COMPOSITE CONES 



871 



by lava (Figs. 234. 235). The lava sometimes extended through 

 these fissures, building up secondary cones or monticules on the flank 

 of the main cone, as in the case just cited. Fissuring of the cone is 

 of common occurrence in volcanoes, the lava of many of them rarely 

 or never overflowing the crater, but finding an outlet at a lower 

 level through the side of the volcano. If parasitic cones (monti- 

 cules) are built up over such a fissure these may remain the site of 

 eruption for a long period, but sooner or later they are likely to 

 become extinct, and then they may be buried by later flows and 

 ejectamenta. Cinder cones, which are relatively weak structures, 

 will be breached if a subsequent lava stream is poured out, and this 



Fig. 234. Diagram illustrating the 

 formation of parasitic cones 

 (monticules) along lines of fis- 

 sures formed in the flanks of a 

 great volcano. (After Judd.) 



Fig. 235. Basaltic dikes projecting 

 from stratified scoria or tuff in 

 the walls of the Val del Bove, 

 ^tna. 



will issue from their sides. (Fig. 227.) Large composite cones 

 may be breached by explosive eruptions and the shifting of the 

 center of the eruption. A new cone may be built up within the 

 breached outer rim of an original large caldera, as in the case of 

 V'^esuvius, which was built within the breached rim of the extinct 

 Monte Somma. The displacement of the eruptive point may be 

 a gradual one, when a series of adjoining cones will result, all but 

 the youngest being breached on the side toward the direction of 

 migration of the cones. Examples of such consecutively breached 

 cones are found in the volcanic region of central France (Mont 

 Dore Province), and elsewhere. Many variations and combinations 

 occur, and the student is referred for the details of these phenom- 

 ena to the numerous general treatises, some of which are listed at 

 the end of the chapter. 



Compound volcanoes, such as Vesuvius, have alternating periods 

 of light (or Strombolian) and violent (or Vulcanian) activity. Dur- 



