^36 GEOLOGY. 



'parasitic cone of Etna during 100 days was equal to 2,100;000 cubic 

 meters of water. If this were ground-water, and the lava from which 

 it issued had an excess of 500° Fahr. above the fusion-point, the forma- 

 tion of this steam would congeal a column 400 feet in diameter and 

 3000 feet deep in the time given. If this case is typical, and if 

 Fouque's estimate is not greatly exaggerated or very exceptional, the 

 view that any large portion of the steam from volcanoes comes from 

 surface-waters seems to be incompatible with the persistence of ebul- 

 lition and explosion which many of them exhibit. Stromboli has been 

 in constant eruption as far back as the history of the region runs. It 

 is now exploding every three to ten minutes, and yet the mass of lava 

 seems to be small and its outflow inconsiderable. Is it possible that a 

 current of steam, given out with this activity for so long a period, was de- 

 rived from adjacent ground- waters, and has not yet solidified the lava? 



The problem takes on a very different aspect if the steam, or at least 

 some large part of it, rises from great depths and brings thence an 

 excess of heat. It then becomes an agency for the maintenance of the 

 Uquidity of the lava, for giving it convective motion, and for promot- 

 ing explosive action, so long as it continues to rise. 



For these and other reasons the balance of present evidence seems 

 to us to favor the view that most of the steam and other gases come 

 with the lava from its original source deep in the earth. 



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 Theoretische Geologic, 1888. Fouque, Santorin et ses Eruptions, Paris, 1879, Sar- 

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