1906.] 



SEE— THE CAUSE OF EARTHQUAKES. 401 



by a "tidal wave," for the reasons and in the manner above ex- 

 plained; and thus to the afflicted inhabitants all nature seems to be 

 convulsed at once. From the observed course of events, we cannot 

 doubt that this connection actually exists ; and unfortunately it 

 seems to be abundantly illustrated in the annals of the East as well 

 as of the West Indies. 



In the same way we explain Alexis Perrey's w^ell-known laws 

 of earthquakes. The forces assigned to account for these disturb- 

 ances, however, are not the cause of the convulsions of nature, but 

 only the occasions for outbreaks of highly unstable conditions de- 

 pending on subterranean forces easily set off.' 



In like manner the violent outbreak of a volcano or the occur- 

 rence of a great earthquake in one part of the world may tend to 

 bring on similar phenomena in another remote region. 



The order of events often observed in the development of the 

 volcanic and seismic phenomena following great outbreaks seems 

 to support this view. While we do not regard it as proved that 

 an eruption like the recent great outbreak of Vesuvius, for example, 

 could indirectly bring on the earthquake in California and other 

 similar disturbances, yet we do not regard such an influence as 

 at all impossible. Conditions of instability once existing are con- 

 tagious and tend to spread like a conflagration. As there are on 

 the average over 60 world-shaking earthquakes every year, or 

 more than one a week, it is evident that if one should break out in 

 a region where it might accelerate the outbreak of others, several 

 might be grouped into a small space of time, and these in turn might 

 exercise wade influence on unstable conditions throughout the world, 

 as often seems to be the case.^ Our knowledge of these teleseismic 

 effects, however, is still far from complete, and the settlement of the 

 question must be left to the future. 



§ 61. Internal state of the earth. 



The investigations of Lord Kelvin and Professor Sir G. H. 



^ In his " Journal of Researches during the Voyage of the Beagle," 1845, 

 p. 291, Charles Darwin mentions this remarkable coincidence of phenomena: 

 After a long slumber, Conseguina in Central America, and Aconcagua and 

 Corcovado (S. lat. 32^° and 43^°) in Chile, broke out the same day! His 

 suspicion that this coincidence was not accidental would be confirmed by 

 our knowledge of the Andean trough, in the Hne of which all these volcanoes 

 are situated. 



