262 C. E. BUTTON 



not indeed a precise or accurate estimate of its depth, but a roughly 

 approximate one. 



A quaHfication of the foregoing may be introduced here. The 

 earthquake is no doubt the fracturing or sudden yielding of the rock 

 masses immediately above the lava reservoir. We can only vaguely 

 conjecture the distance which separates the zone of fracture from the 

 zone of melting. But in no case could it be so great as a mile without 

 making itself sensible in the greater depth of the quake. We must, 

 however, increase slightly our estimate of the depth of the lava beyond 

 the estimated depth of the quake. 



We may now proceed to state the probable cause of volcanic erup- 

 tions. They are caused, I conceive, by a development of heat, 

 resulting from radioactivity, in limited tracts at a depth of one to three 

 — at the very utmost not over four — miles from the surface, which is 

 sometimes sufficient to melt the rocks affected by it. The melting is 

 gradual, and when a sufficient quantity is melted, the water which it 

 contains becomes explosive and usually suffices to break through the 

 covering, constituting an eruption. When all the lava is erupted, and 

 the reservoir is exhausted, it closes up for a time. If the heat con- 

 tinues to be generated, more lava is melted, and in due time another 

 eruption occurs. The process may be repeated again. It may be 

 repeated hundreds or thousands of times. The volcanic action may 

 continue in the same place for hundreds of thousands, or even millions, 

 of years, or it may repeat itself only a few times, or may even occur 

 only once. Indeed, it may fail altogether to erupt to the surface, and 

 in many cases does fail. In other words, it goes through the entire 

 process of preparing for an eruption and does not consummate it. 



This view enables us to explain the repetitive character of volcanic 

 eruptions, which is, perhaps, their most striking and characteristic 

 feature. It is in strong contrast with the view long held that the lava 

 reservoirs are a part of the original constitution of the earth, and have 

 lain in their present position through all the vast period of the earth's 

 evolution, waiting for a convenient occasion to explode and pour forth 

 their fiery contents. It regards the reservoirs as having no real exist- 

 ence as such, and as containing no liquid eruptible contents until some 

 source of heat acts upon them and liquefies a portion of the strata, thus 

 giving rise to the reservoir. When a sufficient quantity of the lava is 



