106 WALKS AND TALKS. 



earthquake of translation. This species may become destructive, 

 especially in a secondary way, when occurring either on land 

 or under the sea, in such relation to sea-level as to cause a 

 rush of the sea upon the land. With this discrimination in 

 view, let us consider what causes may be assigned. 



The earthquake of vibration is evidently caused by a sud- 

 den blow or jar-, the earthquake of translation, by a lift, either 

 permaneut or transient. 



From time immemorial earthquakes have been connected 

 in the minds of men with volcanic action ; but careful study 

 shows no uniform correlation between them. Volcanic action, 

 moreover, is too local and too feeble. * Some, in modern times, 

 attribute earthquakes to the movements of the molten interior 

 of the earth, acting against the walls of its prison; or as re- 

 sulting from some other mechanical action within the crust. 

 This opinion is supported by most reputable names Hum- 

 boldt, Scrope, Sir William Thomson. Movements of trans- 

 lation are undoubtedly produced by volcanic forces. Portions 

 of mountains are lifted or even blown away ; fissures are 

 caused and many distinct movements result, which are com- 

 monly embraced under earthquakes. Undoubtedly, it some- 

 times happens, also, as an incident of volcanic action, that 

 sudden blows, or violent explosions occur which impart vibra- 

 tory tremors on a narrow scale, in character like those which 

 sometimes spread over kingdoms and work vast destruction. 

 But it can not be admitted that earthquakes as best known 

 earthquakes of vibration are ascribable to any volcanic 

 agency. 



It may be mentioned, also, that the fall of large rocks, 

 mountain-slides, great explosions, whether natural or artificial, 

 sometimes occasion genuine earthquake tremors. The jar of 

 a train of freight cars, or of a loaded wagon on the city pave- 

 ment, generates real earthquake tremors; but in all these 

 cases, on a scale too insignificant to be dignified with the name. 



What is it, then, which stands as the physical cause of 

 those blows or shocks which, originating at certain foci in the 

 earth, spread radially in earth vibrations which sometimes 



