400 SEE— THE CAUSE OF EARTHQUAKES. [October 19. 



the crust are already highly unstable ; but there is probably no com- 

 munication through the depths of the planet, except a wave motion 

 which spreads in every direction, the earth acting as an seolotropic 

 elastic solid, because of the great pressure to which the matter is 

 subjected. In adopting the view that, because sympathetic effects 

 may be aroused at great distances, the disturbances are therefore 

 very deep-seated, Humboldt appears to have been somewhat misled 

 by the ordinary effects of tremors on unstable conditions even at 

 great distances. 



Yet there can scarcely be any doubt that the connection of the 

 Andean trough from Valdivia to Guayaquil is so intimate that a dis- 

 turbance once started under parts of it may easily be propagated 

 over the whole length of this great trough, or even to another part 

 of the globe; and thus we conclude that seismic activity easily 

 extends, and has a widespread effect which was formerly supposed 

 to be transmitted through the deep interior of the earth. It is 

 impossible to doubt that Charles Darwin was entirely correct in 

 concluding that a subterranean connection generally exists between 

 a continent and its outlying islands, for both are often on the 

 borders of the same continental trough. Occasionally the extent 

 of this connection may be even wider, and sometimes cover a whole 

 region or run from one region into another, as in the events men- 

 tioned above by Professor Milne; but the disturbances are trans- 

 mitted principally by waves and by strains through the crust, and 

 not by means of any currents through the deep interior of the earth. 

 As regards the general question of slight disturbances bringing on 

 greater catastrophes when the conditions of subterranean steam 

 pressure are already highly unstable we may go even further. We 

 occasionally read in History of the West Indies, especially the 

 group including St. Vincent, Martinique, St. Lucia, Guadaloupe, 

 Barbadoes and Trinidad, being visited by a terrible hurricane, fol- 

 lowed by an earthquake and a " tidal wave " ; so that it seems as 

 if all the worst elements 'in nature were suddenly let loose to devas- 

 tate these islands. In view of the cause assigned in this paper, it 

 is evident that unstable conditions of subterranean steam pressure 

 may not require anything more violent than the raging of a hurri- 

 cane to bring on an earthquake, which in turn may be followed 



