Part 24

In the fifth place, let me plead for the virtue of Patience. In a scientific career we encounter two dangers, for the avoidance of which patience is our best support and guide. When life is young and enthusiasm is boundless; when from the details which we may have laboriously gathered together we seem to catch sight of some new fact or principle, some addition of more or less importance to the sum of human knowledge, there may come upon us the eager desire to make our discovery known. We may long to be allowed to add our own little stone to the growing temple of science. We may think of the pride with which we should see our names enrolled among those of the illustrious builders by whom this temple has been slowly reared since the infancy of mankind. So we commit our observations to writing, and send them for publication. Eventually we obtain the deep gratification of appearing in print among well-known authors in science. Far be it from me to condemn this natural desire for publicity. But, as your experience grows, you will probably come to agree with me that if the desire were more frequently and energetically curbed, scientific literature would gain much thereby. There is amongst us far too much hurry in publication. We are so afraid lest our observations or deductions should be forestalled--so anxious not to lose our claim to priority, that we rush before the world, often with a half-finished performance, which must be corrected, supplemented, or cancelled by some later communication. It is this feverish haste which is largely answerable for the mass of jejune, ill-digested, and erroneous matter that cumbers the pages of modern scientific journals. Here it is that you specially need patience. Before you venture to publish anything, take the utmost pains to satisfy yourselves that it is true, that it is new, and that it is worth putting into print. And be assured that this reticence, while it is a kindness to the literature of science, will most certainly bring with it its own reward to yourselves. It will increase your confidence, and make your ultimate contributions more exact in their facts as well as more accurate and convincing in their argument.

The other danger to which I referred as demanding patience is of an opposite kind. As we advance in our career, and the facts of our investigations accumulate around us, there will come times of depression when we seem lost in a labyrinth of detail out of which no path appears to be discoverable. We have, perhaps, groped our way through this maze, following now one clue, now another, that seemed to promise some outlet to the light. But the darkness has only closed around us the deeper, and we feel inclined to abandon the research as one in which success is, for us at least, unattainable. When this blankness of despair shall come upon you, take courage under it, by remembering that a patient study of any department of nature is never labour thrown away. Every accurate observation you have made, every new fact you have established, is a gain to science. You may not for a time see the meaning of these observations, nor the connection of these facts. But their meaning and connection are sure in the end to be made out. You have gone through the labour necessary for the ascertainment of truth, and if you patiently and watchfully bide your time, the discovery of the truth itself may reward your endurance and your toil.

It is by failures as well as by successes that the true ideal of the man of science is reached. The task allotted to him in life is one of the noblest that can be undertaken. It is his to penetrate into the secrets of Nature, to push back the circumference of darkness that surrounds us, to disclose ever more and more of the limitless beauty, harmonious order and imperious law that extend throughout the universe. And while he thus enlarges our knowledge, he shows us also how Nature may be made to minister in an ever-augmenting multiplicity of ways to the service of humanity. It is to him and his conquests that the material progress of our race is mainly due. If he were content merely to look back over the realms which he has subdued, he might well indulge in jubilant feelings, for his peaceful victories have done more for the enlightenment and progress of mankind than were ever achieved by the triumphs of war. But his eye is turned rather to the future than to the past. In front of him rises the wall of darkness that shrouds from him the still unknown. What he has painfully accomplished seems to him but little in comparison with the infinite possibilities that lie beyond. And so he presses onward, not self-satisfied and exultant, but rather humbled and reverential, yet full of hope and courage for the work of further conquest that lies before him.

Such is the task in which you may be called to share. When you have entered upon it and have learnt something of its trials and responsibilities, as well as of its joys and rewards, you will look back with gratitude to the training you received within the walls of this College. You will feel even more keenly than you do now how much you owe to the patient kindness and educational skill of your teachers and to the healthy stimulus of contact and competition with your class-fellows. Most heartily do I wish you success in your several careers. Following up the paths which have been opened for you here, may it be yours to enlarge still further the circle of light which science has gained, and to wrest from Nature new aids for the service of mankind.

FOOTNOTE:

An address to the students of Mason University College, Birmingham, at the opening of the session, on Tuesday, 4th October, 1898.

The Roman Campagna

Among the capitals of Europe Rome has long had the unique distinction of standing in the midst of a wide solitude. Other cities in their outward growth have incorporated village after village and hamlet after hamlet. As their streets and squares merge insensibly into a succession of villas and gardens, cottages and hedgerows, followed by the farms and fields of the open country, so the noise and stir of causeway and pavement gradually give way to the quieter sounds of rural life. But with the Eternal City this normal arrangement does not hold good. For sixteen centuries she has kept herself within her ancient walls which still surround her with their picturesque continuity of rampart and tower. Inside these barriers we still encounter, by day and by night, the 'fumum et opes, strepitumque Romæ.' But outside the gates we find ourselves on a lonely prairie that sweeps in endless grassy, almost treeless, undulations up to the base of the, distant hills. The main roads, indeed, that radiate from the city, are bordered on either side, for the first mile or two, with a strip of suburban osterie, booths and shops, varied here and there, perhaps, by a villa and its grounds. But these fringes of habitation are too narrow and short, and cling too closely to their respective arteries of traffic, seriously to affect the solitariness which broods over the intervening landscape up to the very foot of the walls.

This surrounding district, known as the Roman Campagna, possesses a singular fascination, which has been often and enthusiastically described. The endless and exquisite variety of form and colour presented by the plain and its boundary of distant mountains, together with the changing effects of weather and season on such a groundwork, would of themselves furnish ample subjects for admiration. But the influence of this natural beauty is vastly enhanced by the strange and solemn loneliness of a scene which living man seems to have almost utterly forsaken, leaving behind him only memories of a storied past which are awakened at every turn by roofless walls of long-abandoned farm-buildings, mouldering ruins of medieval towers, fragments of imperial aqueducts, decayed substructures of ancient villas and the grass-grown sites of ancient cities whose names are forever linked with the early struggles of Rome. European travel offers few more instructive experiences than may be gained by wandering at will over that rolling sward, carpeted with spring-flowers, but silent save for the song of the larks overhead and the rustle of the breeze among the weeds below; when the mountainous wall of the Sabine chain from Soracte round to the Alban Hills gleams under the soft Italian sky with the iridescence of an opal, and when the imagination, attuned to the human associations of the landscape, recalls with eager interest, some of the incidents in the marvellous succession of historical events that have been transacted here. If, besides being keenly alive to all the ordinary sources of attraction, the visitor can look below the surface, he may gain a vast increase to his interest in the ground by finding there intelligible memorials of prehistoric scenes, and learning from them by what slow steps the platform was framed on which Rome rose and flourished and fell. He will thus discover that, as befitted the city which was to rule the world, its birthplace was fashioned by the co-operation of the grandest forces in Nature; that, on the one hand, subterranean upheaval and stupendous volcanic activity combined to build up the plain and hills of the Campagna, and that on the other, the universal and ceaseless working of the subaërial agencies has carved it into that varied topography which is typified in the isolation of the Seven Hills of Rome and of the many crags and ridges that served as sites for the towns of Latium and Etruria.

Seen from the crest of the Vatican ridge, the Roman Campagna stretches as a plain from the base of the steep front of the Apennines to the coast of the Mediterranean--a distance of some thirty English miles. To the north it is bounded by the ridge of Soracte and the nearer heights of Bracciano and Tolfa. To the south it runs up to the base of the Alban Hills and sweeps between them and the sea onwards till it merges into the flat and pestilential Maremma. Even from such a commanding point of view, however, this apparent plain can be seen to be far from having an even surface. Not only does it slope upward and inland from the coast, until, where it abuts against the foot of the hills, it has reached heights of 600 or 800 feet, but when looked at more closely it presents a somewhat diversified topography. Though the heights and hollows never vary much from the general average level, they include not only smooth, grassy ridges but also low cliffs that run along the declivities, rising sometimes into craggy scarps; likewise narrow gullies and ravines with steep walls, as well as wide, open, smooth-sided valleys. The surface is for the most part clothed with pasture; yet the brown and yellow rock that forms most of the plain protrudes in many places, not only where it has been laid bare by natural causes, but where it has been artificially cut away or scooped into subterranean recesses.

Such a varied form of ground was eminently favourable for human settlement. The earliest races could find or make rock-shelters almost anywhere. The fertility of the soil afforded to their successors good pasturage and fields for tillage, while the hillocks, girt round with cliffs, and the flat-topped ridges, shelving precipitously to lower ground, offered excellent sites for fortification and defence. Owing to the porous nature of the ground, much of the rain sinks at once beneath the surface, instead of flowing off in brooks. Hence many of the valleys are usually dry, unless in wet seasons. But water can be obtained all over the district by sinking wells, and that this source of supply has been in use from a remote period and to an almost incredible extent, has been strikingly shown by the recent excavations beneath the pavements of the Roman Forum. It would be difficult to find anywhere a form of ground which shows better the influence of geological structure upon the early fortunes of a people.

With some portion of what has been written by Italian and other observers on this district, I have made myself acquainted, and having had the advantage of tracing on the ground the records of the successive stages through which the Campagna has come to be what it is, I propose in the following pages to give an outline of this prehistoric chronicle. I should like to attempt to present to the reader such a picture of the whole sequence of events as has vividly impressed itself on my own mind, avoiding, as far as may be practicable, technicalities and details. Three distinct successive phases can be recognised in this sequence. First came a time when the waves of the Mediterranean broke against the base of the steep front of the Apennines, and when all the low grounds around Rome, and for leagues to the north and south, lay sunk many fathoms deep. Next followed the chief period in the building up of the Campagna. A host of volcanoes rose along the sea-floor on the west side of Central Italy, when ashes, dust and stones were thrown out in such quantity and for so prolonged a time as to strew over the sea-bottom a mass of material several hundred feet thick. Partly from this accumulation and partly by an upheaval of the whole region of Italy, the sea-bottom with its volcanic cones was raised up as a strip of low land bordering the high grounds of the interior, and a few huge volcanoes were subsequently piled up to a height of several thousand feet. Lastly succeeded the epoch in which the volcanic platform, no longer increased by fresh eruptions, was carved by subaërial agencies into the topography which it presents to-day. Each of these three phases has had its history legibly graven in the rocky framework of the Campagna, and some of its memorials may be recognised even within the walls of Rome.

I. The records of the first period lie beneath the Seven Hills on the left bank of the Tiber, but rise high above the plain on the right bank, where they form the chain of heights that culminates in Monte Mario, 455 feet above the level of the Mediterranean. These records, forming the series known to geologists by the name of Pliocene, consist of a lower bluish-grey clay and an upper group of yellow sands and gravels, the whole being probably a good deal more than 450 feet thick. The clay (Plaisancian) has been found to extend, with a remarkable persistence of aspect and contents, from the north to the south of Italy. It has yielded several hundred species of mollusks and other organisms, which show it to be a thoroughly marine silt, deposited on the bottom of a sea of some little depth.

At the time of the deposition of this clay the mountainous backbone of the country had already undergone the greater part of that prolonged series of terrestrial disturbances whereby solid sheets of limestone were folded, crushed, ruptured and driven together into a series of parallel ridges, having a general trend from northwest to southeast, and forming the nucleus of what is now the chain of the Apennines. At the epoch when our story begins, however, this chain was still incomplete, and probably a good deal lower, as well as narrower, than subsequent upheaval has made it. Instead of forming, as it now does, the lofty axis of a broad peninsula, it then consisted of a series of parallel islands and islets, separated from each other by long and often narrow sounds or channels. In general appearance it must have resembled parts of the coast of Dalmatia on the opposite side of the Adriatic. Many of the more prominent mountains of the region stood then entirely surrounded by the sea. The Sabine Hills, for example, rose as an island, while Soracte formed another island farther west. A long strait ran northwards by Rocca Sinibalda and Rieti to Terni; another of narrower width stretched towards Perugia and formed then the estuary of the Tiber. All the Roman Campagna, together with the low grounds on both sides of the Apennines, was at that time submerged under the sea. The great band of volcanic heights and cones that extends from Aquapendente to the Bay of Naples had not yet come into existence, but over their site the waters of the Mediterranean lay many fathoms deep.

The climate of Europe had for ages been of so genial a character that sub-tropical types of life had long flourished both in the sea and on the land of this quarter of the globe. But in the period of geological history with which we are now concerned, a remarkable diminution of temperature was in progress all over the Northern hemisphere. As the warmth grew less, the distribution of plants and animals came to be seriously affected. Many southern forms were extirpated from districts which they had long inhabited, while in their place came migrations of northern species. This modification made itself felt both on terrestrial and marine life. Thus in the Atlantic Ocean a number of northern shells, which had pushed their way southward even as far as the coasts of the Spanish peninsula, were able to enter the Mediterranean when a connection was opened between that inland sea and the main ocean outside. It is interesting to note that among the shells introduced into the Mediterranean basin at this time were Astarte borealis, Buccinum groenlandicum, Cyprina islandica, Panopaea norvegica and others whose appellations sufficiently indicate the latitudes where they now find their chief home. On the land, too, such quadrupeds as the reindeer and the now extinct mammoth wandered from the plains of Lapland and Russia to the shores of Italy. Eventually when the refrigeration gave way to the return of more genial conditions, the northern invaders died out. They have no living descendants now in the south of Europe.

The grey clay which forms the lower division of the Roman Pliocene series is best seen on the right side of the Tiber where it forms the lower half of the ridge of the Vatican and Monte Mario, and where for more than five-and-twenty centuries it has supplied material for making the bricks of which ancient and modern Rome has been so largely constructed. The same clay has been found at lower levels on the opposite side of the river. On the flanks of the Pincian Hill, at the Piazza di Spagna, it was exposed about twelve years ago in some excavations connected with the adjustment of the aqueduct of the Aqua Vergine. Only a few feet below the crowded pavements of that busy thoroughfare lies the old sea-bottom with its abundant relics of marine life. In borings for water which have been made around Rome the same deposit has been ascertained to extend below the later volcanic formations of the Campagna. Thus at the Appia Antica fort, near the tomb of Cecilia Metella, the clay was entered at a depth of about 300 feet from the surface or eighty feet below the level of the sea. As the upper limit of the clay at Monte Mario lies about 200 feet above sea level and the distance from that outcrop to the fort in question is about six miles, it might be inferred that there is here evidence of a southeasterly dip of the deposit amounting to forty-six feet in a mile. But before any inference of this kind can be accepted, some considerations should be taken into account, of the highest interest and importance in relation to the early history both of the Campagna and of the Apennine chain.

Before dealing with these questions, however, let us complete the examination of the marine deposits of Monte Mario. In the valuable section disclosed on the slopes of that hill, the grey clay is seen to become sandy towards the top and to include seams of sand which rapidly increase in thickness, until, with their included layers of gravel, they form nearly the whole of the upper part of the ridge. These yellow sands, generally distinguished by the name of 'Astian,' have been traced, like the clay below them, along nearly the whole length of the Italian peninsula. The striking contrast which, in the nature of their material, they present to the clay, plainly points to a great alteration of the geography of the coasts at the time when they were deposited. The sea must have become rapidly shallower. Not improbably one of the uplifts now took place, whereby the land has been raised at intervals to its present height. The steepness of the descent of the mountains into the sea might not lead at once to much gain of land along the western coast; but instead of the grey mud that had previously accumulated in the deeper water, coarser sediment, brought down by numerous torrents from the hills, now spread out over the sea-bottom. Such a transition from the finest silt to gravel and sand could not fail to affect the distribution of the animals living along the coast-line. Accordingly, on comparing the fossils in the sands with those of the clay, we see that while some of the shells, especially the larger and more massive kinds, continued to flourish in abundance; others, which found their most congenial haunts in tranquil waters, were driven further out to sea.

As the Pliocene deposits so well displayed at Rome are known to 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.