The slow draining away of the water some time after the earthquake shows that the sea bottom has sunk and the water rushes in from all sides to fill up the depression; the currents meet at the centre and raise a ridge where there was formerly a depression and the ridge then collapses and sends ashore the first great wave. After this has swept the shore the sea slowly withdraws again to fill up the depression, leav ing bare the land as before and after the same interval again returns and sweeps the coast. At Arica in 1868 and at Iquique in 1877 it is said that the sea continued to oscillate for nearly two days before it finally quieted down and meanwhile the waves were propagated around the world.
In seismic sea waves of the second class, where the water rises suddenly without recession from the shore, the cause is an upheaval of the ocean bed, which also lifts the water resting upon it. The result is the sudden appearance of a great wave. This class of waves appears to be less frequent than the preceding class and therefore is not so famous as those in which great distances from the origin, waves of the first class, in being propagated round the world, resemble those of the second class; so that to classify sea waves intelligently a careful inves tigation must be made of the place of origin of the disturbance.
A quite different theory of earthquakes, which includes also the cause of volcanoes and the mode of mountain formation, has recently been very fully developed by Prof. T. J. J. See. This is in brief, that under the enormous pressures existing at the bottoms of the ocean beds a portion of the water is forced down ward to a considerable depth within the earth's crust; here, becoming diffused among the molten rocks and having its explosive power greatly increased through superheating, it penetrates under the lands along the coast lines where it may elevate mountain ranges parallel to the coast, giving rise in its explosive pressure to faults, volcanic outbreaks, and, as a consequence of each of these, to earthquakes. Admitting
the possibility of water penetrating the crust in this way, this theory explains the situation of the great mountain ranges of the earth, the general absence of seismic disturbances beneath the interior of continents (since here the seep age of the water would be less), the observed fact that the density of mountain masses is often small and many other associated phenomena. Consult 'The Cause of Earthquakes, Mountain Formation and Kindred Phenomena Connected With the Physics of the Earth,' (Proceedings of the American Philosophical Society, 1906).
Dutton, 'Earthquakes in the Light of the New Seismology' (London 1904) and numerous memoirs in the (Transactions> of various learned Societies and Government Surveys; Humboldt, ; Judd, 'Vol canoes' ; Lyell, 'Principles of Geology' (12th ed.) ; Milne, 'Earthquakes> (1903), and 'Seismology' ; Russell, 'Volcanoes of North America' and numerous other works on Geology and related subjects; Hobbs, 'Earthquakes, an Introduction to Seismic Geology> (London 19138); Suess, 'Face of the Earth. Papers by Milne and others in the Philosophical Transac tions and Proceedings of the Royal Society, the papers of See above referred to, and the Re ports of the British Association; Publications of the Seismology Society of Japan; BeitrizgThe zur Geophysik.