Quite in contrast with the weak cinder cone is the lava dome with its rock walls and relatively flat slopes. Considered as a retaining wall for lava it is much the strongest type of volcanic mountain, and it is likely that the hydrostatic pressure of the lava within the crater would seldom suffice to rupture the walls, were it not that the molten rock first fuses its way into old stream tunnels buried under the mountain slopes (see _ante_, p. 112). Composite cones have a strength as retaining walls for lava which is intermediate between that of the other types. Their Vulcanian eruptions of the convulsive type are initiated by the formation of a rent or fissure upon the mountain flanks at elevations well above the base, the opening of the fissure being generally accompanied by a local earthquake of greater or less violence. From one or more such fissures the lava issues usually with sufficient violence at the place of outflow to build up over it either an enlarged type of driblet cone, referred to as a “mouth”, or _bocca_[1] (Fig. 123), or one or more cinder cones which from their position upon the flanks of the larger volcano are referred to as _parasitic cones_ (Fig. 124). The lava of Vesuvius more frequently yields _bocchi_ at the place of outflow, whereas the flanks of Etna are pimpled with great numbers of parasitic cinder cones, each the monument to some earlier eruption (Fig. 125). [Illustration: FIG. 125.—View looking toward the summit of Etna from a position upon the southern flank near the village of Nicolosi. The two breached parasitic cones seen behind this village are the Monti Rossi which were thrown up in 1669 and from which flowed the lava which overran Catania (after a photograph by Sommer).] It is generally the case that a single eruption makes but a relatively small contribution to the bulk of the mountain. From each new cone or _bocca_ there proceeds a stream of lava spread in a relatively narrow stream extending down the slopes (Fig. 126). [Illustration: FIG. 126.—Sketch map of Etna, showing the individual surface lava streams (in black) and the tuff covered surface (stippled).]