426 



FIRST TEAR SCIENCE 



the rock mass to the surface, where it causes sudden 



and often tremendous 

 shocks. These slippings 

 may occur occasionally 

 for ages along the same 

 fault line. Sometimes 

 they are intense enough 

 to cause great damage ; 

 at other times only a 

 slight tremor is felt. 



The rapidity of the 

 transmission of the 

 shock differs with the 

 kind of material through 

 which it is transmitted, 

 varying from a few 

 FAULT LINE OF AN EARTHQUAKE. hundred ' feet to several 



thousand feet per second. The nearer a place is to the 



break or slip the greater 



is the intensity of the 



shock. Sometimes the 



crack or fault along 



which the movement 



occurs reaches to the 



surface and makes the 



displacement apparent. 



If an earthquake orig- 

 inates under the sea, a 

 great wave may be de- 

 veloped which rushes 

 inland from the coast, 

 causing great destruc- FENCE BBOKEN BY SLIPPING.. OF THBT 



00 EARTH ALONG A FAULT LINE. 



tion. One of the most 



fearful of these waves occurred at Lisbon, Portugal, in 



