Great Neapolitan Earthquake of 1857/Part I. Ch. XIX
CHAPTER XIX.
THE PHYSICAL AND GEOLOGICAL FEATURES OF THE COUNTRY SHAKEN.
Before proceeding to Part II. some remarks are required upon the general physical and geological features of the earthquake region of December, 1857, in order that the references as to their modifying effects, upon the directions, local variation of intensity, reflection, &c. of the shock, to be made in Part III. may be understood.
The notion commonly formed, from our books of geography and maps, of the physical configuration of the surface of Italy, is that of a long strip of land, separating into two at the south, and divided right down the midst of each strip, by the ridge of the Apennines, with a steep watershed to either shore.
This is but a very inadequate representation of the facts, and only to a limited extent true. Confining ourselves to the kingdom of Naples—i.e., starting on the north, with a line reaching from the mouth of the Tronto on the east, to that of the Tiber on the western shore; from Monte Pennino (in Roman territory) down through the summits of Monte Como and the Majella, to near Monte Acuto, south of Melfi—the highest ridges of the southern Apennine chain, are found following a wavy line, at about one-third the breadth of the peninsula, from its northern coast. Again, from Monte St. Angelo, just above Amalfi, to a point approaching the Adriatic coast, some miles south-east of Bari, a transverse ridge stretches nearly west to east and from sea to sea, and which bending southward, to the north of Taranto, continues with decreasing development, down into the extremity of Otranto.
Returning to Monte Acuto, the great central ridge is continued, in a direction almost due north and south, for nearly 150 miles, and then stretches in a waving line, down to the southern end of the Calabrian peninsula, where it culminates in Cocuzzo and Aspramonte.
At the north-western end of the first ridge, we have Monte Corno, nearly as high as Etna in Sicily, with several summits, between that and Acuto, of from 7,000 to 9,000 feet in height. In the transverse ridge, Acuto is the highest crest, probably; but Monte St. Angelo, in the little peninsula of Cape Campanello, terminates the western end, as a rampart to the Bay of Naples, at an elevation of 4,770 feet, the elevation gradually declining from Monte Acuto to the Adriatic.
Again, between Monte Acuto and Capo del Armi, at the toe of Calabria, we have Cocuzzo, 5,620 feet, and Aspramonte, variously stated at from 5,830 to 4,380 feet, The little peninsula of Gargano forms a small mountain system of its own, an elevated well-studded table land, of a lumpy, roundish form, with radiating stream channels, in which Monte Cairo is said to be the highest point, reaching 5,088 feet.
These ridges, in lines far from straight, and broken by many differences of elevation, are, indeed, the spine and ribs of Southern Italy; but our notions thus limited, convey no true idea of the physical features of the country. These ridges determine, the great forms and directions of the water-sheds, but by no means those of the vast tract of subordinate mountain ranges and culminations, by which these axial chains are surrounded and buttressed.
From the Tronto, to Gargano, the lateral mountains tend on the whole, to stretch parallel to the lines of the rivers, which fall with north-eastern courses into the Adriatic; and hence, the lines of mountain and valley, are generally transverse to the axial chain on this side.
On the opposite side, between the great axial chain and the transverse axis, from Naples to Monte Acuto, the great rivers, such as the Carigliano and the Volturno, take in tributaries from every point of the compass, and indicate, the extreme irregularity that prevails, in the alignment of the secondary ridges. This is also, to a less extent, true of the great trapezoidal area, between Salerno and Monte Acuto on the north, the gulfs of Salerno and Policastro on the west and south, and the southern continuation of the axial chain from Monte Acuto on the east. The southern branches, however, of the largest river within this boundary, the Salaris or Sole, have a nearly south to north course. Eastward of the axial chain of Monte Acuto, and over the whole province of Basilicata, the rivers all run, nearly parallel to each other, and in a direction almost exactly from N.W. to S. E. into the Gulf of Tarauto; but the directions of the secondary ridges are, on the whole, distinctly transverse to the river courses, which make their way through breaks or depressions, or wind round the teminals of the short and abrupt ridges. So that on the whole the mountainous country south of the transverse axis, and down to the parallel of 40°, at Policastro, may be viewed largely, as a surface furrowed in parallel ridges, running north and south with a trend westward, but twisted, broken through by gaps, and irregular in a high degree.
South of parallel 40°, to the extremity of Calabria Ultra, the lateral chains, tend to place themselves at right angles to the axial chain, except about the boundary separating the two Calabrias, between Cape Suvero, on the west, and Capes Alice and Colonne on the east coasts, where a mountain knot is formed by the intersection of a short but well-defined transverse axis, running east and west, and nearly parallel with the great transverse axis to the northward; the road over which at Petrania, between Cosenza and Nicastro, reaches an elevation of nearly 3,400 feet.
This transverse chain is, in fact, the dam, that absorbs the earthquake movements of Calabria Ultra, and prevents their full spread northwards, and vice versâ, just as the great transverse chain of Monte Acuto, partially stops the propagation northward, of the shocks from the Principatas and Basilicata. At the intersection of the transverse axis of Monte Acuto, with the north and south one, there is a great mountain knot, comprised between Laviano and Oppido, east and west, and Venosa and Potenza, north and south. Within this space, which presents some of the grandest scenery of the Apennines, Muro and Bella occupy almost the central position. The mountainous country thus described, extends over about one half of the entire surface, of the kingdom of Naples. The remainder consists of vast plains, (relatively at least) of two distinct sorts—one, the Photo Pl. 108.
Vincent Brooks, lith. London
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rolling, rounded, hilly country, constituting the vast grazing downs, of Capitansta and Basilicata, on which countless sheep and goats are pastured in winter and spring; the other the rich corn plains, level as the sea almost, of which the largest are in Otranto and Bari; next to which come those of the Terra di Lavoro, the plain of Pæstum, and of Calabria Ultra, from Rosarno to St. Euphemia (the scene of the great earthquake of 1784). All these great plains (piani) are on the seaboard, but almost every mountain valley of any magnitude, consists of a piano, almost perfectly level, from the sides of which, the mountains spring abruptly, as from a sea shore. The largest of these, is the Piano di Diano, in Principato Citeriore, the scene of some of the worst disasters of the earthquake of December, 1857, in early spring presenting, as do all these valley plains, characters of the richest and most enchanting country. The general aspect of the Val di Diano may be gathered from the Photogs. Nos. 108 (and 109 Coll. Roy. Soc.), being views of the town of Diano, from which the Vailone and Piano take their name; the other of St. Arsenio, on the west side of 'the same plain. The smaller valley bottoms, present the same characters upon a less scale—many are partially in forest. The mountain cincture of the piani, usually consists, of one or several sloping terraces of small elevation, having frequently the character, more or less perfect, of "parallel roads," tracing round the margins. Those in the piano of the Bay of St. Euphemia, have been described by Meissonier, 'Comptes Rendus for 1858,' and such terraces are observable around a large portion of the Val di Diano.
The piani are not always, or necessarily, strictly level plains, however; some slope, gently but continuously in one direction.
Of the geology of the kingdom of Naples very little is accurately known. Within the parallels of 40° to 42°, the following are the leading facts so far as I have observed them, and learned from the sketch map of Italian geology of Colligno. Probably the lowest and most ancient visible stratified rock is the jurassic limestone, which constitutes the central mass of the axial, and all the higher lateral chains.
Lithologically, it is usually in heavy and well-marked beds, the line of strike being very commonly in the general direction of the chain, and the beds tilted to a high angle, so that a very large proportion of the whole mountainous surface of the country, consists of highly inclined beds, running about north and south. There are, however, large exceptions to this: in the mountain knot, of Muro and Bella, amongst other places, for example, the beds, nearly vertical, often cross the lines of valley at right angles. Again, in the great range of La Scorza, or Monte Albano,—south of the Salaris, and between the Val di Diano and the Plain of Pæstum—the beds support a large elevated and nearly level table land, with an east and west strike, and inclined at various angles dipping to the south, and are piled up fully 3,000 feet above the valley of the Rio Negro. They seem to dip inwards, towards the centre of the table on top, so as to rampart it all round: it is the largest surface of mountain table land in the kingdom.
Everywhere this lower limestone presents truces, of immense disturbance and dislocation, and of enormous denudation.
Its colour is most commonly yellowish ash gray, and when most compact, it has quite a liassic aspect in hand specimens; it varies much in colour, however; red, purple, variegated, and nearly white, are to be found. In many places it presents metamorphic characters, and becomes for limited areas, flinty and hard.
A great band of this limestone, extends from around the Terra di Lavoro, southwards to the Gulf of Taranto, from thirty to forty miles wide on the west side; it winds about, forming the summits of all the hills that rise out of the level bed of tufa, that surrounds Naples, and then stretches away northward in a still wider band to the eastward, and a narrower one to the westward, sides of the peninsula. A large region from Barletta to Gioia, on the Adriatic coast, also consists of it. Within the first-mentioned band, and resting upon this limestone, are scattered immense deposits of a coarse calcareous breccia, consisting of rounded masses of various sizes, (sometimes, as north of Potenza, very large, reaching eight or ten feet in diameter,) and cemented together, with similar but softer material, in ill-defined heavy beds, usually much less inclined, than those of the limestone beneath. Whether this rock belongs to the cretaceous series, or to what other, I am unable to say: it occupies the bottom of many of the narrow valleys, and in one place on Collegno's map, appears to be assigned to the pliocene tertiaries, but probably in error.
Above the lower limestone, reposing upon it, laid against its highy-inclined beds, and often mixed with it, in perplexing confusion—we find the nummulitic and hippurite limestones of the cretaceous formation, always characterized to the eye, even far away, by the want of clear bedding, the more rounded outlines, of the lower mountains composed of it, and by its brighter colour. It is usually nearly white, often sufficiently hard and dense to work well, as a beautiful building material, capable of a good polish; but also passing insensibly, within a few miles, into a soft, sandy stuff, of little coherence, like a compound of English chalk, and fine white Dorsetshire sand, but still forming rocky eminences several hundred feet in height.
Upon this again appear to lie, chiefly in the bottoms and on the flanks of the valleys, beds of marls of various tints, of enormous thickness—600 or 700 feet in some places. These I presume to be the sub-Apennine marls of Collegno.
In the lowermost portion of these marls, beds of yellow and brown sandstone occur, here and there of great thickness; in some places, they are traversed by beds of indurated, highly ferruginous and magnetic, dark gray calcareous rock,—by beds of gypsum,—and in very many places, give evidence of metamorphism. The beds, usually soft and sectile, and acted on with immense rapidity by river erosion, being converted into masses of striped jasper, often of great beauty and extreme hardness.
In the neighbourhood of Potenza, there are large developments of indurated argillaceous slaty beds, of dark blue gray colour, which Collegno appears to refer to these sub-Apennine marls, but which appeared to me widely different, in lithological character at least, from those last referred to.
Above these marls, reposes, the usually great depth of alluvial clays, which constitute the valley bottoms.
The tops and flanks, of the upper and lower limestone mountains, are almost always nearly bare of soil; it has been all swept off from them and levelled under water in the valley bottoms, to form these rich plains of agriculture. The soil is commonly a dense tenacious brown or red brown loam, often free from stone for large areas, and composed of the detritus of the calcareous and argillaceous rocks. That combining the sand of the cretaceous rocks, and the clay of the marl beds, is of high fertility.
Where the fall is small, the rivers (as the Tanagro in the Val di Diano) run slowly, upon beds of these clays, and form for themselves a permanent bottom, pared by the angular fragments and boulders of limestone washed from the soil; but when the fall is rapid, as in the Rio Agri—a view in the valley of which, is given in Photog. No. 110 (Coll. Roy. Soc.), then erosion takes place upon a scale of great grandeur. The alluvial soil is cut through to the bottom; the marl beds follow it; and the river runs upon a bed of loose blocks of limestone, resting on the bare rock, at the bottom of a ravine or "nullah", with steep sloping banks, which are continually washing away and slipping in, and at a level often of 300 to 700 feet below the flat of the piano, or valley bottom, that spreads out above, for miles at either side. The rocks in the foreground of this photograph, are of the calcareous breccia described. The rivers themselves, fed by the heavy rains of the wet season, which fall with something of the regularity and violence of the tropical rainy seasons, by the wet of winter, and by the sudden melting off, of the vast masses of snow that fall on the higher ridges, (and at elevations exceeding 2,000 feet above the sea, lie accumulating during winter,) are of a torrential character, and almost all flow thick, and turbid with brown alluvial sediment, to the sea.
North of the great transverse ridge about Atella, we begin to encounter the great tufa deposit, of the ancient volcanic region around Monte Vulture, and extending northward to beyond Melfi, where it is evident that denudation, has levelled and spread out, over the limestone, prodigious quantities of the tufas and decomposed lavas, at a period probably long anterior to the ejection, of the earliest of the tufas of the Vesuvian tract, and where torrent and rain erosion, present features of the largest, and most instructive character.
Details, either of that, or of the Vesuvian volcanic tract, are beside our present purpose, however, except to remark, that in both, the limestone laps in under the superficial volcanic products to an immense distance; indeed, in the Terra di Lavoro—as we travel, for example, from Naples to Caserta and Capua—it is obvious that the level plain of tufa over which we pass, out of which the limestone mountains rise sheer and abrupt on all sides, has been run in and levelled between them, and has traced out by its surface, the contour, of every sinuosity of their narrow winding valleys, when all were under water, and that the limestone of the hills, in reality underlies almost the whole of the great tufa bed. Earthquake vibrations, therefore, penetrate both these volcanic regions, through the intervention of the harder and more elastic limestone beneath, the tufas being thus shaken like plastic clay in a saucer, just as the great alluvial beds in the more southern valleys, are shaken by the vibrations, primarily propagated through the limestone surrounding them. Of the two, probably the tufa is the worse material, for the easy propagation of earth wave.
The connection between one valley and another, at a not greatly different level, is not unfrequently through a gorge or serrated cleft, fractured through the rock, in the bottom of which a torrent mars, while the road or mule tract is over the shoulder above; such are Campostrina, between the valleys of the Calore and Tanagro—and that of the Gioija, at Muro. In general, however, the valley bottoms are at different levels, and are reached by passing over low shoulders between them, the streams finding vent in sinuous, but not very deep ravines.
The lower mountain ranges, of the cretaceous limestone are not very steep, though the slopes can nowhere, except in the rolling plain country, be called gentle. The higher ridges, are always steep, and frequently characterized by a shaggy bristling grandeur, of crest and outline, greater than one is prepared to find in mountains, of a formation so recent.
The fall of the rivers generally, throughout the kingdom is rapid, the mean breadth of the land, not giving an average length of bed, of much above a hundred miles, in which the average fall is probably above 3,000 feet; and even the great rivers, have their volume so augmented in winter, that on reaching the seaboard plains, their velocity is still very great, on debouching into the sea. Thus, the Salaris, after having traversed the plain of Pæstum, retains a mid-surface, winter velocity, of about eight feet per second.
The towns in the earthquake region to which this Report refers, are nearly all built, as stated, upon rocky eminences, within the mountainous region; in some cases, however, within it, they are (or were) built upon the alluvial clay deposit, on the level of their respective Piani.
In the Capitanatas, Basilicatas, and Bari, there are many towns—some of them large and important, that stand upon the plain, or on elevated knolls upon it—none, with the exception of some on the coasts, such as Amalfi, are found nestling into valley bottoms of small size, as in colder climates, and such towns are usually of extreme antiquity.
In the Appendix (No. 1) to this part, a translation is given, of that portion of the report of Professors Palmieri and Scacchi, on the Melfi earthquake of 1851, which comprises their general account of the geology of Southern Italy. Although still leaving much to be desired, it is the best sketch I have been able to meet with.
A few monographic memoirs on Neapolitan geology exist, such as Elie de Beaumont's, and others, on the Lignite formation of Calabria ('Comptes Rendus,' 1858).
Collegno's 'Elemente di Geologia Practica e Theoretica,' Torino, 1847, contains a good deal of information as to the geology of Central and Northern Italy, with sections of the Northern Apennines; and so also does Pilla's 'Saggio Comparativo die Terreni d'Italia,' and his 'Trattato di Geologia' (Pisa, 1847-51); but for Southern Italy I have met with no corresponding information.