RISE OF MOLTEN ROCK TO THE EARTH'S SURFACE 125 



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 inter- 

 mediate between that of the 



other types. Their Vulcanian 

 eruptions of the convulsive 

 type are initiated by the forma- 

 tion of a rent or fissure upon 

 the mountain flanks at eleva- 

 tions 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 



FIG. 124. A row of parasitic cones raised 

 above a fissure which was opened upon 

 the flanks of Mount Etna during the 

 eruption of 1892 (after De Lorenzo). 



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). 



cones (Fig. 124). The lava of Vesuvius more frequently yields 

 bocchi at the place of outflow, whereas the flanks of Etna are 



1 Italian for mouth ; plural bocchi. 



