224 LIGHT SCIENCE FOR LEISURE HOURS. 



or the other, the balance between upheaval and de- 

 pression must be restored. Hence, if it can be shown 

 that for the most part the forces of upheaval act under- 

 neath the land, it follows though we may not be able 

 to recognise the fact by obvious visible signs that 

 processes of depression are taking place underneath the 

 ocean. Now, active volcanoes mark the centre of a 

 district of upheaval, and nearly all volcanoes are found 

 near the sea. It seems as if nature had provided 

 against the inroads of the ocean by seating the earth's 

 upheaving forces just where they are most wanted. 



Even in earthquake districts w r hich 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 appa- 

 rently disconnected from 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 land, we might quote many re- 

 %iarkable instances, but considerations of space compel 

 us to confine ourselves to two or three. The slow pro- 

 cesses of upheaval or depression may, perhaps, seem 



