I 



THE CAUSE OF EAETHQUAKES. 2.79 



Flurrounding stations, it would be recorded at each of 

 these as having the same direction. To those situated on 

 a line forming a continuation of the strike of the fissure, 

 the vibrations would advance so to speak end oil, whilst 

 to those stations lying in a line perpendicular to the strike 

 of the fissure, the motion would advance broadside on. 



Motions like these latter have been recorded in Tokio, 

 where earthquakes which from time observations were 

 known to have come from the faulted and rising region 

 to the south have been registered as a series of east and 

 west motions, or vibrations transverse to this line of 

 propagation. 



It must, however, be here mentioned that the registra- 

 tion of only transverse motion may possibly be due to the 

 extinction of normal motion, although this is not generally 

 regarded as probable. 



It would therefore appear that certain earthquakes 

 and faults are closely related phenomena, the former 

 being an immediate effect of the latter. Faults are due 

 to earth oscillations, and to a variety of causes producing 

 disturbances in the equilibrium of the earth's crust ; the 

 principal cause of all these phenomena being alterations 

 in the distribution of heat, and the attractive force of 

 gravity. 



Earthquakes consequent on the explosion of steam. — 

 Humboldt regarded volcanoes and earthquakes as the 

 results of a common cause, which he formulated as * the 

 reaction of the fiery interior of the earth upon its rigid 

 crust.* Certain investigators, who have endeavoured to 

 reduce Humboldt's explanation to definite limits, have 

 suggested that earthquakes may be due to sudden out- 

 bursts of steam beneath the crust of the earth, and its 

 final escape through cracks and fissures. 



Admitting that steam may accumulate by separating 



