124 



IGNEOUS AGENCIES. 



Fig. 105.— Shocks in Mines 



would reach, the surface by refraction, but a portion would be reflected 

 and return into a b, concurring and interfering with the advancing 

 waves, and producing great commotion in these strata. 



5. Shocks less severe in Mines. — This case is probably more common 

 than the last. It was notably the case in the earthquake of 1872 in 



Inyo County, Califor- 

 nia. While the sur- 

 face was severely 

 shaken, many houses 

 destroyed, and large 

 fissures formed in the 

 earth, the miners, sev- 

 eral hundred feet be- 

 low the surface in the 

 hard rock, scarcely 

 felt it at all. This is probably, at least partly, explained as follows : 

 As long as the wave travels within the earth, motion of the particles is 

 restrained by the work of elastic compression ; but, as soon as the sur- 

 face is reached, the motion becomes free, and the velocity of shock is far 

 greater than before, often so great as to throw bodies high in the air. 

 The phenomenon is exactly like that in the familiar experiment of the 

 ivory balls : when the first in the series is struck, an elastic wave of 

 compression passes through all, but only the last one moves. 



6. Bridges. — In a manner somewhat similar are to be accounted 

 for the phenomena of bridges. In the earthquake regions of South 

 America there are 



certain favored 

 spots, often of small 

 extent, which are 

 partially exempt 

 from the shocks 

 which infest the 

 surrounding coun- 

 try. The earth- 

 quake-wave seems to pass under them as under a bridge, to reappear 

 again on the other side. The mere inspection of Fig. 106 will explain 

 the probable cause of this exemption, viz. : reflection from the under 

 surface of an isolated mass of soft, inelastic strata, c c. 



7. Fissures. — The ground-fissures, so commonly produced by earth- 

 quakes, are sometimes of the nature of the great fissures of the crust, 

 which are the probable cause of earthquakes. Such great fissures are 

 usually wholly beneath the surface at great depth, but sometimes may 

 break through and appear on the surface. This is certainly the case 

 when decided faults occur with elevation or depression of large tracts 



