From the supposed centre of volcanic action just suggested, it may be traced northward through the districts of Macomer, Bonorva, Giavesu, Keremule, with the hillock on which Ardara stands, and Codrongianus, to its termination in the cliffs of Lungo Sardo. But its most salient feature is the detached group of mountains on the western coast between Macomer and Orestano, which are entirely volcanic. This group has the name of “Monte del Marghine,” in the small map prefixed to Captain Smyth's survey, but I do not find that or any other distinct name attached to it in La Marmora's large “Carta dell'Isola.” The village of St. Lussurgiu is literally built in a crater connected with this group, as is also that of Cuglieri. The highest point, Monte Articu, the summit of Monte Ferro, entirely volcanic, rises 3442 feet above the Mediterranean, and the Trebia Lada, 2723 feet high, is one of the three basaltic feet forming the _Trebina_, or Tripod, on the summit of Monte Arcuentu, a mountain between Orestano and Ales formed of horizontal layers of basalt. Further south at Nurri, closely approaching the primitive chain, are two hills, called “pizzè-ogheddu,” and “pizzè ogu mannu,” or peaks of the little and great eye, which were certainly ignivomous mouths, and the peasants believe that they still have a subterraneous communication. A volcanic stream has run from them over a calcareous tract, forming an elevated plain nearly 1600 feet above the level of the sea, called, “_Sa giara e Serri_.” It overlooks Gergei, and is covered with oaks and cork trees, while the northern side of its declivity affords rich pasture. North-west from this place is the “_Giara di Gestori_,” of similar formation, proceeding from a crater at Ales, but strewed with numerous square masses of stone—principally fragments of obsidian, and trachytic and cellular lava—so as to resemble a city in ruins. At Monastir there is a distinct double crater, now well wooded; and a bridge constructed of fine red trap, with the bold outline of the neighbourhood, render the entrance to the village by the Strada Reale singularly picturesque. The volcanic current, flowing westward from Monastir by Siliqua and Massargiu, again approached the coast towards the southern extremity of Sardinia, extending across the deep gulf of Palmas to the islands of S. Pietro and S. Antonio, which are entirely composed of trachytic rocks. Their bold escarpments arrested our attention on approaching the coast, near Cape Teulada, in one of our excursions to Sardinia.