THE USEFULNESS OF EARTHQUAKES. 245 



and depression must be restored. Hence, if it can be 

 shown that for the most part the forces of upheaval act 

 underneath the land, it follows though we may not be 

 able to recognize the fact by obvious visible signs 

 that processes of depression are taking place under- 

 neath the ocean. Now, active volcanoes mark the 

 centre of a district of upheaval, and nearly all volca- 

 noes are found near the sea. It seems as if Nature 

 had provided against the inroads of the ocean by seat- 

 ing the earth's upheaving forces just where they are 

 most wanted. 



Even in earthquake districts which have no active 

 vent, the same law is found to prevail. It is supposed 

 by the most eminent seismologists that earthquake re- 

 gions around a volcano, and earthquake regions appar- 

 ently disconnected form any outlet, differ only in this 

 respect, that in the one case the subterranean forces 

 have had sufficient power to produce the phenomena of 

 eruption, while in the other they have not. In " earth- 

 quakes," says Humboldt, "we have evidence of a vol- 

 cano-producing force ; but such a force, as universally 

 diffused as the internal heat of the globe, and proclaim- 

 ing itself everywhere, rarely acts with sufficient energy 

 to produce actual eruptive phenomena; and when it 

 does so, it is only in isolated and particular places." 



Of the influence of the earth's subterranean forces 

 in altering the level of the land, we might quote many 

 remarkable instances, but considerations of space com- 



