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so violent that the solid crust is fractured, one portion thrown up and another thrown down, the sea-bed uplifted into dry land, and the dry land submerged beneath the waters. The phenomena that accompany earthquake convulsions are extremely varied. Occasionally they are preceded by an unusual stillness and sultriness of the atmosphere; low hollow rumblings, more audible than felt; and great restlessness and terror among birds and mammals, as if the instincts of these were keener than human perception. At other times there is no premonition, but all at once a few smart concussions, passing away in a certain direction, or not unfrequently spreading from a central point in diminishing intensity. On other occasions, however, the momentary concussions return after a short pause with increased vehemence, and then there is a perceptible undulation of the earth's crust, as if it were passing away from beneath the feet of the spectator—an uplift, a shock, a series of giddy shocks, and the work of destruction. Though continuing at most for a few seconds, these violent shocks generally result in extensive fracturing of the rocky crust. Yawning rents and fissures, gaseous discharges, bursting forth of new springs, absorption of streams, changing the course of rivers, elevation of the sea-bed, submerging of the dry land, and the conversion of populous cities into masses of ruin and rubbish, are the not unfrequent but destructive effects of the earthquake. At times too, as the water in a vessel that has been agitated and then brought suddenly to rest strikes forcibly over its margin, so the sea, its floor having been shaken, is frequently thrown into violent waves (earthquake - waves), which rush forward against the land to the height of 40, 50, or 60 feet, and sweep everything into destruction before them. The wave that rolled in upon the coasts of Portugal after the great Lisbon earthquake in 1755 was estimated at 60 feet high,
and the succession of such waves (three in number) that desolated the town of Simoda (Japan) in 1854 were little inferior in violence and dimensions.
On thewhole, the effects of the earthquake are much more disastrous than those of the volcano. The discharges of the one, being at considerable altitude, are chiefly felt for a few miles round its crater or in long narrow streams down its sides; but the other convulses for leagues, and this at all levels and alike over land and over sea. "The earthquake," says Humboldt, "appears to men as something omnipresent and unlimited. From the eruption of a crater, from a stream of lava running towards our dwellings, it appears possible to escape; but in an earthquake, whichever way flight is directed, the fugitive believes himself on the brink of destruction!" The two, however, are usually in close connection; and in centres of igneous activity, when the volcano begins to discharge, the convulsions of the earthquake cease, or at all events lose much of their intensity. The one acts as a sort of safetyvalve to the other, and this necessarily so if we regard them as both arising from the same deep-seated source of igneous intensity. This connection was long ago noticed by Dr Hutton, who quaintly but somewhat bitterly remarks: "A volcano is not made on purpose to frighten superstitious people into fits of piety and devotion, nor to overwhelm devoted cities with destruction. A volcano should be considered as a spiracle to the subterranean furnace, in order to prevent the unnecessary elevation of land and fatal effects of earthquakes. And we may rest assured that they, in general, wisely answer the end of their intention, without being in themselves an end, for which nature had exerted such amazing power and excellent contrivance."
earth's interior upon its external crust consists in those slow movements by which certain portions of the land are stage by stage elevated above the waters, and other portions as gradually depressed beneath them. To this manifestation we may apply the name of Crust-Motion, as indicating a slow and long - continued movement of the solid crust in the region where it occurs, in contradistinction to the volcano and earthquake, whose operations are sudden and convulsive. Whether these gradual crustmovements result from the same igneous forces that give rise to the earthquake and volcano is a matter open to question, but in the present state of our knowledge we can perceive no other adequate cause, and aie therefore compelled to associate them with the same pervading vulcanicity. Along the coasts of our own islands, and indeed along the coasts of every other country, the attentive observer will perceive at various levels above the present sea-beach several shelves or terraces, which meet the eye like reaches of former shore-line. On closer inspection, their parallelism, the sand and gravel of which they are composed, the shells, bones, and other marine exuviae which they contain, prove incontestably that the sea formerly stood at these levels, and that the land to this extent has been successively elevated above the waters. Whether such elevations took place suddenly or by slow degrees it is often impossible to tell, but it is readily seen from the old beaches and the cliffs that guard them that the sea must have long stood at their successive levels. If the movement takes place suddenly—like that which during the present century elevated the coast of Chile to the height of eight or ten feet, or that which depressed the Eun of Cutch, or that which more recently uplifted part of the north island of New Zealand to the height of six feet —it is generally ascribed to earthquake convulsion; but if it occurs by slow stages it is regarded as crust-motion, the proximate cause of which is at present unknown.
As notable instances of this crust-motion we may point to the shores of Scandinavia, which have been long known to be rising at a slow and equable rate ; to the raised beaches of Spitzbergen, as described by Mr Lamont and other recent visitors; to the ancient shore-lines of Siberia, as amply illustrated by Von Wrangell; the numerous terraces of uprise noticed by Dr Kane and other Arctic voyagers on the coasts of Greenland and the Arctic islands; the terraced shores of Patagonia, long since observed by Mr Darwin; as well as to the lass distinct, because more ancient, shore-lines that encircle our own islands and the opposite coasts of France and the Spanish peninsula. As with these uprises, so also with several depressions that have been noticed, though these from their nature are generally less perceptible. Such tracts of subsidence have been observed along the west coast of Greenland, the southern coasts of the United States, and generally in the basin of the South Pacific. Whatever the nature of these uprises or depressions—whether at the rate of four or five feet a century, as in the case of Scandinavia, or with greater or less rapidity; whether recent, like those of Spitzbergen and Greenland, or ancient, like those of our own country—they all belong to the same class of phenomena, and are evidently the result of some great but unknown law. Physicists have attempted an explanation, some attributing the phenomena to oscillations of the hypothetical molten interior, and others to secular expansions and contractions of portions of the crust, as arising from changes in the axis of rotation and centre of gravity. In either case the oscillation of a few thousand feet is insignificant as compared with the diameter of 'the globe; and as elevation in one region seems to be counterbalanced by subsidence in another, the general relations of our planet may
be regarded as standing for ages unaffected by the amount of its superficial changes.
Insignificant, however, as may be the effect of such oscillations upon the general relations of the earth, they are all-important to the climate, and consequently to the flora and fauna of the region in which they occur. A few hundred feet of elevation or depression in the higher latitudes of the north is tantamount to a loss of several degrees of annual temperature; and we can readily conceive what would be the effect of another thousand feet of uprise on the existing flora and fauna of Siberia, Greenland, or the Arctic islands of America. In fine, it requires no great stretch of the imagination to conceive to what extent the climatology of the globe may be influenced by a system of extensive elevations and depressions of its surface; how the cold and warm currents of the ocean might be diverted; how one region might be elevated so as to be permanently enveloped in snow and ice, while another in the same latitude might lie at so low a level as to enjoy the amenities of a temperate climate. On the whole, whatever may be the origin of these slow and gradual crust-motions, we behold in them a system by which the distribution of sea and land is changed, by which climate is modified, and consequently by which the plant-life and animal-life of our planet is materially affected.
Such are the three great manifestations of vulcanism— the volcano, the earthquake, and the gradual crust-motion; and though their origin be obscure, the human mind seldom rests satisfied with mere description, but must attempt a solution of cause and origin. There are two principal hypotheses that have been advanced to account for vulcanic phenomena, and which may be respectively termed the mechanical and the chemical. By the former, the whole