530 



GEOLOGY. 



tion^ rather than its length, that is effective. Instrumental investi- 

 gations indicate that the oscillations, after they have left their points of 

 origin, are usually only a fraction of a millimeter in amplitude ; at most 

 they seldom exceed a few millimeters. A sudden shock with an ampli- 

 tude of 5 or 6 millimeters is sufficient to shatter^ a chimney. It is true 

 that estimates assigning amplitudes of a foot or more have been made, 

 but their correctness Is open to serious doubt. It should be understood 

 that it is the length of oscillation of the particles of the sub-surface rock 

 transmitting the vibrations that is referred to, not the movement of the 

 free surface, or of objects on the surface. The throw at and on the sur- 

 face is much greater. Just as a slight, quick tap of a hammer on a floor 

 is sufficient to make a marble lying on it bound several inches, so a suffi- 



FiG. 447. — Illustration of the destructive effects of the Charleston earthquake, showing 

 definite direction of throw. (W J McGee.) 



ciently sudden rise of the surface of the earth, though but a fraction of an 

 inch, may project loose bodies many feet. 



Destructive effects. — The interpretation of the disastrous results of 



