THE EXTRUSIVE PROCESSES. 623 



original or is merely a special form of convective circulation, has an 

 important bearing on the supply of the atmosphere, which is constantly 

 being depleted by the oxidation of the rocks and by the formation of 

 carbonates and carbonaceous deposits. This vital phase of the subject 

 will receive further consideration. While recognizing the lack of deci- 

 sive proof, it would seem that the preponderance of evidence lies in 

 favor of the view that a notable portion, at least, of gaseous volcanic 

 emanations is derived from the interior of the earth, and is really a 

 contribution to the atmosphere and the hydrosphere. The hydrogen, 

 on coming in contact with the atmosphere, ignites and adds itself to 

 the hydrosphere. The carbon dioxide is in part decomposed by plants, 

 and adds to the supply of atmospheric oxygen. The nitrogen, being 

 comparatively inert, doubtless gradually accumulates in the air and has 

 thus come to be its preponderant constituent. 



THE CAUSE OF VULCANISM. 



The extraordinary facts involved in volcanic phenomena cannot 

 well be discussed fully until the origin of the earth is considered, and 

 the great agencies, as well as the peculiar conditions, which the earth 

 inherited from its birth, are duly weighed, for these were, with little 

 doubt, the true causal antecedents of vulcanism. We shall return to 

 the subject after a sketch of the early conditions of the earth, but the 

 views that have been entertained may be reviewed here while the 

 phenomena are fresh in mind. 



The explanation of vulcanism involves two essential elements. 

 These are (1) the origin of the lavas, which involves a consideration of 

 the necessary temperatures, pressures, and other conditions, and (2) 

 the forces by which the lavas are expelled. 



Nearly all current explanations of vulcanism are founded upon 

 conditions supposed to be derived from a molten globe, and fall under 

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

 portions of the original molten mass, and (II) those which assign the 

 lavas to the local melting of rock. 



I. On the Assumption that the Lavas are Original. 



In this case it is not necessary to assume any special accession of 

 heat, but merely to account for extrusion. There are two phases of 



