_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. Great catastrophes are recorded in the shape of some volcanoes which consist of a circular rim perhaps miles in diameter, inclosing a vast crater or a caldera within which small cones may rise. We may infer that at some time the top of the mountain has been blown off, or has collapsed and been engulfed because some reservoir beneath had been emptied by long-continued eruptions (Fig. 230). The cone-building stage may be said to continue until eruptions of lava and fragmental materials cease altogether. Sooner or later the volcanic forces shift or die away, and no further eruptions add to the pile or replace its losses by erosion during periods of repose. Gases however are still emitted, and, as sulphur vapors are conspicuous among them, such vents are called _solfataras_. Mount Hood, in Oregon, is an example of a volcano sunk to this stage. From a steaming rift on its side there rise sulphurous fumes which, half a mile down the wind, will tarnish a silver coin. [Illustration: 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?] =Geysers and hot springs.= The hot springs of volcanic regions are among the last vestiges of volcanic heat. Periodically eruptive boiling springs are termed geysers. In each of the geyser regions of the earth--the Yellowstone National Park, Iceland, and New Zealand--the ground water of the locality is supposed to be heated by ancient lavas that, because of the poor conductivity of the rock, still remain hot beneath the surface. [Illustration: Fig. 233. Old Faithful Geyser in Eruption, Yellowstone National Park]