282 EARTHQUAKES. 



ejectian of ashes and lava from volcanic vents, there is 

 an extensive evisceration of the neighbouring ground. 

 When we look at a volcano like Fujiyama, 13,000 feet 

 in height, and at least fifty miles in circumference, and 

 remember that the mass of cinders and slag of which it 

 is composed came from beneath the area on which it 

 rests, the point to be wondered at is, that earthquakes, 

 consequent on the collapse of subterranean hollows, are 

 not more frequent than they are. At the time of a single 

 eruption of a volcano, the quantity of lava ejected amounts 

 to many thousand millions of cubic feet. In 1783 the 

 quantity of lava ejected from Skaptas Joknee, in Iceland, 

 was estimated as surpassing *in magnitude the bulk of 

 Mont Blanc' ^ Admitting that hollow spaces are the 

 results of these eruptions, and that in consequence of 

 this evisceration the ground is rendered unstable, the 

 instability being increased by the additional load placed 

 above the eviscerated area, it would seem that from time 

 to time earthquakes are inevitable. 



Facts, however, teach us that volcanoes act as safety 

 valves, and that, as a rule, at or shortly after an eruption, 

 earthquakes cease. The relationship of earthquakes to 

 volcanic eruptions would therefore indicate, notwithstand- 

 ing the arguments put forward to show that an area 

 loaded by a volcano has in consequence of the evisceration 

 and the load a quaquaversal dip, that evisceration does 

 not take place beneath volcanoes as is usually supposed, 

 and we may conclude that it is but few earthquakes 

 which have an origin due to these causes. 



Earthquakes and evisceration by chemical degrada- 

 tion. — A powerful agent, which tends to the formation 

 of subterranean hollows, is chemical degradation. The 

 effects of this have been often measured by quantitative 

 ' Lyell, Principles, vol. ii. p. 61. 



