At three places in the old sea basin the subterranean fires burst forth. Near the north shore rose the great volcano of Rocca Monfina, which added itself to the Volscian Mountains, and heaping the products of its eruptions upon Mons Massicus,--once an island,--formed with this the northern boundary of the plain. Toward the middle the numerous small vents of the Phlegraean Fields threw up the low heights, to which the north shore of the Bay of Naples--Posilipo, Baiae, Misenum--is indebted for its incomparable beauty of landscape. Finally, near the south shore, at the intersection of the fissures, the massive cone of Vesuvius rose, in complete isolation--the only volcano on the continent of Europe still remaining active. Its base on the southwest is washed by the sea, while on the other sides a stretch of level country separates it from the mountains that hem in the plain. On the side opposite from the sea, however, Vesuvius comes so near to the mountains that we may well say that it divides the Campanian plain into two parts, of which the larger, on the northwest side, is drained by the Volturno; the small southeast section is the plain of the Sarno. [Illustration: Fig. 1.--Map of Ancient Campania.] The Sarno, like the Umbrian Clitumnus, has no upper course. At the foot of Mt. Taburno, bounding the plain on the northeast, are five copious springs that soon unite to form a stream. Since 1843 the river has been drawn off for purposes of irrigation into three channels, which are graded at different levels; the distribution of water thus assured makes this part of Campania one of the most fertile districts in Italy. In antiquity the Sarno must have been confined to a single channel; according to Strabo it was navigable for ships. In Roman times three cities shared in the possession of the Sarno plain. Furthest inland, facing the pass in the mountains that opens toward the Gulf of Salerno, lay Nuceria, now Nocera. On the seashore, where the coast road to Sorrento branches off toward the southwest, was Stabiae, now Castellammare. North of Stabiae, at the foot of Vesuvius, Pompeii stood, on an elevation overlooking the Sarno, formed by the end of a stream of lava that in some past age had flowed from Vesuvius down toward the sea. Pompeii thus united the advantages of an easily fortified hill town with those of a maritime city. "It lies," says Strabo, "on the Sarnus, which accommodates a traffic in both imports and exports; it is the seaport of Nola, Nuceria, and Acerrae." [Illustration: Fig. 2.--Vesuvius as seen from Naples.] A glance at the map will show how conveniently situated Pompeii was to serve as a seaport for Nola and Nuceria; but it seems hardly credible that the inhabitants of Acerrae, which lay much nearer Naples, should have preferred for their marine traffic the circuitous route around Vesuvius to the Sarno. However that may have been, Pompeii was beyond doubt the most important town in the Sarno plain.