Apennines, which bound the valley of the Po on the south. Hydrogen gas
escapes from fissures in the rocks to the south of Modena and Bologna,
and is utilised in several instances in the manufacture of lime,
and for other purposes. These gas springs of Pietra Mala, Porretta,
and Barigazzo were known by the ancients and during the Middle Ages
as “fiery springs,” and they illuminated the path of the traveller
overtaken by the night. Lower down the slope, almost on the verge of
the plains, we meet with a line of mud volcanoes, or _bombi_, the most
famous of which are those of Sassuolo, near Modena. The largest of
these, that of Mirano, has no less than forty craters. The ancient gulf
of the sea, now converted into a plain, is thus skirted by volcanic
cones, mud volcanoes, hot springs, and deposits of sulphur. As high up
as Piemont, and notably at Acqui, we meet with hot springs, attesting
that volcanic activity is not yet altogether extinct.
[Illustration: La Dent blanche, 14,319 ft.; Château des Dames, 11,998
ft.; Mt. Cervin, 14,705 ft.; Mischabel Hōrner, 14,942 ft.; Breithorn,
13,680 ft.; Monte Rosa (Dufour Spitze, 15,217 ft.).
THE PENNINE ALPS, AS SEEN FROM THE BECCA DI NONA (PIC CARREL), 10,380
FEET.]
The valleys of the Alps and the plains extending along their foot were
filled, in a former geological epoch, with huge glaciers, descending
from what was anciently the immense glacial region of Central Europe.
There is not a valley between that of the Tanaro in the west, and that
of the Isonzo descending from the mountains of Carinthia, but contains
accumulations of débris carried down by the {195} glaciers, and now
covered with vegetation. Most of these ancient glaciers exceeded those
of the Monte Rosa and the Finsteraarhorn in extent, and several of them
rivalled the existing glaciers of the Himalaya. If we would gain a
notion of what the Alps were like during this glacial epoch, we must go
to Greenland or to the Antarctic regions.
[Illustration: Fig. 56.—THE ANCIENT GLACIERS OF THE ALPS.
Scale 1 : 4,800,000.]
One of the smallest of these ice streams, that which descended from the
mountains of Tenda in the direction of Cuneo, had a length of thirty
miles. That which brought down the ice of Mont Genèvre, Mont Tabor, and
Mont Cenis had twice that length, and its moraines formed a veritable
amphitheatre of hills, locally known as _regione alla pietre_, or stony
region. Farther north the streams of ice descending from the Pennine
Alps between the Grand Paradis and Mont Blanc united in a single stream
eighty miles in length, and spread over the plain far beyond Ivrea.
The alluvial accumulation of this ancient glacier rises 1,100 and even
2,130 feet above the valley through which the Dora Baltea now flows.
One of its lateral moraines, known as the _Serra d’Ivrea_, forms a
regular rampart to the east of the river, eighteen miles in extent.
Its slopes are now covered with chestnuts. The western ravine (Colle
di Brossa) is less prominent, because it is inferior in height; but
the frontal ravine, forming a complete demicircle, can still be traced
readily. In the débris accumulated at the foot of this ancient glacier,
rocks derived from Mont Blanc are mixed with others brought down
from Mont Cervin. And yet it was but a dwarf when compared with the
ancient twin glacier of the Ticino and the Adda, which extended from
the Simplon to the Stelvio, filled up the cavities now occupied by the
Lago Maggiore {196} and the Lake of Como, sent a lateral branch to the
tortuous bed of the Lake of Lugano, and finally, after a course of from
100 to 120 miles, debouched upon the plain of Lombardy. The glacier of
the Oglio was small in comparison with it, but it was exceeded by that
of the Adige, the most considerable of all on the southern slope of
the Alps. This river of ice, from the mountains of the Oetzthal, where
it originated, to its terminal moraine to the north of Mantua, had a
length of 175 miles. One of its branches descended towards the east,
down the valley of the Drave, as far as where the town of Klagenfurt
now stands. Its main stream filled up the cavity of the Lake of Garda,
pushing along a formidable rampart of elevated moraines.
[Illustration: Fig. 57.—THE SERRA OF IVREA AND THE ANCIENT GLACIER
LAKES OF THE DORA.
From the Sardinian Staff Map. Scale 1 : 250,000]
The hand of man is scarcely able to make an impression upon the vast
accumulations heaped up by the action of the glaciers. The hills
of Solferino, of Cavriana, and Somma Campagna, so often named in
connection with battles, are nothing but débris brought down from the
flanks of the Alps, and they were much higher formerly than they are
now. {197}
Some of the erratic blocks were as large as houses, but, being used as
quarries, they are fast disappearing. One of them at Pianezza, at the
mouth of the Susa valley, is 80 feet long, 40 feet broad, and 46 feet
high, and a chapel has been built upon it. The huge erratic blocks
in the hills between the two arms of the Lake of Como have supplied
materials for the monolithic columns of the churches and palaces in the
environs. The slopes of the hills of Turin facing the Alps are likewise
covered with erratic blocks.
* * * * *
When the glaciers retired into the upper valleys of the Alps, the soil
which they covered was left bare, and the depressions now occupied by
the beautiful lakes of Lombardy were revealed. These depressions, whose
bottom even now sinks down below the level of the ocean, were formerly
arms of the sea, in character very much like the fiords of Norway.
That such was the case is proved by the presence, in every one of the
Lombard lakes, of a sardine (the _agone_), which naturalists consider
to be a sea fish. In Garda Lake, moreover, there still dwell two marine
fishes which have adapted themselves to their new condition of life, as
well as a small marine shell-fish.
[Illustration: Fig. 58.—ANCIENT LAKES OF VERBANO.]
The number of these Alpine lakes was much larger formerly, and those
which still exist shrink from year to year. In Upper Piemont alluvial
deposits have long ago filled up the lakes, and there now only remain
a few pools of {198} water to indicate their site. The first sheets
of water to which the term “lake” may fairly be applied are met with
on both banks of the Dora Baltea (see Fig. 57). The little basin of
Candia and the shallow Lake of Azeglio, to the west and east of the
river, are the only remains of _Lacus Clisius_, which covered an area
of several hundred square miles until its waters broke through the
semicircular terminal moraine which bounded it on the south. The Dora
Baltea formerly escaped from this lake in the south-east, its present
course only dating from the fourteenth century.
[Illustration: Fig. 59.—THE UPPER EXTREMITY OF THE LAKE OF COMO.
Scale 1 : 148,000.]
Since this reservoir has been drained, the first lake of importance in
the west is that of Verbano, very inappropriately called Lago Maggiore,
or the “principal lake,” as that of Garda exceeds it in extent. Ancient
beaches, at an elevation of 1,300 feet above the sea, prove that the
waters of the lake have considerably subsided, and that its area was
much larger formerly; and it curiously ramified with neighbouring lake
basins, now merely connected with it by rivers. The ancient moraine
at the foot of this lake, and through which the Ticino has excavated
itself a passage, still rises to a height of 980 feet. {199}
Centuries elapsed before the changes which we now perceive were
accomplished. Still they proceeded at a sufficiently rapid rate. Even
now the alluvium carried down by the Ticino and the Maggia continually
encroaches upon the Lago Maggiore. Seven hundred years ago the village
of Gordola stood on the shore of the lake: it is now nearly a mile away
from it. The landing-places of Magadino, at the mouth of the Ticino,
have to be continually shifted, for the lake retires steadily. Only
sixty years ago barges were able to receive their cargoes at a wharf
nearly half a mile higher up than the present one. The Gulf of Locarno
is gradually being separated from the main sheet of water by alluvial
deposits brought down by the Maggia.
[Illustration: Fig. 60.—SECTION OF THE NORTHERN PORTION OF LAKE COMO.
Scale 1 : 25,000.]
[Illustration: Fig. 61.—SECTION OF THE LAKE OF LECCO, NEAR THE
BIFURCATION.
Scale 1 : 25,000.]
[Illustration: Fig. 62.—LONGITUDINAL SECTION OF LAKE COMO.
Horizontal scale 1 : 50,000. Vertical scale 1 : 500,000.]
The Lario, or Lake of Como, which rivals the Maggiore by its beauty,
is likewise being gradually silted up. In the time of the Romans the
navigation extended as far as Summolacus (lake-head), the modern
Samolaco. But the torrent of Mera gradually converted most of the upper
extremity of the lake into an alluvial plain, whilst the alluvial
deposits carried down by the Adda cut off the remainder from the main
body of water. There now remains only the _Lacus_ {200} _Dimidiatus_,
or Lake of Mezzola, which is shrinking from year to year, and will
finally disappear altogether. The miasmata rising from the swamps
at the mouth of the Adda have frequently depopulated the environs,
and the ruined fort of Fuentes, at the mouth of the river, built to
defend the Val Tellina, was hardly ever more than a hospital for its
fever-stricken garrison.
The south-eastern arm of the lake, that of Lecco, through which the
Adda makes its escape to the south, has likewise been divided into a
series of separate basins. Nature, which would convert these lakes into
bottom-lands at no distant date, is being aided here by the works of
man. The barrier which obstructed the free egress of the Adda has been
cleared away, the structures of fishermen have been removed, and, in
consequence of these and other engineering measures, the once-dreaded
rises of the lake have been reduced to a minimum, and the southernmost
of the lake basins, that of Brivio, has been converted into dry
land. The large Lake of Brianza, which extended formerly far to the
south-west, has likewise been partially drained, and there now remain
only a few lakelets of small extent.
We know sufficient of the bottom of the Lake of Como to enable us to
judge of the manner in which it is becoming gradually filled up with
alluvium. The mud deposited in its northern portion has filled up all
the original inequalities of the soil, and even in the centre of the
lake, and in its south-eastern arm, the bottom is almost a perfect
level. In the Como arm, however, which receives no tributary river
of any importance, the bottom is still full of inequalities. These
differences amply prove to us the geological agency of the rivers,
which must terminate in the lake being converted into a bottom-land,
with a river flowing through its centre. The third of our diagrams
(Fig. 62) shows that the greatest depth now hardly exceeds 1,300 feet,
whilst, if we may judge from the slopes of the hills which bound it,
the depth in former times cannot have been less than 2,300 feet.
The Sebino, or Lake of Iseo, and the lakelet of Idro, which are fed
by the glacier streams of the Adamello, exhibit the same features as
the lakes farther to the west. The Benaco, or Lake of Garda, however,
the most extensive of these Alpine lakes, is very stable as regards
its outline and the configuration of its bottom, a fact sufficiently
explained by the small size of its tributary streams as compared
with its vast area. The old Alpine lakes of the Venetian Alps have
disappeared long ago, and there remain only a few ponds, filling
cavities in the dolomitic rocks and peat bogs, to indicate their
ancient sites.[64] {201}
[Illustration: Fig. 63.—VILLA SERBELLONI, ON THE PENINSULA OF
BELLAGIO, LAKE OF COMO.]
These lacustrine basins, like all other reservoirs of the same kind,
regulate the outflow of the torrents which empty into them. During the
freshets they store up the superabundant waters, and only part with
them in the dry season, and upon their difference of level in different
seasons depend the oscillations of the emissary rivers which issue from
them. In the case of the Lake of Garda, which drains but a small area
in proportion to its size, this difference is small, and throughout
the year the pellucid waters of the Mincio flow tranquilly beneath
the blackened ramparts of Peschiera. Such is not the case as regards
either the Lago Maggiore or the Lake of Como, for the volume of water
discharged into them is so considerable that their level in summer
and winter varies to the extent of several yards, and corresponding
differences may be observed in the rivers issuing from them. Lake Como
rises no less than 12 feet, and increases 70 square miles in area,
whilst the Lago Maggiore sometimes rises 22 feet, and {202} increases
to the extent of one-fifth. The volume of the Ticino, when at its
highest, almost equals the average volume of the Nile, and if it were
not for the regulating influence of the lake from which it issues, it
would alternately convert the plains of Lombardy into a sheet of water
and leave them an arid tract of land.[65]
The Alpine lakes of Italy thus play an important part in the economy of
the country. They render the climate more equable, serve as high-roads
of commerce, and, being the centres of animal life, attract a dense
population. But it is not this which has rendered these lakes famous,
which has attracted thousands of wanderers ever since the time of the
Romans, and caused villas and palaces to rise on their shores: it is
their incomparable beauty. And, indeed, there are few spots in Europe
which bear comparison with the delightful Gulf of Pallanza, over which
are scattered the Borromean Islands, or with the peninsula of Bellagio,
which may be likened to a hanging garden suspended within sight of
the snow-clad Alps, and affording a prospect of the rock-bound shores
of the Como Lake, cultivated fields, and numerous villas. Perhaps
even more delightful is the peninsula of Sermione, jutting out into
the azure waters of the Garda Lake, like the tender stalk of a flower
developing into a many-coloured petal.
Most of the lakes in the plain have been drained into the neighbouring
rivers. The Lake of Gerondo, mentioned in mediæval records, has
dwindled down into a small swamp, or _mosi_, now, and its populous
island of Fulcheria has become merged in the plain of Lombardy. The
lakes on the southern bank of the Po, above Guastalla, have likewise
been drained; and if the two shallow lakes of Mantua still exist, this
is entirely due to the embankments raised in the twelfth century. It
would have been much better, and would have saved the city the horrors
of many a siege, if these lakes had been allowed to disappear likewise.
* * * * *
The lagoons along the Adriatic have decreased in extent in the course
of centuries, and whilst new lagoons are being formed, the old ones
are gradually being converted into dry land. The old maps of the
Venetian littoral differ essentially from our modern ones, and yet
all the vast changes they indicate have been wrought in the course
of a few centuries. The swamps of Caorle, between the Piave and the
Gulf of Trieste, have changed to an extent which prevents us from
restoring the ancient topography of the country; and if the lagoons
of Venice and Chioggia exhibit a certain permanence of contour, this
is only on account of the incessant interference of man. The ancient
lagoon of Brondolo has been dry land since the middle of the sixteenth
century. The large lagoon of Comacchio, to the south of the Po, has
been cut up into separate portions by alluvial embankments formed by
the agency of rivers and torrents. For the most part it consists now of
_valli_, or alluvial deposits, but there still remain a few profound
cavities, or _chiari_, which the rivers have not yet succeeded in
filling up. Formerly these {203} lagoons extended far to the south in
the direction of Ravenna, and, according to Strabo and other ancient
writers, that ancient city once occupied a site very much like that of
Venice or Chioggia in our own days.
[Illustration: Fig. 64.—BEECH AND PINE WOODS OF RAVENNA.
Scale 1 : 2,470,000.]
There can be no doubt that these lagoons were anciently separated from
the Adriatic by a narrow strip of land over 120 miles in length, and
similar to what we still meet with on the coasts of Carolina and of
the Brazils. This ancient barrier still exists in the _lidi_ of Venice
and Comacchio, which are pierced at intervals, admitting the vivifying
floods of the open sea. Elsewhere the traces of this ancient beach must
be looked for on the mainland. The low delta of the Po is traversed
from north to south by a range of dunes constituting the continuation
of the lidi of Venice, and extending into the swamps of Comacchio,
where they form a natural embankment running parallel with the coast.
These dunes, between the Adige and Cervia, are covered with sombre pine
woods, replaced here and there by oaks. The underwood mainly consists
of hawthorns and juniper-trees, and wild boars still haunt it.
No sooner have the lagoons protected by these barriers been converted
into dry land than the sea seizes upon the sand, and forms it into new
curvilinear barriers similar to the former ones. The principal range of
dunes to the east of Ravenna, which is about 20 miles in length, and
varies in width between 50 and 3,300 yards, has thus two other ranges
of dunes running parallel with it, one of them being still in course of
formation. Signor {204} Pareto has estimated the annual advance of the
land at 7½ feet, and at much more near the mouths of rivers.
The sea thus marks by a series of barriers its successive recoils.
Sometimes, however, the sea gains upon the land in consequence of a
gradual subsidence of the Venetian shore, the cause of which has not
yet been elucidated. Thus the gravel bank of Cortellazzo, opposite
the swamps of Caorle, appears to have anciently been a _lido_ which
has sunk nearly 70 feet below the level of the sea. The islands which
fringed the littoral of Aquileja during the Middle Ages have almost
wholly disappeared. In the time of the Romans these islands were
populous; there were forests and fields upon them, and the inhabitants
built ships. The chronicles of the Middle Ages tell us that the Doge
of Venice and the Patriarch of Aquileja hunted stags and wild boars
upon them, much to the scandal of the inhabitants. At the present day
the dunes which of yore protected these islands have almost wholly
disappeared, the forests have been supplanted by reeds, and Grado is
the only place on the littoral which may still boast of a certain
number of inhabitants. Piers, walls, mosaic pavements, and even stones
bearing inscriptions, which are found occasionally at the bottom of the
sea or of swamps, prove that the mainland was formerly more extensive
there. Farther to the west the littoral of Venice bears evidence of
a similar subsidence. Artesian wells sunk in the city of the lagoons
have led to the discovery of four beds of turf, the deepest no less
than 420 feet below the level of the sea. The subterranean church of
St. Mark has within historical times been converted into a submarine
church, and streets and buildings are gradually sinking beneath the
waters of the lagoons. If it were not for the alluvium brought down by
the rivers, the sea would continually encroach upon the land. Ravenna,
too, participates in this subsidence, which Signor Pareto estimates to
amount to 0·60 inch in the course of a century.
* * * * *
Amongst the geological agents constantly at work to modify the surface
of the earth, the rivers and torrents irrigating the plain lying at the
foot of the Alps are the most active, and no other country of Europe,
Holland alone excepted, can compare in this respect with Northern Italy.
The torrent of Isonzo offers one of the most striking instances of
these geological revolutions. It is said to have formerly communicated
through subterranean channels with the Istrian Timavo, and that its
existence as a separate river does not date very far back. Ancient
writers do not enumerate the Isonzo amongst the rivers flowing into
the Adriatic. It is first mentioned in a document of the sixth century
as a river irrigating some inland valley. On Peutinger’s Table we
meet with a station, Ponte Sonti, far to the east of Aquileja, and
near the sources of the Timavo. The chronicles are silent with
respect to the peripatetics of this river, but a careful examination
of the surrounding hills justifies the assumption that the valley of
Tolmein, on the Upper Isonzo, was formerly a lake which overflowed
towards the north-west through the narrows of Caporetto, and that its
pent-up waters found their way through the Natisone into the Adriatic.
Subsequently they opened themselves a passage to the south, and another
lake was {205} formed at the confluence of Isonzo and Wippach. This
lake communicated by subterranean channels with the Timavo, but it has
now disappeared, and the Isonzo flows directly into the sea, its bed
wandering continuously towards the east. The alluvium carried down by
this river has formed the peninsula of Sdobba, and joined several old
islands to the mainland.
[Illustration: Fig. 65.—SHINGLE BEDS OF THE TAGLIAMENTO, THE MEDUNA,
AND ZELLINE.
From the Austrian Staff Map. Scale 1 : 290,000]
The Tagliamento is even a more active geological agent than its
neighbour just beyond the frontier. The débris deposited at the mouth
of the narrow gorge in which it rises covers many square miles of
a once fertile plain. In summer its waters trickle through these
accumulations of shingle, but after heavy rain the river is converted
into a powerful torrent several miles in width, and all the more
formidable as its bed lies higher than many parts of the surrounding
country. The Meduna and Zelline, to the west of the Tagliamento, are
equally destructive, and an extensive tract at their confluence is
covered with shingles. Lower down, in the lagoons, these torrents have
thrown up huge embankments of sand on either side of their ancient
beds. The alluvium brought down by these torrents to the sea is in
every instance deposited to the west, a circumstance accounted for by
the direction of the coast current.
The Piave, the most considerable river to the east of the Adige, is
likewise a most active geological agent, converting fertile fields
into sterile shingle tracts, filling up swamps, and carrying large
quantities of matter into the sea. At its {206} mouth the land
gains rapidly upon the sea, and Heraclea of the Veneti, now known as
Cittanova, which was a seaport once, at the present time lies far
inland.
The Piave was formerly supposed to have changed its bed in the same
manner as the Isonzo. Below the Capo di Ponte, a wild defile in the
Dolomite Alps, the Piave flows towards the south-west, past Belluno,
and lower down is joined by the Cordevole. It was, however, supposed
that the river originally flowed through the valley of Rai, immediately
to the south of the Capo di Ponte, and that the Meschio and Livenzo
constituted its lower course. Earthquakes or landslips were supposed to
have created a barrier across that valley, and the small lakes still
seen there were looked upon as remains of the ancient river bed. But
M. de Mortillet has shown that this hypothesis is untenable, for the
barrier referred to is merely the moraine of an ancient glacier, and
there exist no traces whatever of landslips.
[Illustration: Fig. 66.—THE SUPPOSED OLD BED OF THE PIAVE.
From the Austrian Staff Map. Scale 1 : 550,000.]
At the same time it cannot be doubted that extensive changes have
taken place in the basin of the Piave. Thus in 1771 the course of the
Cordevole, its most important tributary, was obstructed for a time by
a landslip which carried the verdant terraces of Pezza down into the
valley. Two villages were destroyed, and two others overwhelmed by the
rising floods of the river.
[Illustration: VENICE.]
[Illustration: Fig. 67.—THE LAGOONS OF VENICE.
Scale 1 : 394,000.]