Vol. 6] Reiti: The Elastic-Rebound Theory of Earthquakes. 415 



vibrations must be of the nature of waves, elastic and not 

 gravitational waves; and the question which presents itself to 

 us is: what produces these waves? Their origin lies, of course, 

 in the region of greatest disturbance, but its exact position and 

 the causes which produce the disturbance are not so easily dis- 

 covered. Many theories have been advanced. The earlier ideas, 

 suggested largely by the outbursts of volcanoes, were that the 

 earth was a fluid mass surrounded by a thin liquid crust floating 

 upon it, and that movements of the fluid interior caused earth- 

 quakes in the crust. These vague notions have been rendered 

 somewhat more precise by Pilar, who, in his book on "Abysso- 

 dynamik" in 1881, supposed that the crust was broken into 

 sections by cracks which were inclined and not vertical, and that 

 those blocks which had a broader base and contracted upwards 

 were raised, and the intervening blocks lowered, under the action 

 of gravity and the pressure of the fluid interior, whenever the 

 disturbance allowed this readjustment, and in this way earth- 

 quakes were produced. The idea that volcanic outbursts were 

 always accompanied by earthquakes and that generally the 

 regions of the earth where volcanoes were common were also 

 regions of many earthquakes, led to the belief that these were 

 but two phases of the same phenomena, and that earthquakes 

 themselves were due to explosions in the fluid interior. The 

 downfall of rock in caverns was, one hundred years or so ago, 

 looked upon as the most important cause of earthquakes, but 

 we shall see that this cause is insufficient to bring about strong 

 earthquakes and indeed it has gradually received less and less 

 support. The ideas that many earthquakes are independent of 

 volcanic action and that they are due to movements of some 

 kind in the crust of the earth became stronger, and in 1850 

 C. F. Naumann divided earthquakes into two classes, the volcanic 

 and the tectonic; the first being due to volcanic explosions and 

 the second to movements in the rock-mass. The importance of 

 this latter cause has become more and more apparent, so that 

 now we feel quite certain that all the really great earthquakes 

 are of the tectonic class, and that earthquakes connected with 

 volcanic outbursts are of comparatively little importance. It 

 is always found that volcanic earthquakes, although they may 



