﻿352 AN EARTHQUAKE MODEL. [Aug. I9IO, 



conditions in Nature was so obvious, that no model could convey 

 information or prove anything. The Author had adopted and 

 illustrated a theory which was generally accepted, but which the 

 speaker himself had found reason to abandon for the supposition 

 that great earthquakes are as a rule due to the sudden development 

 of strain, or what might be crudely described as an explosion, 

 rather than the sudden relief by fracture of a slowly accumulating 

 strain. This, however, did not affect the use of the Author's model 

 as an illustration of the deformation which may be the result of an 

 earthquake. 



The Author said that he was strongly opposed to the views of the 

 school of seismologists, to which Mr. Oldham had, it seemed, now given 

 his adherence, who thought that the great earthquakes represented 

 explosions of igneous material in the earth's crust (misslungene 

 Ausbruchsversuche, to use the expressive phrase of Branca). 

 Such earthquakes as were known to be due to explosive volcanic 

 action were local in character, and had little effect on distant seismo- 

 graphs. Important earthquakes like the Californian earthquake of 

 1906, as well as the majority of the smaller disturbances, appeared 

 to be closely related to lines of tectonic weakness, and it was 

 reasonable to suppose that they were incidents of the readjustment 

 of the earth's crust, which is always proceeding, though more active 

 in some periods than in others. 



He would like to add that a sudden explosion of an imprisoned 

 magma could only occur in the neighbourhood of the surface. It 

 was dynamically impossible under the pressure of a great thickness 

 of superincumbent strata. 



