Plains of lava, called “_giare_” by the natives, are often found reposing on the large tracts of recent formation, such as those of Sardara, Ploaghe, and other places; and considerable extents of trap and pitchstone are frequently met with on limestone strata, while others, tending fast to decomposition, are incorporated with an earth formed of comminuted lava. Vestiges of craters, though generally ill defined, still exist in the vicinity of Osilo, Florinas, Keremule, St. Lussurgiu, Monastir, &c. Some of these are considered, from their less broken and conical shape, and from the surrounding country consisting of fine red ashes, slaggy lava, scoria, obsidian, and indurated pozzolana, with hills of porphyritic trap,—all lying over tertiary rock,—to have been of a much more recent formation than the others, which in form present a lengthened straggling appearance, and in composition resemble those of Auvergne.
The tertiary formation lies on the west side of the principal granitic chain, and, besides forming the Campidano and the bases on which the volcanic substances rest, constitutes the hills of Cagliari, Sassari, and Sorso. The tertiary limestone seldom ranges more than 1313 feet above the level of the sea, though at Isili and some other places it is 1542 feet high. La Marmora considers it analogous to the upper tertiary formations found in the south of France, central and southern Italy, Sicily, Malta, the Balearic Islands, and Africa. The plains generally consist of a deep alluvial silt, interspersed with shingly patches, containing boulder stones. Such is the valley of the Liscia, occupying nearly the whole surface from sea to sea towards the northern extremity of the island. This, it may be recollected, we crossed north of the Limbara. Then succeeds the series of _Campi_ or _Campidani_, properly so called. We have already spoken of the vast plain of Ozieri, terminating in the south-west with its minor branches, the Campi di Mela, St. Lazarus, and Giavesu, to which it spreads transversely from the Gulf of Terranova, on the eastern coast. The bottom of this gulf forms one of the finest harbours in the island, with some trade, but the town of that name is a wretched place, remarkable for its insalubrity and the truculent character of the inhabitants.