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. [Sidenote: Freshwater formation.]