244 LIGHT SCIENCE FOR LEISURE HOURS. 



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. 



ISTow, 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. Kot 

 necessarily at the same instant, it should be remarked. 

 The process of upheaval may be either momentarily 

 accompanied 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 or the other, the balance between upheaval 



