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very small angle, so as to occupy wide tracts. The abrupt cones consist chiefly of the more viscous lavas which have coagulated immediately round the focus of eruption. The depressed cones, on the other hand, are built up of the more liquid lavas, which flow out rapidly, and reach relatively greater distances from the focus of eruption. Not infrequently the cones formed by the outwelling of very viscid lava show no crater—the lava coagulates around and above the vent. In other cases the top of the abrupt dome-shaped cone is blown out by escaping gases, and a crater-shaped hollow is thus formed. The volcanoes of the Hawaiian Islands present the grandest examples of the eruption of liquid lavas. Hawaii itself is made up of five volcanic mountains, ranging in height from some 4000 feet up to nearly 14,000 feet. All these are depressed cones. Mauna Loa (r3.075 feet), for example, has a broad, flattened summit, sunk in which is the great cauldron-like crater, some 3^ miles in length by \\ in width, and 800 feet deep. From the lip of this crater the mountain slopes outwards at an angle of 30, which gradually increases to 70, the diameter of the mountain at its base being not less than 30-40 miles.
But composite cones, built up of lava and loose ejecta, are of far more common occurrence than cones composed of lava alone. To this class belong most of the better-known volcanic mountains. Their general characters have already been outlined in the short description we have given of a typical volcano. It remains to be noted that many composite volcanoes show a cone-in-cone structure. During some paroxysmal eruption the upper portion of a volcano may be destroyed—shattered and blown into fragments. Or, as a result of long-continued activity, the mountain becomes partially eviscerated, and the upper part of the cone eventually caves in, and a vast cauldron is formed, after which a protracted period of repose may ensue. When the volcanic forces again come into action a younger cone, or it may be several such cones, gradually grow up within the walls of the old crater. The younger cones may rise in the middle of the great hollow, or they may be eccentric, as in the case of Vesuvius, which has grown up upon the rim of the large crater of Monte Somma.
Of comparatively little importance from our present point of view are mud-volcanoes. Some of these owe their origin to the escape of steam and hot water through disintegrated and decomposed volcanic materials, either tuff or lava, or both. They are usually of inconsiderable size, many being mere monticles, while others may exceed ioo feet in height. They show craters atop, and have the general form of tuff-cones. Their origin is obvious. The mud is simply flicked out as it bubbles and sputters, and the material thus accumulates round the margins of the cauldron, until a cone is gradually built up. Other so-called mud-volcanoes have really no connection with true volcanic action, but owe their origin to the continuous or spasmodic escape of various gases, such as marsh-gas, carbonic acid, sulphuretted hydrogen, etc. The mud of which they are chiefly composed is saline, and usually cold. Now and again, however, stones and ddbris may be ejected. These "volcanoes" (variously known as salses, air-volcanoes, and maccalubas) usually form groups of conical hillocks like miniature volcanic cones. Here also may be noted, in passing, the sinter-cones formed by those eruptive fountains of hot water and steam which are known under the general term of geysers. When the geyser erupts on level, or approximately level, ground, the sinter tends to assume a dome-shape; when, on the other hand, the springs escape upon a slope, the silicious deposits are not infrequently arranged in successive terraces.
All the volcanic eruptions to which we have been referring have proceeded from isolated foci. Some volcanoes are quite solitary, others occur in irregular groups, while yet others appear at intervals along a given line. These last are obviously connected with great rectilinear or curved dislocations of the earth■s crust; not a few of the former, however, apparently indicate the sites of funnels or pipes which have been simply blasted out by the escape of elastic vapours. There is yet another class of volcanic eruptions which have played a prominent part in geological history, although they are not now so common. These are the fissure or massive eruptions, of which the best examples at the present time are furnished by Iceland. Lavas;, usual-) of the more liquid kind, well out sometimes simultaneously from more or less numerous vents situated upon lines of fracture, or from the lips of the fissures themselves. Usually such floods and deluges of lava are not accompanied by the discharge of any fragmental materials. Sheet after sheet of molten rock has been discharged in this manner so as to completely bury former land-surfaces, filling up valleys, submerging hills, and eventually building up great plains and plateaux of accumulation. The basalt-plains of Western North America, which occupy a larger area than France and Great Britain, are the products of such massive eruptions, the lavas reaching an average thickness of 2000 feet. The older basalts of Iceland, the Faroe Islands, the Inner Hebrides, and Antrim are the relics of similar vast fissure eruptions. And of like origin are the basaltic plateaux of Abyssinia and the Deccan in India. The volcanic phenomena of the Hawaiian Islands have also much in common with fissure or massive eruptions.
The forms assumed by the materials accumulated at the surface by subterranean action are all more or less distinctive and characteristic. Hills, mountains, plains, and plateaux, which owe their origin directly to volcanic activity, agree in this respect, that their internal structure and external form coincide. Even the most perfectly preserved examples of volcanic accumulation, however, are seldom without some trace of the modifying influence of epigene action. The shape of a volcanic cone, for example, during its period of growth is subject to modification. Wind affects the distribution of loose ejecta, while rain and torrents sweep down materials, and gullies and ravines furrow the slopes of the mountain. The ravages thus caused continue to be repaired from time to time so long as the volcano remains active. But when its fires die out and the mountain is given over to the undisputed power of the epigene agents, the work of degradation and decay proceeds apace. The rate of this inevitable destruction is influenced by many circumstances—by the nature and structure of the materials, for example, and the character of the climate. Thus, cones built up of loose scoriae are likely to endure for a longer time than cones composed of fine tuff and hardened mud. Rain falling upon the former is simply absorbed, and consequently no torrents scour and eat their way into the flanks of the cones, while tuff- and mud-cones are more or less rapidly washed down and degraded. Again, a composite volcanic mountain of complicated structure, the product of several closely associated vents, buttressed and braced by great pipes of crystalline rock and an abundant series of lamer and smaller dikes, is better able to withstand the assaults of epigene agents than a cone of simpler build. Sooner or later, however, even the strongest volcanic mountain must succumb. Constantly eaten into, sapped, and undermined, it will eventually be levelled.