284 PHYSICAL GEOLOGY 



a minute. The velocity depends to a large degree upon the density 

 and elasticity of the rock, being much slower in sand and loose sand- 

 stone than in slate, schist, or granite. This has been shown experi- 

 mentally by noting the velocity of shocks produced by explosions of 

 gunpowder, and it has been found that the velocity is 825 feet a 

 second in sand, and 1088 feet a second in slate and schist. In all such 

 experiments, however, account must be taken of the presence of fis- 

 sures and whether or not the fissures are filled with water. 



Amplitude of Vibration. — By the amplitude of vibration is meant 

 the distance each rock particle is moved from its position of rest 

 during an earthquake (Fig. 282). It is a common notion that the 

 amplitude is very great, but measurements show that they are minute, 



Fig. 282. — Wire model showing the motion of an earth particle during an 



earthquake. 



an amplitude of 20 millimeters (three fourths of an inch) being suffi- 

 cient to destroy a city ; one of 10 millimeters (three eighths of an inch) 

 constituting a severe earthquake ; and one of 5 or 6 millimeters being 

 adequate to shatter a chimney. Amplitudes much greater than the 

 above have been recorded. It should be remembered in this con- 

 nection that it is the suddenness of the shock that makes it effective. 

 This can best be illustrated by a simple experiment. If a stone or 

 metal slab upon which a marble rests is struck a sharp blow„ the 

 marble will be thrown into the air, but it is evident, that the actual 

 movement of the particles composing the slab, and through which 

 the vibrations were transmitted to the marble, was 3. very small frac- 

 tion of an inch, the projection of the marble being due to the great 

 suddenness of a small movement. This phenomenon is well illus- 



