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VULCANISM—ITS NATURE AND FUNCTION.
DEFINITION OF THE TERM—MODES OF MANIFESTATION—VOLCANOES, THEIR VARIOUS ASPECTS, CHARACTERS, PRODUCTS, AND FUNCTIONS —EARTHQUAKES, THEIR MODES OF ACTION AND EFFECTS—CONNECTION WITH VOLCANOES—CRUST-MOTIONS, THEIR CHARACTER AND RESULTS—RECENT EXAMPLES—THEORIES OF VULCANISM OR INTERNAL FIRE-ACTION—ITS SHIFTINGS FROM AREA TO AREA— APPARENT FUNCTIONS OF—POWER AND' PERMANENCE OF, AS A NATURAL FORCE—LAW BY WHICH REGULATED—NECESSARY PORTION OF WORLD-MECHANISM.
Whatever be the nature and origin of the thermal forces that operate within the crust of our earth—whether deepseated or near the surface—whether arising from chemical actions or dependent on some primordial condition—it is convenient to arrange them under one general term, and that of Vuleanism or Vulcanicity, suggested by Humboldt in his 'Cosmos,' seems by far the most comprehensive and appropriate. In this way, not only the volcano proper, but the earthquake, hot-springs, gas-springs, mudsprings, and all kindred phenomena, are brought under one category; and "it is really advantageous," as remarked by the great German philosopher, "to avoid the separation of that which is causally connected, and differs only in the strength of the manifestation of force, and the complication of physical processes." Here, then, we give to the constantly active reaction of the interior of the earth upon its external crust or surface the name of Vuleanism; and the object of the present Sketch is to describe the operations of this internal heat, to explain what is known of its origin, and to define its apparent function in the economy of nature. To render a subject intelligible it is necessary to be methodical; and we shall therefore, without any assumption of technical exactitude, arrange all the thermal phenomena of our globe under the three great heads of Volcanoes, Earthquakes, and Crust-Motions.
Every one is less or more acquainted with the aspect and nature of a Volcano or burning-mountain. Whether he has seen one or not, his readings and hearings lead him to associate with his idea a mountain of a conical form, having a crater or orifice of eruption at top, and from which at intervals are emitted clouds of vapour and flame, showers of dust and ashes, and streams of lava or molten rock-matter. The outline may not be strictly conical, yet such is the form which it usually assumes; there may be more craters or orifices than one, some being central or near the top, and others lateral or placed along the sides; and the substances discharged may be very heterogeneous—now highlyheated steam and sulphurous vapours, now dust and cindery matters called scoria;, now fragments of rock (lapilli and volcanic bombs), and anon wellings-out of lava, sometimes extremely fluid, and at others slaggy and cindery. Notwithstanding such local and periodic differences, there is, on the whole, a great similarity in aspect and operation between all volcanoes, which leads to the belief that they belong to one brotherhood, and to the same system of causation. It is usual to speak of lava-cones, tufa-cones, cinder-cones, and mixed cones, according as a mountain is chiefly composed of one or other of these substances, or of a mixture of all of them; but, as might be anticipated, the mixed cones are by far the most prevalent, and the distinction is mainly valuable in assisting the observer to determine the composition of distant or inaccessible volcanoes. Thus the slope of a lava-cone is very gentle—from 3 to 10 degrees; that of a tufa-cone from 15 to 30 degrees; of a cinder-cone from 35 to 45 degrees; and that of a mixed cone usually gentle beneath, but topped with a steep peak of loose and scoriaceous materials. Again, some are strictly sub-aerial—that is, take place on dryland; and others sub-aqueous—that is, operate under the waters, or but rarely manifest themselves at the surface; yet both seem to act in a similar way, and to discharge similar products. Further, some are ceaselessly active; others become active only at long intervals, and are said to be dormant; while others have been so long dormant and shown no symptoms of activity that they are regarded as extinct. Between the existing and the extinct there is every grade of activity, just as among the extinct there is every degree of antiquity. We are thus led from the active craters of Etna and Vesuvius back to the extinct cones of Central France and the Ehine, which, in their crateriform domes and rugged lava-streams, still retain the aspect of volcanoes; from these back through the Apennines and Alps; from the Alps to the Pyrenees; and from the Pyrenees to the Scandinavian and older mountainranges, which have all had the same origin, though now their craters and domes are obliterated, and their outlines have undergone a thousand modifications from those denuding agencies of air and water which have operated upon them during untold ages. And here the reader cannot be too strongly impressed with the fact that the profiles of all our existing Mil-ranges are more the results of waste and denudation from without, than of upheaval and accumulation from within. When a mountain is first presented to us, the natural idea is, no doubt, that of upheaval and accumulation; but a little reflection will soon correct the misconception. All the older lands have been repeatedly under the sea, and have suffered at each depression and reelevation the denuding effects of wave and tidal action. Even since their latest re-elevation, the rains, frosts, and rivers have been incessantly wearing, wasting, and eroding —so much so, that the older mountain-ranges are but the merest skeletons of what they once were. Vulcanic agency may block out, as it were, the contours and profiles of the land; but the meteoric and aqueous agents—the frosts, rains, rivers, and waves—are the busy chisellers that are for ever conferring upon it its latest features—the latest, but never the last.
In looking upon the more ancient hills, as well as upon existing volcanoes, a question naturally arises—Are these elevations chiefly upliftings of the earth's solid crust, or are they accumulations of igneous matter that have been discharged from its interior 1 In other words, are mountains and mountain-ranges mainly produced by upheavals or swellings-up of the earth's rocky crust, or have they been accumulated on the surface by repeated discharges of volcanic matter? Much controversy has existed on this point, and many arguments adduced on both sides; but the truth seems to be, that the forces from within have acted in both ways—partly by elevation of the stratified crust, but chiefly by the accumulation of erupted materials. In this view every mountain and mountain-range becomes a matter of slow and gradual growth, every shower of ashes and stream of lava adding to the bulk of the isolated cone, and every new cone adding another link to the mountain-chain. We have no exact measure of this slow and gradual accumulation, but judging from the small amount that has been added to Etna and Vesuvius during the historical period, many of the existing volcanoes must be of vast antiquity; and when we carry our retrospect back through the extinct volcanic hills to the ancient mountain-ranges, the mind is altogether unable to grasp the cycles that must have elapsed since their formation.
Presuming, then, that volcanic hills are chiefly masses of accumulation and not of upheaval, the repeated eruptions that take place must necessarily fracture and derange the continuity of the surrounding district, and thus every igneous centre is marked by such accompaniments as hotsprings, boiling mud-springs, discharges of sulphurous gases, and the like, better known perhaps as the suffioni and solfataras of Italy, and the salses, hornitos, and Tiervideros of Mexico and South America. Such minor discharges are the normal accompaniments of all active volcanoes, and long after activity has ceased they form the residual phenomena, and indicate by their declining force and numbers the distance both in time and place of the fiery forces that once operated below. No doubt springs of considerable temperature may exist in districts long since quiescent, and now far removed from volcanic activity (those of Bath and the Pyrenees, for example) ; but the monticules thrown up by mud-volcanoes and escapes of heated and sulphurous vapours generally mark either the proximity of igneous activity or the comparative recentness of its manifestations in the area. The whole are merely indications of the same thermal agency—that internal fireforce which Humboldt has so appropriately included under the name of Vulcanism or Vulcanicity.
The next great manifestation of vulcanism is the Earthquake, a distinction made in scientific as well as in everyday language; for though the earthquake is generally the close concomitant of the volcano, yet its throes may be felt in districts where no volcano has existed for ages. This motion, as the name implies, is a quaking or trembling of the earth—varying from the gentlest tremor to agitations