Most volcanoes are built, like Vesuvius, both of lava flows and of tuffs, and sections show that the structure of the cone consists of outward-dipping, alternating layers of lava, scoria, and ashes.
Fig. 231. Section of Vesuvius
V, Vesuvius; S, Somma, a mountainous rampart half encircling Vesuvius, and like it built of outward-dipping sheets of tuff and lava; a, crystalline rocks; b, marine strata; c, tuffs containing seashells. Which is the older mountain, Vesuvius or Somma? Of what is Somma a remnant? Draw a diagram showing its original outline. Suggest what processes may have brought it to its present form. What record do you find of the earliest volcanic activity? What do you infer as to the beginnings of the volcano?
From time to time the cone is rent by the violence of explosions and by the weight of the column of lava in the pipe. The fissures are filled with lava and some discharge on the sides of the mountain, building parasitic cones, while all form dikes, which strengthen the pile with ribs of hard rock and make it more difficult to rend.
Fig. 232. Crater Lake, Oregon
How wide and deep is the basin which holds the lake? The mountain walls which enclose it are made of outward-dipping sheets of lava. Draw a diagram restoring the volcano of which they are the remnant. No volcanic fragments of the same nature as the materials of which the volcano is built are found about the region. What theory of the destruction of the cone does this fact favor? W´, Wizard Island, is a cinder cone. When was it built?