A comparison of these vast crater-rings leads us to the conclusion that in the majority of cases, if not in every instance, they are composed almost entirely of volcanic tuff and dust. In the case of the more solidly-built composite volcanic cones, the volcanic forces, as we have seen, produce fissures in the mass, and along these fissures parasitic cones are thrown up, the tension of the mass of imprisoned vapours below the mountain being thus from time to time relieved. But in the case of a volcanic cone composed of loose fragmentary materials, such temporary relief is impossible. The cracks, as soon as they originate, will be filled up and choked by the falling in of materials from above and at their sides. In this way the eruptive action will be continually repressed, till at last the imprisoned vapours acquire such a high state of tension that the outburst, when it occurs, is of the most terrible character, and the whole central mass of the volcano is blown into the air. It may often seem surprising that the ejection of such vast masses of material from the centre of a volcanic cone does not effect more in the way of raising the height of the crater-walls. But it must be remembered that, in the case of craters of such vast area, the majority of the ejected materials must fall back again within its circumference. By repeated ejections these materials will at last be reduced to such an extreme state of comminution that they can be borne away by the winds, and spread over the country to the distance of hundreds or thousands of miles. After great volcanic outbursts enormous areas are thus found covered with fine volcanic dust to the depth of many inches or feet. [Illustration: Fig. 73.--Peak of Teneriffe in the Canary Islands (12,182 ft.), surrounded by great crater-rings.] Sometimes, as in the case of the Lago di Bracciano, the eruptive forces appear to have entirely exhausted themselves in the prodigious outburst by which the great crater was produced. But in other cases, as in that of the Lago di Bolsena, the eruptive action was resumed at a later date, and small tuff-cones were thrown up upon the floor of the crater; these now rise as islands above the surface of the lake. In other cases, again, the eruptive action was resumed after the formation of the great crater-ring, with such effect that bulky volcanic cones were built up in the midst of the crater-ring which surrounds them like a vast wall; examples of this are exhibited in the extinct volcanoes of Rocca Monfina and Monte Albano. Some of the grandest volcanoes of the globe, such as Teneriffe (fig. 73), the volcanoes of Mauritius and Bourbon (figs. 74 and 75), and many others that might be cited, are thus found to be surrounded by vast crater-rings. Vesuvius itself is surrounded by the crater-ring of Somma (fig. 76). [Illustration: Fig. 74.--The volcano of Bourbon, rising in the midst of a crater-ring four miles in diameter.]