162 EARTHQUAKES. 



Sometimes these changes have taken place gradually, 

 and sometimes with violence. Mountains have been 

 toppled over, valleys have been filled, cities have been 

 submerged or buried. 



With the records of these convulsions before us, we 

 see that seismic energy yet exhibits a terrible activity in 

 changing the features of the globe. 



Reason of these moveraents. — To formulate a single 

 reason for these catastrophes would be difficult. Where 

 they are of the nature of landslips, or materials have been 

 dislodged from mountain sides, the cause is evidently the 

 sudden movement of this ground acting upon strata not 

 held together in a sufficiently stable condition. A similar 

 explanation* may be given for the sudden elevations or 

 depressions of strata in a district removed from the centre 

 where the disturbance had its origin. The seismic effort 

 exhibits itself in a certain area round its origin as a sudden 

 push, and by this push, strata are fractured and caused to 

 move relatively to each other. 



At or near to the origin of an earthquake it might 

 be argued that it was the sudden falling of rocky strata 

 towards a position of stable equilibrium that caused the 

 shaking, and in such a case the movements referred to 

 may be regarded as the cause rather than the effect of an. 

 earthquake. 



A subject closely connected with the sudden dislocation 

 of strata, is the production of secondary or consequent 

 earthquakes, due to the disturbance of ground in a critical 

 state (see p. 248). 



