THE EXTRUSIVE PROCESSES. 633 



of the rock of the lower horizons. The ascending column may reach 

 hydrostatic equilibrium before it reaches the surface, and may then 

 form underground intrusions of various sorts without superficial erup- 

 tion, or it may only find equilibrium by coming to the surface and pour- 

 ing out a portion of its substance and discharging its gases. 



Additional Considerations Relative to the Gases. 



The question whether the volcanic gases are a contribution to the 

 atmosphere and hydrosphere is so important in its bearings on the whole 

 history of the atmosphere as to merit additional consideration here. 

 As already noted, if the volcanic gases arise from water and absorbed 

 air that have previously passed down through the strata, there is no 

 real contribution to the hydrosphere and atmosphere, but merely circu- 

 lation. If the gases are chiefly derived from the deep interior, they are 

 an important accession to the atmosphere and hydrosphere. 



Most views are more or less intermediate, assigning a part of the 

 gases to the interior and a part to the exterior. No one will question 

 that some part at least of the steam is due to the contact between the 

 ground-waters and the hot lava, and probably no one will question 

 that some gas comes from the interior if the lavas originate there. The 

 vital question is, whence comes the major portion? Are the constant 

 ebullitions of some volcanoes and the terrific explosions of others due 

 mainly to surface-waters, or to interior gases? 



It seems to be certain that in most cases the gases are diffused 

 through the substance of the lava, and are not simply in contact with 

 the walls of the column or with its summit. Without doubt steam is 

 generated around the lava- column by external contact, and perhaps 

 some explosions are due to the entrance of the rising lava upon a crevice 

 or cavern filled with water, or to the invasion of a lake gathered in an 

 old crater; but it still remains a question whether the importance of 

 such explosions has not been exaggerated. Such action does not seem 

 competent to produce inflated lavas, but merely shattered ones. Water 

 thus ''suddenly flashed into steam" could scarcely diffuse itself inti- 

 mately through the lava, for the process of diffusion is exceedingly 

 slow. But inflated lavas, pumice, scoriae, and cinders are the typical 

 products of explosive vulcanism. Not only in the ordinary Vesuvian 

 type, but in the extraordinary Krakatoan t^^pe, inflated lavas are the 



