The more intimate earthquake-phenomena are more uniform. Sometimes there is nothing else felt than a trembling or gentle motion of the surface, without producing any injury. In severe earthquakes, the almost invariable succession of phenomena is first a trembling, then a severe shock, or a succession of shocks, and then a trembling, gradually becoming insensible. The violent shocks are instantaneous, and very few in number, sometimes only one, usually not more than three or lour. In the intervals between these, smaller shocks or tremblings take place. The severe shocks do the mis chief. At the point or line of greatest disturbance, the shock has a distinctly vertical direction, coming from below upwards. As we leave this point, the direction of the motion becomes more and more horizontal, gradually also decreasing in intensity until it becomes insensible. This progressive movement is produced by an earth-wave or true undulation of the solid crust of the earth. The whole mass of the area is not moved at once, but only the wave-crest. In the case of the earthquake at Lisbon, the progress of the wave was roughly calculated; it was shown to have had a very great velocity, and to have lasted only for an instant at any one spot. The area affected on this occasion was very extensive. The shock was felt on the one side as far as the southern shores of Finland, and on the other it reached 'beyond the St. Lawrence in Canada, and was observed in some of the West India islands—an area of no less than 7,500,000 sq. miles. The force required to move this must have been enormous, for, suppose the thickness of the earth's crust moved to have been no more than 20 m., then 150,000,000 cubic m. of solid matter was moved. The influence of this earth-wave is communicated to the sea, when the E. is near the shore, or on the bed of the ocean. The sea swells, and slightly retires from the beach, and then a great wave rolls in upon the shore. At the Lisbon K, this wave rose to a height of 60 ft. at Cadiz. It carries with it sea-spoil, scattering it over the surface of the earth, far beyond the ordinary reach of the sea.
Of the various theories as to the nature of earthquakes, we can only refer to the most important. All theorists are agreed as to the connection between volcanoes and earth quakes; that they are produced by the same subterranean agency. The existence of molten matter in the interior of the earth, is the starting-point in all except the chemical theory propounded by Davy, which, though in the end abandoned by him, still finds supporters. When he discovered the metallic bases of the earths and alkalies, he threw out the idea that those metals might abound in an unoxidized state in the subterranean regions, to which water must occasionally penetrate. When this occurred, gaseous matter would be set free, sufficient to produce the E., the metals would combine with
the oxygen of the water, and heat enough would be evolved to melt the surrounding rocks.
,Mr. Mallet, in an elaborate report on the subject presented to the British association, proposed an ingenious theory. He assumes that volcanoes, and the centers of E. dis turbances, are near the sea, or other large supplies of water; and he says that when an irruption of igneous matter takes place beneath the sea-bottom, the first action must be to open up large fissures in its rocky material, or to lift and. remove its incoherent portions, such as sand, mud, gravel, etc. The water on meeting the heated surfaces assumes the spheroidal state; while in this condition, the intestine motion may be great, but little steam is generated; but no sooner have the surfaces cooled, than the water comes into close contact with them, and a vast volume of steam is evolved explosively, and blown off into the deep and cold water of the sea, where it is condensed, and thus a blow"of the most tremendous sort is given at the volcanic focus, and being transferred outwardly in all directions, is transmitted as the E. shock, The surfaces of the ignited material, however, now cooled down below the point at which steam can be generated rapidly, merely keep up a gentle ebullition, which is transmitted as the trembling after the shock, On the surfaces again becoming heated by conduction from the molten mass, the various phases are again repeated. This he considers the chief cause of earthquakes, but he supposes they may also be due to the evolution of steam through fissures, and its irregular and per :Whim condensation under pressure of sea water; or to great fractures and dislocations in, the rocky, crust, suddenly produced by pressure acting in( it from beneath, or in.aiif dthei'direetion:. ' (Pk The old assumption that the earth consists of a molten fluid core with a cooled and hardened rind floating upon it, is inconsistent with the rigidity that astronomers have proved the earth to possess. But although the earth must be mainly solid, it is yet believed to be of a honey-combed structure, and that the cavities contain in many places lakes of molten rock, between which and the surface volcanoes are orifices of communi cation. Into these cavities, water sinking down through crevices from the ocean or the land must be constantly finding its way; and the steam thus generated exerts such enormous pressure as to force the molten matter to the surface, itself mingling and escaping along with it. When a mass of water is suddenly precipitated iuto a hot cavern, the explosion of steam will cause an earthquake concussion, and where there is no vent, may be sufficient to convulse and rend the superincumbent strata.