74 



PHYSICAL GEOLOGY 



tain slopes, and, where they affect cities, may become highly 

 destructive to human life and property. The Alaskan earth- 

 quake of 1899 resulted from a sudden displacement of more 



FIG. 57. Overthrusts in the Highlands of Scotland. (H.M. Geol. Surv.) 



than 40 feet; and the San Francisco disaster in 1906 was 

 caused by a horizontal displacement of 5 to 20 feet (Fig. 58), 

 which took place along a line many miles in length. Other 



FIG. 58. Displacement of a road where crossed by a horizontal fault. 

 (Jones.) The San Francisco earthquake of 1906 was a result of the fault- 

 ing. 



subterranean shocks, such as those attending volcanic erup- 

 tions, may likewise produce violent earthquakes. 



The destructiveness of earthquakes is due to the suddenness of 

 the shock. Thus a sharp blow struck on the side of a table will 

 cause very little motion in the table itself, but it may be sufficient 



