MOVEMENTS AND DEFORMATIONS 341 



effective. Except at their points of origin, they are usually only 

 a fraction of a millimeter in amplitude, and seldom exceed a few 

 millimeters. A sudden shock with an amplitude of 5 or 6 milli- 

 meters is sufficient to shatter a chimney. It is the oscillation of 

 the rock particles transmitting the vibrations that is here meant, 

 not the movement of objects on the surface, which is often much 

 greater. Just as a marble lying on a floor may be made to bound 



Fig. 275. Track of electric railway, between South San Francisco and San 

 Bruno Point. (Photo, by Moran.) 



several inches by a slight tap of a hammer some distance away, so 

 a sufficiently sudden rise of the earth-surface for a fraction of an 

 inch only, may project loose bodies many feet. 



Destructive effects. The disastrous effects of earthquake shocks 

 result from (1) the suddenness and strength of rather minute vibra- 

 tions of earth-matter, and from (2) the freedom of motion of the 

 bodies affected. The deeper rocks, where sensibly continuous, 

 transmit seismic vibrations without appreciable disruptive effect 

 so far as known (though the origin of crevices has been assigned 

 to this cause) ; but bodies at the surface are fractured, overturned, 

 and notably displaced. The tap of a hammer sends an almost 

 imperceptible vibration along the floor, but this vibration throws 

 well into the air the glass ball beneath which it runs. Similarly 

 the minute seismic vibrations travel miles from their origin through 

 continuous substance with little result, and yet may then hurl a 

 loose or unstable body to destruction. Earthquake waves striking 



