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).
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).
FIG. 126.—Sketch map of Etna, showing the individual surface lava streams (in black) and the tuff covered surface (stippled).]
=The caldera of composite cones.=—Because of the varied episodes in the history of composite cones, they lack the regular lines characteristic of the two simpler types. The larger number of the more important composite cones have been built up within an outer crater of relatively large diameter, the Somma cone or _caldera_, which surrounds them like a gigantic ruff or collar. This caldera is clearly in most cases at least the relic of an earlier explosive crater, after which successive eruptions of lesser violence have built a more sharply conical structure. This can only be interpreted to mean that most larger and long-active volcanoes have been born in the grandest throes of their life history, and that a larger or smaller lateral migration of the vent has been responsible for the partial destruction of the explosion crater. Upon Vesuvius we find the crescent-like rim of Monte Somma; on Etna it is the Val del Bove, etc. It is this caldera of composite cones which gave rise to the theory of the “elevation crater” of von Buch (see _ante_, p. 95, and Fig. 127).
