THE EXTRUSIVE PROCESSES. 621 



duce noxious effects. Volcanic and meteoric data seem to indicate 

 that steam is held less tenaciously than the other gases in the magmas 

 as they solidify into rocks. 



The source of the gases. — As already noted, it is one of the outstand- 

 ing problems of geology to determine (a) how far the gases of lavas were 

 possessed by them from their origin, whatever that may be, and (b) 

 how far they have been acquired in the lava's ascent to the surface. It 

 is recognized that lavas have the power of absorbing gases, and one of 

 the views entertained is that surface-waters, percolating through the 

 rocks and coming in contact T\ith the ascending colunm of lava, are 

 converted into steam, which is absorbed into the lava and rises with it 

 to the surface. There are two phases of this ^^ew. (1) The more 

 conservative one supposes that the water merely penetrates the frac- 

 ture zone of the surface of the earth through the ordinary means of 

 passage of underground-waters, and so makes a comparatively short 

 circuit. Under this view, the steam and other gases given forth are not 

 a contribution to the atmosphere and hydrosphere, but merely a resto- 

 ration to them of water and dissolved gases previously carried do^^m from 

 the surface. An even narrower view is sometimes entertained which 

 supposes that the larger part of the water descends through the volcanic 

 cone itself, or immediately about its base. The presence of chlorine 

 gases in the volcanic emanations and the nearness of most existing 

 volcanoes to the sea have been the basis for the idea that sea- 

 water, penetrating to the laA'a, is a chief source of the volcanic gases. 

 (2) The broader phase of the view assumes that the waters penetrate not 

 only the outer fracture zone of the earth, which is probably limited to 

 five or six miles in depth, but that they diffuse themselves through the 

 continuous unfractured zone doT\TL to depths where temperatures of 

 fusion prevail, and that they there enter into combination vrith the 

 lavas or with hot rock to form lavas. It is weU knoT\Ti that aqueous 

 vapor facilitates the fusion, or more accurately, the mutual solution of 

 the minerals. This view is a part of one of the hypotheses concerning 

 the origin of the lavas themselves. 



The opposing view supposes that the gases were in the main original. 

 Of this view there are two phases: (1) One supposes that the lavas are 

 remnants of an original molten globe which absorbed gases from the 

 primitive atmosphere and retained them till the time of their eruption. 

 The possible absorption of steam and air into the supposed molten 



