Part 23

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.

As a consequence of the prolonged eruptions, the sea along the west coast of Central Italy must have become increasingly shallow. This result may not improbably have been expedited by that uplift of the whole region to which reference has above been made. In course of time, not only would volcanic cones appear as islands above sea-level, but the action of winds, waves and tidal currents would throw up bars or _lidi_, like those of Venice or those of more ancient date which traverse the alluvial plain on either side of the mouth of the Tiber. Further deposition of sediment, either from the volcanoes or from the torrents of the Apennines, would lead to the silting up of the lagoons between these bars. The hollows on the newly gained land would eventually become fresh-water lakes, and the drainage from the mountains would find its way by numerous channels across the low plain into the sea. Thus, the Tiber, escaping from its narrow estuary among the hills not improbably continued its southwesterly course, so as to pass across what afterwards became the great volcanic district of Bolsena and to enter the sea somewhere between Civita Vecchia and Orbetello. The Anio would thus at that time be the main stream in the Roman Campagna.