THE USEFULNESS OF EARTHQUAKES. 221 



the earth. And it will be remarked, that although 

 upheaval is the process which appears at first sight to 

 be the only effectual remedy to the levelling action of 

 rains and ocean-currents, yet the forcible depression of 

 the earth's surface may prove in many instances yet 

 more effective, since it may serve to reduce the sea-level 

 in other places. 



Now, the earth's subterranean forces serve to pro- 

 duce the very effects which are required in order to 

 counteract the continual disintegration of the shores 

 and interior parts of continents. In the first place, 

 their action is not distributed with any approach to 

 uniformity over different parts of the earth's crust, and 

 therefore the figure they tend to give to the surface of 

 that crust is not that of a perfect sphere. This, of 

 itself, secures the uprising of some parts of the solid 

 earth above the sea-level. But this is not all. On a 

 comparison of the various effects due to the action of 

 subterranean forces, it has been found that the forces 

 of upheaval act (on the whole) more powerfully under 

 continents, and especially under the shore-lines of con- 

 tinents, while the forces of depression act most power- 

 fully (on the whole) under the bed of the ocean. It need 

 hardly be said that whenever the earth is upheaved in one 

 part, it must be depressed somewhere else. Not neces- 

 sarily at the same instant, it should be remarked. The 

 process of upheaval may be either momentarily accom- 

 panied by a corresponding process of depression, or the 

 latter process may take place by a gradual action of 

 the elastic powers of the earth's crust ; but, in one way 



