388 GEOLOGY 



The peculiar proportions of the rock-gases, among which hydro- 

 gen and carbon dioxide so greatly preponderate, seem to imply 

 that they are not derived chiefly from surface waters or the atmo- 

 sphere; they appear to be original constituents of the rocks in the 

 main, and when given forth they appear to constitute real additions 

 to the atmosphere. 



The Cause of Vulcanism 



The explanation of the extraordinary facts involved in volcanic 

 phenomena is wrapped up in that of the origin of the earth, for the 

 agencies which the earth inherited from its birth are beyond doubt 

 factors in vulcanism. This phase of the subject will be treated 

 only briefly here. 1 



The explanation of vulcanism involves two essential elements, 

 (1) the origin of the lavas, and (2) the forces by which they are 

 expelled. The . current explanations of vulcanism fall into two 

 general classes: (1) those which assume that the lavas are residual 

 portions of an original molten mass, and (2) those which assign 

 the lavas to the local liquefaction of rock. 



The view that the lavas are residues of an original molten globe 

 formerly prevailed, but is found to encounter grave difficulties 

 because of the independent action of vents which are closely 

 adjacent to one another. When the lava columns vary thousands 

 of feet in height on the same mountain mass, as in the Hawaii vol- 

 canoes, even a resort to the hypothesis of local residual reservoirs 

 is not altogether satisfactory. Another view which has had much 

 currency supposes that surface water and its absorbed gases pene- 

 trate to heated rock and are absorbed by it, rendering the whole 

 liquid, and that the lava thus formed is then forced to the surface. 

 The progress of investigation, however, does not support the belief 

 that water penetrates from the surface to depths below the zone 

 of fracture, and hence is far from reaching highly- heated rocks. 

 The gases of volcanoes and of igneous rocks are not sufficiently 

 like those of water to support this view. 



The relief of pressure, which lowers the melting point of rock. 



1 For fuller statement see the authors' larger work, Vol. I, pp. :W.")-607, 

 Vol. II, pp. 99-106, 116-118, 120, 130. 



