[6] Whymper determined the height to be 20,498 feet; Reiss and Stübel make it 20,703 feet. Whymper thinks there may be a crater concealed beneath the dome of snow.--_Travels amongst the Great Andes of the Equator_, by Edward Whymper (1892). [7] Whymper states that there is a prevalent idea that Cotopaxi and a volcano called Sangai act as safety-valves to each other. Sangai reaches an elevation (according to Reiss and Stübel) of 17,464 feet, and sends intermittent jets of steam high into the air, spreading out into vast cumulus clouds, which float away southwards, and ultimately disappear.--_Ibid._, p. 73. CHAPTER III. LINES AND GROUPS OF ACTIVE VOLCANIC VENTS. The globe is girdled by a chain of volcanic mountains in a state of greater or less activity, which may perhaps be considered a girdle of safety for the whole world, through which the masses of molten matter in a state of high pressure beneath the crust find a way of escape; and thus the structure of the globe is preserved from even greater convulsions than those which from time to time take place at various points on its surface. This girdle is partly terrestrial, partly submarine; and commencing at Mount Erebus, near the Antarctic Pole, ranging through South Shetland Isle, Cape Horn, the Andes of South America, the Isthmus of Panama, then through Central America and Mexico, and the Rocky Mountains to Kamtschatka, the Aleutian Islands, the Kuriles, the Japanese, the Philippines, New Guinea, and New Zealand, reaches the Antarctic Circle by the Balleny Islands. This girdle sends off branches at several points. (See Map, p. 23.) [Illustration: Fig. 3.--Volcanic cone of Orizaba (Cittaltepeth), in Mexico, now extinct; the upper part snow-clad, and at its base forest vegetation; it reaches a height of 16,302 Parisian feet above the sea.--(After A. von Humboldt.)]