156 



Prof. J. Prestwich. 



just escaped, while another portion of the water filling the stratum 

 is pressed forward, to take the place of the exploded portion. If the 

 lava still continues to ascend, the in-forcing of the vapour and the 

 explosions of successive portions of water succeed almost uninter- 

 ruptedly, and the volcanic duct thus becomes the centre to a battery 

 discharging at many levels and keeping up an incessant volley ; for 

 the supply of water at first is comparatively unlimited, and being 

 renewed as quickly as it is exploded, there is no cessation in the action 

 until the source is drained or stopped or the lava ceases to flow. If 

 the water is lodged at such, a distance, n,. from the sides of the duct 

 that it explodes behind blocks or fragments of rocks, or of older 

 lava lodged in the cavities left by previous removals of portions of the 

 rock, then such blocks will be blown and driven into the outflowing 

 lava current. This seems to be a more probable explanation than that 

 of their being torn or wrenched off the sides, although in the first 

 stage of a new volcano there may often be great mechanical action 

 and ejection of debris in clearing the passage, as in the case of the 

 large quantity of debris of Devonian strata in so many of the old 

 volcanoes of the Eifel. 



In considering the various phases of this problem, it is, however, 

 only too apparent that while the hydro-geological conditions admit of 

 investigation on known principles, the thermo-dynamical conditions 

 are involved in much obscurity, and are more hypothetical. The 

 warrant for any hypothesis depends,- however, upon whether the 

 observed phenomena accord with the inferences that should follow on 

 the assumptions, and for this I think sufficient cause can in this 

 instance be shown. For the more or less deep-seated subterranean 

 detonations and thundering that accompany most eruptions, and the 

 paroxysmal explosions accompanied by enormous ejections of steam 

 and ashes from the crater, are the necessary consequences of an influx 

 of water into the volcanic duct under the conditions we have 

 described ; while a term is placed to the continuance of the eruption 

 in the circumstance that the wa.ter supply being external and indepen- 

 dent, whenever that supply is exhausted by expulsion through the 

 escaping lava, the explosions must cease, although the eruption of lava 

 may proceed for a longer period. The hypothesis agrees also with 

 the fact that as a rule the eruptions are more paroxysmal the longer 

 the interval of rest, for the filling of the underground reservoirs 

 exhausted by the previous eruption is a question of time, and the 

 greater their water-stores, the greater and more powerful the explo- 

 sions. 



The apparently conflicting phenomena of Ehrenberg's discovery of 

 fresh- water diatoms in the volcanic ejections of some islands, and of 

 marine diatoms in others, admit also of ready explanation on this 

 hypothesis, for so long as the inland underground waters due to the 



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