348 EAETHQUAKES. 



inasmucli as the rock in which they are found is soft and 

 easily weathered, indicate an exceedingly rapid elevation, 

 earthquakes are of common occurrence. 



From the evidences of elevation which we have upon 

 the South American coast, Japan, and in other countries, 

 it appears that these movements are intermittent, there 

 being periods of rest, when sea cliffs are denuded, and 

 perhaps even periods of subsidence. There is also evi- 

 dence to show that, although these movements have been 

 gradual from time to time, they have been aided by 

 starts occasioned by earthquakes. 



As to whether earthquakes are more numerous during 

 periods of elevation, or of subsidence, or during the inter- 

 mediate periods of rest, we have no evidence. 



Sudden displacements which occasionally accompany 

 earthquakes might, it was said, sometimes be regarded as 

 the cause of an earthquake, and sometimes as the effect. 



The slow elevations here referred to may be looked 

 upon as being one of the more important factors in the 

 production of earthquakes. By various causes the rocky 

 coast is bent until, having reached the limit of its elas- 

 ticity, it snaps, and, in flying back like a broken spring, 

 causes the jars and tremors of an earthquake. 



If this is the case, then the number of earthquakes 

 felt in a district which is being elevated may possibly be 

 a function of the rate of elevation. 



