As a general result of these detailed observations, Captain Dutton states that there is a remarkable coincidence in the phenomena with those indicated by the theory of wave-motion as the proper one for an elastic, nearly homogeneous, solid medium, composed of such materials as we know to constitute the rocks of the outer portions of the earth; but on the other hand he states that nothing has been disclosed which seems to bring us any nearer to the precise nature of the forces which generated the disturbance.[9]
[1] The views of Mr. R. Mallet, briefly stated, are somewhat as follows:—Owing to the secular cooling of the earth, and the consequent lateral crushing of the surface, this crushing from time to time overcomes the resistance; in which case shocks are experienced along the lines of fracture and faulting by which the crust is intersected. These shocks give rise to earthquake waves, and as the crushing of the walls of the fissure developes heat, we have here the vera causa both of volcanic eruptions and earthquake shocks—the former intensified into explosions by access of water through the fissures.—"On the Dynamics of Earthquakes," Trans. Roy. Irish Acad., vol. xxi.
[2] Illustration of the mode of propagation of earthquake shocks will be found in Lyell's Principles of Geology, vol. ii. p. 136, or in the author's Physiography, p. 76, after Hopkins.
[3] "Rep. on Theories of Elevation and Earthquakes," Brit. Ass. Rep. 1847, p. 33. In the map prepared by Prof. J. Milne and Mr. W. K. Burton to show the range of the great earthquake of Japan (1891), similar isoseismal lines are laid down.
[4] Lyell, loc. cit., p. 163. Two Catalogues of Earthquakes have been drawn up by Prof. O'Reilly, and are published in the Trans. Roy. Irish Academy, vol. xxviii. (1884 and 1886).
[5] Ninth Annual Report, U.S. Geological Survey (1888).
[6] A True and Particular Account of the Dreadful Earthquake, 2nd edit. The original published at Lima by command of the Viceroy. London, 1748. Translated from the Spanish.
[7] I take the account from that of Capt. Dutton above cited, [p. 220].
[8] Dutton, Report, Plate xxvi., p. 308.
[9] Ibid., p. 211. On the connection between the moon's position and earthquake shocks, see Mr. Richardson's paper on Scottish earthquakes, Trans. Edin. Geol. Soc., vol. vi. p. 194 (1892).