The south-eastern part of the island consists of four crater-rings, one half of each of Which has been successively destroyed, through the shifting of the centre of eruption towards the north-west, along the great line of fissure shown in the general map (fig. 81). The last formed of these four crater-rings is the one which is now most complete, and culminates in Monte Saraceno (1581 ft.), _a_ in the plan, the highest point in the island. The older crater-rings have been in part removed by the inroads of the waters of the Mediterranean on the shores of the island. In the centre of the great crater, _b_, which we have just described, rises the present active cone of Vulcano, 1,266 feet high, and having a crater, _c_, about 600 yards in diameter and more than 500 feet in depth. From this cone, a great stream of obsidian, _e_, flowed in the year 1775, and a small crater, _d_, the Fossa Anticha, has been opened in the side of the cone. The continuation of the same line of fissure is indicated by a ruined tuff-cone, _f_, known as the Faraglione, and the three scoria-cones of Vulcanello, _g, h_, which have been thrown up so close to one another as to have their lower portions merged in one common mass, as shown in fig. 86. [Sidenote: SYSTEMS OF VOLCANIC FISSURES.] Even in volcanoes of the largest dimensions we sometimes find proofs of the centre of eruption having shifted along the line of fissure. Lyell showed that such a change in the position of the central axis of the volcano had taken place in Etna, and the same phenomenon is exhibited in the clearest manner' by some of the ancient volcanoes of the Inner Hebrides, which have been dissected by the denuding forces. [Illustration: Fig. 86.--Vulcanello, with its three craters. _a._ The most recently-formed and perfect crater, _b_ and _c_. Older craters, the walls of which have been partly removed by denudation, _e._ Lava-currents proceeding from _b_. The section exposed in the cliff at _d_ is represented in fig. 35, p. 116.] In the case of the Lipari Islands, the fissures along which the volcanic mountains have been thrown up radiate from a common centre, and a similar arrangement can be traced in many volcanic regions, especially those in which a great central volcano has existed. In other cases, however, as in the Campi Phlegræi, the volcanic vents appear to be formed along lines which assume a parallel arrangement, and this doubtless marks the relative position of the original fissures produced in the earth's crust when these volcanoes were formed. In some other cases we find evidences of the existence of a principal fissure from the sides of which smaller cracks originated. These three kinds of arrangements of volcano-producing fissures are equally well illustrated when we study those denuded districts, in which, as we have seen, the ground-plans of volcanic structures are revealed to our view.