Old Rome: A Handbook to the Ruins of the City and the Campagna

by Robert Burn

The only places within the actual walls of Rome where these tertiary marine strata are to be found, are the Vatican and the Janiculum. At the base of the Capitoline, in the subterranean vaults of the Ospitale della Consolazione, under the volcanic rock which forms the upper part of the hill, Brocchi found a stratum of calcareous rock and clay, which he affirms to be of marine origin, and to resemble the limestone of the Apennines.

Volcanic formation.

The second group of strata found on the site of Rome is one which is not confined to the neighbourhood of Rome, but is most extensively spread over the whole of the Campagna, the district of Campania, and a considerable part of southern Italy. The great mass of the Capitoline, Palatine, Aventine, Esquiline, Cælian, Viminal, Quirinal, and Pincian Hills, is composed of this formation. Geologists give it the general name of tufa, and divide it into two kinds, the stony and the granular. It is distinguished from lava by not having flowed in a liquid state from the volcano, and is a mechanical conglomerate of scoriæ, ashes, and other volcanic products which have been carried to some distance from the crater of eruption, and then consolidated by some chemical re-arrangement of their constituent elements. The harder kind of tufa, the tufa litoide, is a reddish brown, or tawny stone, with orange-coloured spots. These spots are embedded fragments of scoriaceous lava. It is hard enough to be used as a building stone, and has been quarried largely under the Aventine Hill near S. Saba, at Monte Verde, on the southern end of the Janiculum, and at other places near Rome, as at Torre Pignatara on the Via Labicana, at the bridge over the Anio, on the Via Nomentana, and at the Tarpeian rock.

This tufaceous stone presents itself in very thick banks, traversed by long vertical and oblique fissures, probably produced by the contraction of the mass on passing from a humid and soft to a dry and hard state. The Arch of the Cloaca Maxima, near S. Giorgio in Velabro, is built of this stone, and the inner part of the substruction of the so-called tabularium on the Capitol. Portions of the Servian wall were also built of it, and many stones which were taken from this wall are to be seen at the present day in the walls of Aurelian, near the gate of S. Lorenzo; and others have been laid bare by the railway excavations in the Servian Agger. Brick-shaped masses of it are found in the ambulacra of the Theatre of Marcellus, so that the use of it must not be restricted to the earliest times of Roman architecture. In fact, several buildings of the Middle Ages in or near Rome consist of this stone, as may be seen at the Fortress Gaetani, near the Tomb of Cæcilia Metella, and in the large tower at the side of the palace of the Senator.

Freshwater formation.

Fresh-water formations cover the bottoms of all the valleys in the district of Rome and in the whole of the Campus Martius, and ascend to a considerable height on the flanks of the hills and into the Campagna. They consist chiefly of sand, clay, gravel, and the stone called travertine, and of tufa beds which have been disturbed and then re-deposited. This re-deposited tufa has been the subject of some controversy. It was at one time thought to indicate that the lower tufa was also a fresh-water deposit, since it is sometimes found overlying the fresh-water formations. But no doubt now remains that it must have been formed by a re-arrangement in fresh water of previously deposited marine tufa beds. The water of the Tiber, at the time when these fluviatile formations took place, stood at such a height as to leave deposits upon the intermontium of the Capitol, and as high as the Church of S. Isidoro on the Pincian, and it must have partially removed and shifted the previously existing light and porous volcanic soil of the sea-bottom. Even the top of the Pincian was covered by this fresh water; for modules of calcareous matter, such as are deposited in fresh water alone, were found in digging the excavations for the fountain on the public promenade.

The surface of the broad river which then existed, seems, in fact, to have been at from 130 to 140 feet above the present surface level of the Tiber, and its water must have been more surcharged with alluvium, derived from sources with which the present river is no longer connected.

Among the fluviatile deposits, argillaceous marl beds now play an important part. They intercept the water as it descends from the hills, and impede its descent to the river, thus furnishing supplies to the wells in Rome, but rendering the soil less dry and healthy. The greater portion of these strata consist of a mixture of sand and clay. The ridge between the Campo Vaccino and the Coliseum, on which the Arch of Titus stands, is formed almost entirely of these mixed strata of clay and sand. To prove the fresh-water origin of these deposits, we need only refer to the modules of travertine and the shells of lacustrine animals which they contain. Such species of fresh-water shell-fish could not live in turbid and rapid water like that of the Tiber as it now is, and we must therefore conclude from their presence that the waters of the Tiber valley where such fossils are found were once in a semi-stagnant state. That there was also a period of violent movement during the prevalence of this lacustrine era is testified by the quantities of matter brought from a distance and accumulated at considerable altitudes, and by the size of the pebbles and boulders which have been rolled along by the stream. But before a more accurate investigation of facts shall have been made, it will be impossible to distinguish these two periods of stagnation and rapid movement from each other.

Tiber water.

The river water has no longer the power which it once possessed of depositing the travertine which we find lying in thick beds upon the slopes of some of the hills of Rome, and from which the larger ruins are all built. This travertine is formed from carbonate of lime which the waters take up as they pass through the soil containing it. In order to give the water the power of holding this carbonate of lime in solution, a certain quantity of carbonic acid gas must be present in it. When by means of the rapid movement of the water or from other causes this gas becomes disengaged, it leaves the carbonate of lime behind in the shape of a hard stony deposit. This natural process of petrifaction is familiar to all who have seen the Falls of the Anio at Tivoli, and the way in which the artificial canals of running water in that neighbourhood are choked by limestone concretions, and it may be seen in all vessels made use of to boil water which is impregnated with lime. The more violent the agitation of the water the more rapid is the disengagement of the carbonic acid gas, and the consequent settlement of the lime. This process is accompanied, in most places where it can be seen, by the presence of sulphuretted hydrogen, which produces a white colour in the water by depositing the sediment called gesso by the Italians. Hence an explanation of the ancient name of Albula given to the Tiber is easy. In the period when the Tiber had the power of depositing travertine, its waters were much more strongly impregnated not only with carbonate of lime, but also with gesso, which gave a white tinge to the water as it now does to the sulphureous waters near Tivoli. The same colour was characteristic of “the white Nar, with its sulphureous stream,” Virgil’s description of the chief stream of the central Apennines.