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is resolved into an aboriginal igneous condition of the earth's mass, on which, after the lapse of ages, a cooled and rocky crust has been formed over a molten interior. oscillations in this molten interior, to its reactions upon the crust, to the cavernous structure of the crust, and to chinks and fissures that admit the percolation of water down to the incandescent mass, are ascribed the tremors and convulsions of the earthquake and the sudden explosions of the volcano. By the latter hypothesis it is presumed that the solid crust contains abundance of metallic elements, such as potassium, sodium, calcium, magnesium, and the like; and that the percolating waters coming in contact with these produce instantaneous combinations, which result in uncontrollable manifestations of heat, and the conversion of these metals into their oxides-potash, soda, lime, magnesia, &c.-which enter largely into the composition of the rock-matter ejected at the surface. Such is a brief and general view of the two leading hypotheses that have been advanced to account for vulcanic phenoThe adherents of the former question the presence of these metallic elements in such abundance as to produce such gigantic results, and point to the universality of volcanic action and the uniformity of its products as evidences of its arising from the same great interior source. The adherents of the latter contend for a system of action and reaction, without which the globe would gradually lose its supposed interior heat, and become so cool that in process of time volcanic action would cease-a result incompatible with the maintenance of a diversified and habitable surface. According to the mechanical theory, say they, the interior heat must be gradually declining, and must finally come to an end; but according to the chemical, there is a round of incessant action and reaction, a system of compensation and endurance, which accords with the other appointments

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of the universe. This is not the place to do more than merely allude to these contending views, our object in these sketches being rather to explain what is known than to discuss what is questionable.

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But whatever be the origin of volcanic force, we see it abundantly manifested in various regions of the globehere in isolated centres like those of the Mediterranean and Iceland, there in linear directions like the Andes, and occasionally over wide areas like the Indian and Chinese Archipelagoes. At the present moment there are between three and four hundred active volcanoes (with as many more in a dormant or semi-extinct state), chiefly fringing, as it were, the Pacific, or scattered over its surface. latter instance they appear in insular centres, as in the Sandwich and other groups; in the former they occur in great lines, as in the Andean, Mexican, and Columbian Mountains, the Aleutian Islands, Kamtschatka, Japan, the Philippine Islands, and the Indian Archipelago. Compared with these the display of volcanic energy in other regions is insignificant; while over immense tracts like the north of Asia and Europe, and the Atlantic slopes of both Americas, the internal forces have been still and stationary for ages. We know from the mountain-ranges of these regions that the forces once were there; we see their effects in the primary granitic mountains, in the secondary hills of basalt and greenstone, and in the tertiary domes of trachytic lava;* but of the law that has regulated this shift

* It may be of use to the general reader to mention that the principal rocks in recent volcanic hills are-lavas of various aspect and compactness; tufas, or consolidated cindery matters; pumice, or light vesicular lava; obsidians, of glassy texture; and trachytes, or granular-crystalline masses: and that their differences depend partly on chemical composition, but chiefly on the rapidity with which they have been cooledrapid cooling producing a compact glassy texture, and slow cooling the reverse. The rocks of the secondary hills, on the other hand, though originally consisting of the same volcanic ejections, are now converted

ing from area to area, and now restricted it to its present centres, we know next to nothing, and are merely on the threshold of the inquiry.

Little, however, as we know either of the cause or of the course of vulcanism, we can readily perceive its function, and behold in it one of the great means by which the crust of the globe is held in equilibrium, and by which the diversity and variety of its surface is maintained. Were there no adequate force acting from within, the powers of waste and degradation from without would in time reduce the surface to one dreary monotony of level, incompatible with that diversity of condition and of life which appears to be one of the great aims of creation. But just as the meteoric and aqueous forces wear and waste from without, so the vulcanic renovate and upheave from within; and thus the rocky crust is held in perfect equipoise, and its surface diversified by all that irregularity of hill and valley, of table-land and plain, which is indispensable to variety in the plant-life and animal-life by which it is adorned. Wherever large expanses of the earth's surface, like the prairies and pampas of America, are characterised by sameness of condition, there is a consequent want of variety in their vegetable and animal existences; but as the great design of Creation seems to be variety in space as well as variety in time, this uniformity of surface is incessantly broken up by the operations of the earthquake and volcano. Locally disastrous as may be the throes of the one or the discharges of the other, we thus behold in each a necessary part of the world's mechanism, and powerful in proportion to the work it has to accomplish.

into crystalline greenstones, basalts, felstones, the softer trap-tuffs, and amygdaloids, or those whose original vesicles have got filled with almondshaped infiltrations of lime-spar, agate, and other minerals. In the older mountains the conversions are still more complete, and highly-crystalline granites, syenites, and porphyries are the prevailing compounds,

It has been frequently discussed, and with some is still a question, whether this power of internal vulcanism be steadfast or declining, and whether it did not manifest itself with greater intensity during the earlier geological periods? Locally we may perceive that it has ceased in some areas, and in others seems gradually declining; but at the same time we behold it breaking forth in new areas, and on a survey of the whole world, see no reason to conIclude that it is now either less extensive in its distribution or diminishing in its intensity. The Andes, through whose extreme length the earthquake and volcano are ceaselessly active, are as gigantic as the Himalaya, where they have long since ceased to exist; the Mexican Cordilleras broader and loftier than the Alps; the Alps more imposing than the older Pyrenees; and the Pyrenees as decided in their character as the primitive ranges of Scandinavia. The cincture of volcanic action that girdles the Pacific (in the Andes, Californian Sierras, Aleutian Isles, Kamtschatka, Japan, Philippine Islands, the Indian Archipelago, and New Zealand, to say nothing of the groups that stud its bosom) is as gigantic as any axis or area geology has revealed,*

* In absence of a map the following arrangement may convey to the reader a general idea of the disposition of these volcanic lines and centres. -1. Along the borders of the Pacific:-Throughout the entire length of the Andes from Tierra del Fuego northwards; in Central America; Mexico; Oregon; the Aleutian Islands; Kamtschatkan peninsula; the Kuriles; Japan group; Philippines; East India Islands; New Guinea; east coast of Australia and New Zealand. 2. Over the Pacific:--In the Sandwich Islands; Friendly Islands; Fejees; Santa Cruz group; New Hebrides; and the Ladrones. 3. Over the seas that lie between the northern and southern continents and adjacent regions:-West India Islands; the Mediterranean and its borders; the southern borders of the Caspian and eastward; and the East Indian Archipelago as lying between Asia and Australia. 4. In the Indian Ocean :- Bourbon and the Mauritius; Comoro group; and Madagascar. 5. In the Atlantic:-St Helena; the Cape Verdes; Canaries; Madeira; Azores; and Iceland. The interiors of the great continental masses both in the Old and New Worlds are still and quiescent.

while the individual discharges and irruptions are unexcelled by those of any former period: and in corroboration of this we need only point to the lava-streams of Etna, from twenty to forty miles in length; to those of Iceland, full fifty miles in length, by twelve to fifteen in width; or to those of the Sandwich Islands, nearly seventy in length, and of varying width and thickness, according to the nature of the declivity down which they have flowed. On the whole, there seems no ground for the supposition that vulcanism is now either less extensive or less energetic than in former ages. All that geology perceives is, that it has shifted from area to area; that the old mountains where it once reigned are long since cold and silent; that the secondary hills bear but few traces of its presence; and that now it upheaves along other lines and convulses from newer centres.

That in vulcanism, as in other cosmical forces, there must be a law determining both its time and mode of operation is sufficiently obvious; but in the mean time that law lies altogether beyond the indications of science, and we must content ourselves with mere descriptions of phenomena and explanations of function. What the cause of volcanic energy? what the periodicity of its discharges? what the times and directions of earthquake convulsions? and what the law which regulates the shifting of vulcanism from centres that were once disturbed to newer areas? are questions to which science can give no satisfactory answer, and generations may pass before even the way to a solution is indicated. What we want is, first, a more minute investigation of the products of volcanoes, that it may lead to a knowledge of the decompositions taking place within the earth's interior; second, observations as to the connection between earthquake throes and volcanic discharges; third, a record of the times and directions of earthquake

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