THE EXTRUSIVE PROCESSES. 



607 



less because the increased atmospheric weight on a large area of the 

 adjacent crust aids in forcing out the lava or the volcanic gases. This 

 can only be effective when other forces have almost accomplished the 

 result, and would doubtless have completed it a little later had not the 

 atmospheric wave supplied the little remaining pressure needed. Erup- 

 tion seems also to be more common when the tidal strains favor it, for 

 like reasons. In the same class are probably to be put the effects of 

 heavy rains, whether they act by gravity or by giving rise to steam. 

 Such agencies are to be regarded as mere incidents of no moment in the 

 real causation of vulcanism, but of some value in determining the pre- 

 cise moment of action. This is not to be understood as inconsistent 



Fig. 464. — Crater of Kilauea, 



with the view that the periodic stresses of the body-tides of the earth 

 are important factors in vulcanism, as elsewhere explained, but merely 

 that the special time of surface- eruption is only incidentally connected 

 with the water-tides. 



Periodicity. — Most volcanoes are periodic in their stages of action. 

 Tong donnant periods inter^^ene between eruptive periods. Volcanoes 

 supposed to be extinct occasionally awaken with terrific violence. 

 Sometimes also they awaken quietly. This larger periodicity yet awaits 

 an explanation, but it very likely means a temporary exhaustion of the 

 supply of gas or of lava, or of both, to which the active stage is due. 



