When the height of the cone has grown to a certain extent, it becomes more easy for the volcanic energies below to rend the sides of the cone than to raise the molten materials to its summit. In this way lateral or parasitic cones are thrown up on the flanks of the volcanic mountain, the mass being alternately elevated and strengthened by the ejections from the summit and sides respectively. When the volcanic energies no longer suffice to raise the fluid materials to the summit, nor to rend the sides of the volcano, fissures with small cones may be formed in the plains around the great central volcano. At last, however, this energy diminishes so far that rock materials can no longer be forced to the surface, the fissures become sealed up by consolidating lava, and the volcanic cones fall into a condition of extinction and decay. The existence of heated materials at no great depth from the surface is indicated by the outburst of gases and vapours, the formation of geysers, mud-volcanoes, and ordinary thermal springs. But as the underlying rocks cool down, the issuing jets of gas and vapour lose their high temperature and diminish in quantity, the geysers and mud-volcanoes become extinct, and the thermal springs lose their peculiar character or disappear, and thus all manifestations of the igneous energies in the district gradually die away. [Sidenote: DURATION OF VOLCANIC CYCLES.] Such a cycle of changes probably requires many hundreds of thousands, or even many millions, of years for its accomplishment; but by the study of volcanoes in every stage of their growth and decline we are able to reconstruct even the minutest details of their history. CHAPTER VIII. THE DISTRIBUTION OF VOLCANOES UPON THE SURFACE OF THE GLOBE. It is not by any means an easy task to frame an estimate of the number of volcanoes in the world. Volcanoes, as we have seen, vary greatly in their dimensions--from vast mountain masses, rising to a height of nearly 25,000 feet above the sea-level, to mere molehills; the smaller ones being in many cases subsidiary to larger, and constituting either parasitic cones on their flanks, or 'puys' around their bases. Volcanoes likewise exhibit every possible stage of development and decay: while some are in a state of chronic active eruption, others are reduced to the condition of solfataras, and others again have fallen into a more or less complete state of ruin through the action of denuding forces.