Earthquakes and Volcanoes 



A good deal of attention has been paid by Mr. 

 R. Mallet to the study of earthquake phenomena, 

 or Seismology, and he is led to conclude that the 

 origin of the disturbance is usually not deep- 

 seated in the interior of the earth, probably never 

 exceeding a depth of thirty miles; while in many 

 cases, it is certainly much less. Thus he ascer- 

 tained that the great Neapolitan shock of 1857 

 had its origin at a depth of only eight or nine 

 miles beneath the surface. Dr. Oldham has 

 since found that a great earthquake at Cachar, 

 in India, in 1869, had its focus, or centre of im- 

 pulse, at a depth of about thirty miles. 



Although earthquake-shocks are happily of 

 rare occurrence in this country, it must be re- 

 membered that, in many parts of the world, they 

 are by no means rare phenomena; and, probably, 

 it is not overstating the case to say that earth- 

 quake shocks occur, on an average, about three 

 times a week During the year 1876, for ex- 

 ample, no fewer than 104 earthquakes are re- 

 corded in Professor Fuchs's Annual Report; and, 

 in the preceding year, as many as 100 days were 

 marked by the occurrence of shocks. But, in 

 addition to these, there are no doubt many slight 

 disturbances in unfrequented districts, which are 

 never recorded in such reports. The total effect 

 produced by the causes of such disturbances 

 must consequently be far from insignificant, 

 even in the course of a single year. 



Subterranean disturbances which commence 

 merely with quakings of the ground often termi- 

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