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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.
In regions of extinct volcanoes we may study every stage in the process of demolition. Isolated cones and groups of cones crumble away, until all the lavas and tuffs ejected from the old vents may have disappeared, and the only evidence of former volcanic action that may remain are the basal portions of the dikes that proceeded from the foci, and the solid cores with which the latter were finally plugged up. (See Fig. 75.) As these cores usually consist of more
Fig. 75. View Of Neces = Cores Of Old Volcanoes. (Powell.)
durable materials than the rocks they pierce, they tend to form somewhat abrupt conical hills. It goes without saying that such extreme cases of denudation are met with only in regions where volcanic action has for a long time been extinct. Excellent exam
pies on a relatively small scale are furnished by the so-called "Necks" of Scotland, of which the accompanying section (Fig. 76) shows the general phenomena. Similar structures occur in many parts of Europe and North America.
Fig. 76. Section Of Highly Denuded Volcano. Minto Hill,
A■, throat or neck of volcano plugged up with ejectamenta, angular and subangular stones, grit, dust, etc.; St Silurian rocks; D, Old Red Sandstone strata.
Frequently the products of great volcanic eruptions of vast geological antiquity have been largely preserved, owing to their subsequent burial under sedimentary accumulations. Many of the hill-ranges of Central Scotland, for example, are built up of lavas and tuffs. These are the relics of volcanoes which came into existence in Palaeozoic times, and after erupting molten and fragmental materials for longer or shorter periods, eventually died out, becoming submerged and covered with sedimentary accumulations to depths of several thousand feet. Subsequent elevation of the region brought these sediments under the operation of the agents of erosion, and in time great thicknesses were removed, so that ultimately the ancient volcanic rocks were again laid bare and in their turn exposed to denudation. But if the lat