Landscape in History, and Other Essays

by Archibald Geikie

The explosion that eviscerated the Alban volcano must have caused widespread desolation over the surrounding country. It was not improbably followed by a long interval of repose. But the subterranean energy was not exhausted, though it never again showed itself on so vigorous a scale. We can trace, indeed, the signs of its gradual enfeeblement. When it recommenced its activity the vent, which served as the channel by which its eruptions took place, still retained its central position. Round this vent a new but much smaller cone, bearing witness to less vigour of eruption, was built up in the middle of the crater. This younger mass rises in Monte Cavo to a height of 3150 feet, the highest elevation on the whole mountain. It encloses a well-marked crater with the flat plain of the Campo di Annibale at its bottom. Eventually the central orifice came to be choked up by the lava that had risen and solidified with it, and as the volcanic forces still sought an outlet to the surface, they were compelled to find egress at other and weaker points of the volcano. At least two explosions took place on the old crater-rim and produced the deep-sunk and singularly impressive lakes of Albano and Nemi. Others broke out on the flanks of the great cone. Of these, the largest is marked by the crater of the Valle Arriccia, but at least two dozen of smaller size have been discriminated by the geologists of the Government Survey round the outer slopes of the volcano. These lateral vents not improbably mark the sites of the last eruptions.

While the Alban Mount was heaped up on the southern margin of the Campagna, another independent series of volcanoes rose on the northern border. The Lago di Bracciano marks the position of the vent that lay nearest to Rome. The huge cavity in which this sheet of water lies is some six miles in diameter and not improbably owes its origin to another and still more stupendous explosion than that of the Alban Hills. The level of the lake is 538 English feet above the surface of the Mediterranean, and as the water is as much as 900 feet deep, the bottom is 362 feet below sea-level. The crater wall still rises in the Rocca Romana to a height of 1,437 feet above the sea, or 900 feet higher than the lake which it encloses. Numerous streams of lava have poured down the outer slopes of the cone, especially on the southern flank. A few minor craters have been opened on its east side, and all round there still rise warm springs and emanations of sulphuretted hydrogen. To the north of this great vent lies another of similar character and origin but of smaller size, which now contains the Lago di Vico. The surface of this lake, which stands at a height of 1,663 feet above sea-level, is encircled by a crater-wall which on the west side mounts to nearly 1,600 feet above the sheet of water which it encircles. To the northeast rises the volcanic mass of Monte Cimino, 3,464 feet high. Still farther north is the largest of all the Italian crater-lakes, the Lago di Bolsena, which is no less than twenty-eight miles in circumference.

Having regard to the great variety of material in these different volcanic piles and to the evidence furnished by them that they were formed by many successive eruptions, perhaps separated from each other by long intervals of time, we cannot but be impressed with the antiquity of the great subaërial cones and the protracted period required by each of them for its formation. We must remember, too, that from the very beginning of their history they were ceaselessly attacked by the various agents of subaërial erosion. The first showers of rain that fell on their young slopes of incoherent ashes gathered into runnels which would plough furrows in their descent to the plain. Century after century these watercourses were cut deeper and wider until they have attained the dimensions of the numerous fossi that now radiate from each crater-rim. In some cases these lines of erosion served as channels for the streams of lava that were poured down the slopes, as may be well seen on the southern flanks of the Bracciano volcano.

In most instances the molten rock stopped short on the flanks of its parent mountain, but it occasionally descended into the plain, as in the familiar example on the Via Appia, where the stream flowed from the side of the Alban volcano for some six miles to within a short distance of the site of the future capital of the world. The lava is here a firm, compact, durable stone admirably adapted for pavements, a use to which it has been extensively put for more than two thousand years, both within the walls and on the great high roads that radiate from them. Here, again, we see how bountifully Rome was favoured in regard to the materials needed for the construction of a great city.

The heaping of so much volcanic rock over the surface of the country must have greatly modified its topographical features. The drainage would especially be affected. Streams descending from the Apennines would find their direct passage to the sea blocked by the newly formed ridges, hills and mountains, and they might have to make long circuits before finding an exit. Lakes would gather in the hollows of the irregularly deposited tuff and others would fill up the cavities blown out by explosions, so as to become crater-lakes. The case of the Tiber may be cited in illustration of the deflection of drainage. In earlier times, as I have already remarked, this river probably flowed southwestwards across the site of the volcanic district of Bolsena and Viterbo; but in consequence of the subsequent eruptions, the lower part of its course was buried and the stream, diverted at a right angle, was made to run southeastwards, skirting the volcanic heights until, near Monte Soracte, it reached the plain between the base of the Bracciano and that of the Alban volcano, where it was able at length to find a seaward passage across the site of the future Rome.

That the early races of man witnessed and suffered from the latest eruptions may well be believed. The oldest traces of human occupation are stone implements, found more particularly in the higher river-gravels which, though they must undoubtedly date back to a remote antiquity, are certainly much later than the general mass of the tuff of the Campagna. Traditions of volcanic events seem to have survived into historic times. The pages of Livy, for example, contain references to showers of stones that fell in various places during the early centuries of Rome, and were regarded as portents of divine interposition in human affairs. We are told that in the hundredth year of the city showers of that kind fell on the Alban Mount, accompanied with loud noises from the wood on the summit. Again, in the year 540, fearful storms are said to have been experienced, while a fall of stones on the Alban Mount went on continuously for two days. Nineteen years later, amidst a miscellaneous series of prodigies, it is said to have rained stones at Aricia, Lanuvium, and on the Aventine. Such references have by some writers been interpreted as proofs of true volcanic eruptions, thus bringing the activity of the volcanoes around Rome well down into historic time. A supposed confirmation of this conclusion has been claimed to have been found on the ridge of Castel Gandolfo, where numerous burial urns containing cremated human remains have been unearthed five or six feet below the surface of the ground. Associated with some of these interments were fibulæ and objects in amber and bronze, together with specimens of Etruscan or Italo-Greek pottery of a beautifully Archaic pattern. It has been maintained that the superficial covering of volcanic material (which has even been called 'lava') has been the product of one or more volcanic eruptions, subsequent to the time of the burials, and hence that these eruptions must have taken place not only after the Stone Age, but even so late as after the coming of the Greek colonists. It has even been held that the shepherds of the Alban Hills, driven away from these heights by the violence of the volcanic disturbances, took refuge on the Seven Hills, where they founded the city and empire of Rome. More probably the volcanic detritus which overlies the cinery urns is of much more ancient date, the interments having been made by digging down through it, long after the last eruptions of the volcano had ceased.

It is not necessary, however, to refuse credence to all the portents recounted by Livy. More than two thousand years ago, when the events cited by him are alleged to have happened, the volcanic forces of the region must have been more potent than they now are, and various manifestations by them may have occurred which we do not expect to see repeated at the present day. Though the subterranean fires have been steadily dwindling, they even yet retain heat enough to supply many thermal springs and to discharge large quantities of sulphuretted hydrogen gas. Now and then, also, they show a sudden though local manifestation of energy, and cause disturbances sufficiently alarming to fill the population with superstitious dread. An instance of this kind, which was witnessed within living memory, may here be cited as affording a reasonable explanation of some of the supposed supernatural prodigies recorded in Roman history. On the south side of Monte Soracte lies a dried-up lake which, no doubt on account of its offensive sulphurous exhalations, was called the Lagopuzzo, or Stinking Mere. The late Professor Ponzi has recorded that towards the end of the month of October, in the year 1831, a series of cracks suddenly opened on this old lake-bottom and a large piece of flat ground lying between them gradually sank out of sight. At the same time subterranean rumblings commenced and grew in intensity, mingled with detonations like the thundering of cannon. The surrounding population fled in terror to the neighbouring hills, whence looking back, they could see earth and water thrown up from the fissures, while a thick coating of dust fell over the whole district. The eruption began towards sunset and reached its culmination about seven o'clock in the evening. Next morning it was found that the Lagopuzzo was traversed by a chasm with vertical walls, at the bottom of which lay a sheet of water covered with a white scum and giving off a powerful odour of sulphuretted hydrogen. The ground around the cavity was strewn with pools of water and lumps of erupted earth, sometimes seventy cubic feet in size, which had been ejected to a distance of one hundred feet. The cannon-like detonations continued, but grew gradually less violent, each of them being accompanied by a copious discharge of the ill-smelling gas which threw the water into such commotion as to undermine the surrounding walls of alluvial earth and to cause portions of them to fall into the abyss. After a few days the disturbances ceased, leaving as their memorial a cross-shaped chasm upwards of three hundred feet in diameter, with walls rising some fifteen feet above the water, which was ascertained to be about one hundred feet deep.

Many such incidents as this may have been experienced in the history of ancient Rome. They would be quite enough to fill the minds of the populace with terror, and to call for a nine days' expiation and lustration of the city. The traditional legend of the chasm that opened in the Forum, and into which Curtius threw himself in full armour to propitiate the gods and save the city, may very well have been founded on a real event of this nature. The lake or quagmire in the Forum may have been another Lagopuzzo, rent open by an outburst of gas. The subterranean rumblings and bellowings (boati), which were accounted such dire portents in old times, were exactly repeated near Monte Soracte in the autumn of 1831.

Before we pass from this volcanic period to the consideration of the next phase in the history of the Campagna, it may be noted, as an interesting feature in the growth of the Italian peninsula, that the subterranean energy has been slowly dying out from north to south. The volcanoes of Central Italy have long since entered into the closing or Solfatara stage, when only steam, hot vapours, and gases are emitted. But at the southern end of the chain lies the still vigorous Solfatara of Naples, with the various cones around it, some of which have been in eruption within the last few centuries, while Vesuvius continues to maintain a persistent though variable activity. Still farther south lie the Æolian Islands, where Volcano occasionally breaks out, while Stromboli remains, as it has done since the beginning of authentic history, in a state of constant ebullition and eructation. At the far extremity of the volcanic belt rises the colossal cone of Etna, which from time to time displays an energy worthy of its place among the great volcanoes of the globe.

III. We have traced how the platform of the Campagna has been step by step built up, partly by the accumulation of silt, sand, and gravel on the sea-floor, partly by submarine volcanic ejections, and partly by a widespread uplift of the whole region above sea-level, and by continued subaërial volcanic activity. We now reach the third and last section of our history in which we have to consider how the present topography of the ground has been produced. A little reflection will convince us that, even before its elevation into land, the submerged surface of the district was probably far from presenting a dead flat, though it no doubt approached nearer to that form than it has ever done since. In spite of the levelling action of waves and currents, the sea-floor in front of the Apennine chain must have abounded in inequalities caused not only by the scour of the water, but more especially by the irregular distribution of the volcanic débris and the greater accumulation of the material around the submarine vents. Such inequalities would not only remain but grow more pronounced when the sea-bottom became a land-surface. Every subsequent outbreak of eruptive energy would aggravate them. But ultimately more potent still, because incessant in its operation, would be the influence of the various subaërial agencies by which the land is continually abraded. It is to these agencies that we must mainly ascribe the present topography of the district. By a ceaseless process of sculpture, the volcanic platform has been ultimately carved into hillock and ridge, crag and cliff, valley and ravine. The tools which Nature has employed in this task have been the air, with its wide range of temperature and moisture, frost, rain and running water in all its manifold forms, from the tiniest rill to the broad current of the Tiber.