VULCANISM 377 



of its other outflows were nearly as massive. Besides this mas- 

 siveness, there were extraordinary movements of the lava within 

 the crater, if the testimony of witnesses may be trusted. But 

 throughout these great movements in the higher crater, the lava- 

 column of Kilauea, 10,000 feet lower, continued its quiet action 

 without sensible relation to its boisterous neighbor. No difference 

 in specific gravity that could at all account for a difference in 

 height of 10,000 feet has been observed or can be presumed. It 

 seems a necessary inference, therefore, that the two lava-columns 

 have no connection with each other, or with a common reservoir. 

 The tops of some lava-columns stand about 20,000 feet above the 

 sea, while others emerge on the sea-bottom far below sea-level. 

 The total vertical range of emergence is between 30,000 and 40,000 

 feet, a difference which tells its own story as to their relative in- 

 dependence. 



Trivial agencies. Eruptions seem to be somewhat more com- 

 mon when atmospheric pressure is high than when low, doubtless 

 because the increased atmospheric weight on a large area of the 

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

 only be effective when other forces have almost accomplished 

 the result. Eruptions seem also to be more common when tidal 

 strains favor them, for like reasons. In the same class are proba- 

 bly to be put the effects of heavy rains. Such factors are to be 

 regarded as mere incidents, of no moment in the real causation 

 of vulcanism, but of some value in determining the precise moment 

 of eruption. 



Periodicity. Most volcanoes are intermittent in their action, 

 long periods of dormancy intervening between periods of activity. 

 Volcanoes supposed to be extinct may renew their activity, occa- 

 sionally with terrific violence. Their periodicity awaits an ex- 

 planation, but the temporary quiet very likely means an exhaustion 

 of the supply of gas or of lava, or of both, to which the active 

 stage is due. 



Products of Volcanoes 



Pyroclastic material. The fragmental materials which are 

 blown out of a volcano are, as a rule, portions of lava which solidified 

 before ejection, or during their flight in the air. Masses of rock 



