VOLCANOES. 



95 



more visible by erosion, intersect the strata. 3. After a time, when 

 the mountain has grown to considerable height, the force necessary 

 to raise liquid lava to the lip of the crater becomes so great that 

 it breaks in preference through the fissured sides of the mountain. 

 The secondary craters thus formed immediately commence to make 

 accumulations around themselves, and thus form secondary cones (Fig. 

 80, 6*'), or monticules, about the base and on the sides of the primary 



o 



4T-* 



Section of Volcano, showing Monticules. 



cone. If a secondary cone becomes extinct, it is finally buried (Fig. 

 80, c") in the layers of the primary cone. 4. Finally, in volcanoes of 

 the explosive type, during great eruptions the whole top of the mount- 

 ain is often blown off, and in volcanoes of the quieter type is melted 

 and falls in — in either case forming an immense crater, within which, by 

 subsequent eruptions, another smaller cone of eruption is built up, and 

 in this latter often a still smaller cone is again built. This cone-witliin- 

 cone structure is well illustrated by the present condition, and still 

 better by the history, of Vesuvius. Vesuvius is a double-peaked 



Fig. 81.— Section of Vesuvius and Mount Somma. 



mountain, with a deep, semicircular valley between the peaks. The 

 present active cone of Vesuvius is encircled by a rampart, very high 

 on one side, and called Mount Somma, but traceable to some degree all 

 around, and having the same structure as Vesuvius itself. This ram- 

 part is the remains of a great crater, many miles in diameter. Fig. 81 

 is an ideal section through Mount Somma (S), and Vesuvius ( V). S' 

 is the almost obliterated remains of the old crater on the other side. 

 This is further and beautifully illustrated by the history of this mount- 

 ain, which records the repeated destruction and rebuilding of these 

 cones within cones. Fig. 82 is an outline of Vesuvius as it existed in 

 1756 : * Sis Mount Somma. 



Scrope, Philosophical Magazine, vol. xiv, p. 139. 



