possible to measure the amount of erosion in
various places, and to determine how far the spread of the volcanic
sheet across older formations is due to actual unconformability and how
far to simple overlap. At the same time, the precise height could be
ascertained of the upper limit of the Pliocene deposits, and data would
probably be obtained for determining not only the minimum amount of the
uplift of the land since the Pliocene period, but also how far and in
what directions there may have been any warping of the peninsula in the
course of the elevation. We know from the observations of De Angelis
that the Plaisancian clays, which at Monte Mario do not rise more than
200 feet above the surface of the Mediterranean, reach a height of
as much as 1,050 metres (3,445 feet) in the upper part of the valley
of the Arno, near Subiaco, only about thirty miles east from Rome,
or an upheaval of as much as 108 feet in a mile. It remains still to
discover how far this amount may fall short of the total extent of the
post-Pliocene uplift of the Apennine chain.
Not improbably the deep and extensive erosion of the Pliocene
formations before the deposition of the volcanic tuffs, and their
elevation above sea-level were related phenomena, connected with the
outbreak of the remarkable volcanic episode in the geological history
of Central Italy, which has so profoundly modified the scenery of
the country. In the yellow sands and gravels of Monte Mario no trace
of volcanic detritus has been detected. Their sediments, containing
pebbles of Rhaetic, Jurassic and Cretaceous rocks can hardly have
come from any other source than the Apennine chain. But before their
deposition had quite come to an end the Volcanic period was ushered in
which forms the second stage of the history of the region.
II. We have now to deal with the records of one of the most interesting
phases in the evolution of the framework of Italy--the period that
witnessed the birth, development and extinction of a series of
volcanoes which, starting on the sea-bottom in front of the western
coast, gradually built up a tract of plains in some places thirty to
forty miles broad and altogether perhaps as much as 200 miles long,
finally crowned with majestic cones several thousand feet high. The
chronicles of this episode being tolerably complete in the Roman
Campagna, they enable us to follow the course of events with great
clearness from the beginning to the end.
Nowhere in the district around Rome have the earliest indications
of the oncoming of this volcanic period, the first mutterings, as
it were, of the subterranean convulsions, been more instructively
preserved than in the line of quarries that have been opened along the
edge of the alluvial plain of the Tiber at the Torretta di Quinto, near
the Ponte Molle, about two miles north from the city. The section of
strata there exposed, which has long been known and often described,
is at present undergoing rapid changes from the extensive excavations
required to procure materials for the embankment of the river in its
course through Rome. Nowhere, too, is the geologist more seductively
wooed from the pursuit of his researches by the fascination of crowded
historic associations. From the slopes above the quarries, he sees the
Tiber catching the shadows of the Pons Milvius, where Maxentius met his
doom. Below him rise the roofs of the _Osterie_, which on feast days
repeat the noise and merriment that made the place notorious in the
days of the Empire. Opposite him, a green hill marks the long-deserted
site of Antemnæ, beneath which the Tiber winds as a silver band through
its meadows far up past other old towns that have long since mouldered
into dust. Looking across the green and purple expanse of the Campagna,
dotted with its ruined towers, he sees the whole sweep of the blue
rampart of the Sabine Hills--almost the only feature of the landscape
that has remained the same. If from these memories of the past he turns
to the long line of quarries, he is perchance rudely awakened to the
strenuous present by gangs of workmen, digging, blasting, wheeling; by
the rattle of laden wagons, and by an occasional explosion of gunpowder
or dynamite. Passing through this busy scene, he soon perceives that
the ancient bank against which the Tiber chafed when it was spreading
out its high alluvial plain, has been quarried backward, and as the
ground slopes upward from the plain, the cliff thus artificially cut
open must be continually changing its face and becoming higher. At
present there has been laid open an excellent section of strata forming
part of the upper or sandy group of Monte Mario. These succeed each
other in horizontal bands so diverse in form and colour as to give the
cliff a markedly banded aspect.
At the southern end of this section some layers of coarse gravel may
be seen cemented into a solid calcareous conglomerate full of large
and well-preserved marine shells. A little further over comes a band
of travertine--a compact variety of limestone which is one of the
characteristic and economically important stones of the Campagna,
for where of good quality, it makes an admirable firm cream-coloured
building-stone, which has been largely used from the early times of
Roman history. To its durability the preservation of so many noble
monuments of Republican and Imperial architecture is due. I may remark
in passing that this material can now be seen in actual course of
deposition from solution in the neighbourhood of Rome. The cold waters
of the Anio have formed thick masses of it at the Tivoli Falls, and
the warm springs of Bagni deposit it on the plain below. At the latter
place its accumulation must have been going on for a vast period of
time, seeing that it now covers a tract of the low ground to the north
of the Anio, measuring about six miles from east to west and four miles
from north to south. Here the quarries of ancient Rome were opened,
and the modern city still draws its supplies from the same area. A
smaller tract of similar stone has been worked for building material
at Cisterna di Roma, about twenty-eight miles to the southeast of the
city. A thin band of travertine, possibly a continuation of that at
the Torretta di Quinto, forms a conspicuous cliff along the east side
of the Via Flaminia, between Ponte Molle and the Porto del Popolo, and
other detached masses of it make their appearance at various places
further south.
These various outcrops of travertine, as I shall have occasion to
show, have had an important influence in the excavation of the valley
of the Tiber and the isolation of the hills of Rome. The material was
probably deposited chiefly by hot springs containing abundant carbonate
of lime in solution, and may be regarded as an accompaniment or sequel
of volcanic activity. In most cases, thermal mineral springs depositing
travertine make their appearance in the later phases of a volcanic
period, and often continue long after every other manifestation of
subterranean heat has died out. But at the Torretta di Quinto the
sheet of travertine is found among the records of the beginning of the
volcanic history. Another exceptional feature in this sheet is the
proof that it was laid down on the floor of the sea, for it encloses
the remains of some of the shells that lived at the time in that sea.
More direct and obvious proof of the breaking-out of volcanic eruptions
is to be found in the strata that lie above the travertine. In some
of these may be detected truly volcanic minerals such as felspar,
augite and black mica, derived from the explosion of lava within
eruptive vents and from the falling of the volcanic dust upon the
silt and shells of the sea-bottom. Higher up more pronounced evidence
of successive eruptions is furnished by abundant lapilli and scoriæ
of black slaggy lava, and by bands of true tuff, composed almost or
entirely of volcanic detritus.
This section at the Torretta di Quinto is of great interest as
indicating that the volcanoes of the Campagna began their career under
the sea. Similar evidence obtained at other places makes it probable
that the whole chain of volcanoes in central and southern Italy, from
those of Bolsena on the north to those of Naples and Sicily on the
south, started their eruptions on the sea-floor. When the activity of
this chain was at its height, a band of eruptive vents flanked the
western coast from the neighbourhood of Aquapendente and Ovieto to
perhaps as far as the Bay of Salerno. At first the cones formed round
these vents were probably submerged, and were no doubt more or less
washed down and levelled by the agitation of the sea, but as they were
renewed by successive discharges, the larger examples among them may
have risen above water and scattered their dust and stones into the
air. These volcanic islets would then front the mainland of Italy, much
as the Aeolian islands now flank the northern coast of Sicily. Etna,
Lipari, Volcano, Stromboli and the other islets may be regarded as the
last lineal descendants of the insular volcanoes to whose operations
the scenery of Central and Southern Italy is so largely indebted.
The solid substances ejected by these volcanoes in the earlier stages
of their history consisted mainly of fragmentary material--dust, sand,
stones, scoriæ, and the other discrete forms in which molten lava is
blown out of volcanic vents by the explosion of its absorbed vapours
and gases. Sometimes pieces of limestone or other rock, which were
torn away from the older formations underneath, are found dispersed
through the volcanic detritus. This fragmentary material, now more or
less compacted into the form of Tuff, extends throughout the length
and breadth of the volcanic tract and must thus cover some thousands
of square miles. In the Campagna, which lies upon it and derives
thence its distinctive features, it reaches a thickness of 300 feet
or more, while inland it overlaps the Pliocene deposits in detached
outliers which run far up the Apennine valleys, reaching heights of
1,200 feet and upward. Compared with the Pliocene strata that lie
below it, the tuff presents some characteristic differences which at
once arrest attention. It lacks the rapid alternation and variety of
parallel layers so marked in the Astian sands. Yet it can generally be
seen to possess a stratified arrangement. Here and there, indeed, this
structure gives place to a tumultuous accumulation of coarse detritus,
huddled together as it fell, large and small lumps of lava being
confusedly mingled in the general matrix and forming the rock known
to geologists as Agglomerate. Such coarser intercalations probably
indicate proximity to centres of eruption, and in some cases may
even mark the position of the vents themselves. Alternations in the
character of the successive beds of tuff may be regarded as evidence of
variations in the energy and distribution of the active orifices. It
may be added that the tuff supplied the Romans with various admirable
building materials. In the days of the Kings and of the Republic, its
more compact kinds were quarried in large quadrangular blocks for the
construction of massive walls, while in later times some of its more
incoherent varieties were discovered to be capable of forming the most
durable concrete, which in the hands of Roman architects was employed
with a boldness and skill that have never since been equalled.
That the materials of the tuff were assorted under water is suggested
by their stratified structure. This inference is strengthened by the
intercalation among them of sheets of sand, gravel, clay and marl. The
layers of gravel are especially important, for their component pebbles
of limestone and other non-volcanic stones are unmistakably fragments
of Mesozoic rocks, which have been rolled along by running water from
their original resting places among the Apennines so as to acquire
smoothed and rounded forms. But though the tuff was accumulated under
water, it presents a strong contrast to the clays and sands below it
by its generally unfossiliferous character. Leaves, branches and stems
of ilex, oak and other land-vegetation have been obtained from it at
various places, sometimes as mere hollow moulds or in carbonised forms,
but occasionally with the internal structure still preserved. Less
frequently it has yielded the bones, antlers or tusks of terrestrial
quadrupeds. But both the plants and animals have obviously been
drifted from the land, and did not live where their remains have been
found. It is worthy of remark that though so many observers have been
at work in successive generations among the rocks of the Campagna, no
undoubted example of marine mollusk has been recorded from the tuff
in the interior of the Campagna. The crowds of shells in the Pliocene
strata underneath are there absent. That the conditions required for
the existence of an abundant marine fauna continued over this site
until the beginning of the volcanic period is manifest from the crowded
pteropods, lamellibranchs and gasteropods of the clays and sands. But
as the eruptions increased in area and in intensity these conditions
were eventually destroyed. The descent of continued showers of hot
dust, ashes and stones over the sea-bottom, the rise of mephitic
gases from below, as well as of hot springs that deposited sheets of
travertine, must have made that sea-floor no congenial home for either
plant or animal.
It has often been assumed that the tuff of the Roman Campagna was
derived from the eruptions of the Alban volcano on the one side, and
of the Bracciano volcano on the other. A careful study of the tuff,
however, and a comparison of it with that of more ancient volcanic
districts, the structure of which has been more fully laid open by
prolonged denudation, leads, in my opinion, to a conviction that this
assumption is founded on inaccurate observation. The rapidly varying
and lenticular character of the materials when followed along the
cliffs where they are exposed, and their occasional agglomeratic
character which increases and diminishes in various directions, with
no reference to the two great volcanic centres on each border of the
district, point not to showers of detritus from these centres, or from
any other vents at a distance, but to local eruptions from many and
generally small vents, discharging here fine, there coarse materials,
at different times and independent of each other. I have not myself
been fortunate enough to detect a 'neck,' which would mark the site of
one of these vents, nor so far as I am aware, has any example of this
structure been recorded from the general body of the Campagna tuff. But
this failure of proof, I am disposed to believe, is to be accounted for
rather from the special kind of evidence required not having hitherto
been recognised, or searched for with sufficient experience, than
because it does not exist.
One of the tasks which I think might hopefully be undertaken in regard
to the geological history of this district is that of seeking for
proofs of the distribution of some of the vents whence the tuff was
ejected. Among the numerous crags along the hillsides, and in the
abundant stream-courses or _fossi_, where the naked rock has been laid
bare all over the Campagna, sections might be met with that would help
to solve this problem. The numerous unquestionable 'craters' of the
Alban and Ciminian Hills belong to a much later stage of the volcanic
period than that in which the main mass of tuff was formed. We must
remember also, in considering this question, that the tuff, with its
distinctive and persistent characters, stretches far beyond the limits
within which the materials fell that were discharged from the Alban
or Bracciano volcanoes, even when these were at the height of their
vigour. It can be followed in numerous detached tracts of valley-floor
through the hills eastwards to Sora, and southwards to near Gaeta.
There is reason to believe, indeed, that the type of small submarine
vents extended all through the volcanic tract from its northern to its
southern limit.
A little reflection will show that the sites of these vents may be
expected to be difficult of detection. In the first place, though
numerous, their small size may easily make them escape notice, even
where they may have been wholly or partially laid bare by denudation.
Probably a close parallel to their original forms and to the way in
which they were in some places crowded together is to be found in
the Phlegræan Fields near Naples--a district which well deserves
the careful scrutiny of any one who desires to follow the volcanic
history of the Roman Campagna. Its cones are terrestrial, indeed, not
submarine. Being much younger, they have been far better preserved
than those of the submarine stage of the period. One of them, Monte
Nuovo, though now as cold and silent as the oldest of them, was thrown
up so recently as A.D. 1538. Another, that of the Solfatara, is still
a steaming vent, while Vesuvius from time to time vigorously asserts
its claim to rank in the list of active volcanoes. These Neapolitan
cones probably convey a fair idea of the general distribution and
aspect of those of the Campagna, especially in the later time when the
volcanic platform had eventually been raised above the level of the
Mediterranean. We see, as in the case of the youngest and smallest of
the three craters which have risen through each other to the north
of Astroni, that some of the Neapolitan vents were only a few yards
in diameter. And we learn also that at least one, and probably others
of them, were the product of single eruptions, for Monte Nuovo, which
is nearly 500 feet in height, was thrown up in the course of two
days. Doubtless, these small and rapidly built monticules had many
predecessors of like type on the Roman Campagna.
In the second place, the cones connected with the tuff of the district
around Rome, being composed of loose fragmentary materials, would be
easily washed down. No one can ramble over that area without being
struck with the singular scarcity of solid lava among the endless
exposures of tuff. It is true that around the great craters of the
Alban and Ciminian Hills a good deal of lava can be seen to have been
emitted. But these masses, like the volcanoes that gave vent to them,
belong to that later stage of the volcanic history to which I have
referred. Only to a trifling extent does the tuff of the Campagna
appear to include contemporaneous sheets of lava. If, then, molten rock
has hardly ever poured out at the surface, it may rarely have risen and
consolidated in the upper parts of the throats of the volcanoes, so as
to form there a hard core which would remain as a projecting knob when
the surrounding loose ashes were levelled down by denudation.
In the third place, there can now be no doubt that the greater part of
the sheet of tuff in the Roman Campagna was accumulated under the sea.
This subject was for many years one on which various contradictory
opinions were held. Some writers, from the general stratified
structure, correctly maintained the marine origin of the tuff. Again,
on the evidence of enclosed land-plants and animals, some observers
have regarded it as a freshwater deposit, while others have looked
upon it as a terrestrial formation. It is true, as I shall point out
a little further on, that here and there, especially in its upper
parts, the tuff includes intercalated bands of strata containing land
and freshwater shells as well as bones of terrestrial mammals, and
indicating that the floor of the sea had been converted into low land
with brackish lagoons and lakes of fresh water. But as regards the main
mass of the tuff of the Campagna, the question of its marine origin may
now be considered as definitely settled by the researches of Professor
Portis, of the University of Rome. In specimens of different varieties
of the rock from all parts of the district, and previously supposed
to be entirely unfossiliferous, this careful observer has found that
foraminifera are often abundant and well preserved. These organisms are
unequivocally marine, swimming freely in the upper waters and sinking
when dead to mingle with the silt or to form of themselves an ooze on
the bottom. We can thus understand how they might be borne along above
a seafloor on which molluscan life was hardly possible.
If, then, cones of loose ashes and scoriæ were thrown up on the bottom
of the sea, they would obviously be apt to be rapidly lowered by
the agitation of currents and ground swell, while those which rose
above the surface of the water, as Lipari, Volcano and Stromboli
do now, would be subject also to continual erosion by rain and to
unceasing attack along their shores by wind-waves. They would thus
tend to be ultimately planed down, their materials being strewn over
the surrounding sea-bottom, so as to add to the general accumulating
sheet of tuff. The rapidity with which this kind of demolition may
be completed was impressively exemplified in this very area of the
Mediterranean by the history of Graham Island, which in the summer of
1831 was thrown up by a submarine eruption off the southwest coast of
Sicily. In the course of less than a month, a cone of loose cinders,
scoriæ and pumice was piled up to a height, it is said, of more than
200 feet above sea-level, with a circumference of three miles and a
large crater inside. In about three months, this volcano was levelled
with the surface of the sea.