70 PROCEEDINGS OF THE AMERICAN ACADEMY. 



of the plutonic body, the gases must have high temperature, which 

 increases as the gas-tension increases. The total mass of the gas 

 accumulated at a cupola at any one time may be small, but it may 

 serve to determine a more rapid incorporation of the roof-rock at the 

 cupola than in the area of roof surrounding the cupola. It is obvious 

 also that some roof-rocks are more easily absorbed by a given magma 

 than are other rocks. 



Figure 4. Ideal section showing formation of volcanic vent through the 

 differential rise of assimilating magma. 



The rise of batholithic magma is, therefore, differential. Partly 

 because of gas control its attack on the roof is most efficient at points, 

 rather than along lines or in large areas. (Figure 4.) This deduction 

 seems well matched by the field fact that round intrusive bosses or 

 small stocks are characteristic cupola forms on large batholiths. Some 

 of these bosses have been proved to have very steep contact-surfaces 

 and, in shape, as in their cross-cutting relations, closely simulate vol- 

 canic necks. It is evident that every such cupola increases as well as 

 localizes the danger of true volcanic action. Blowpiping fusion or pure 

 explosion may destroy the relatively thin roof above the cupola. The 

 resulting vent is, then, of composite origin. Its upper part is like the 

 two simple types of central vents already described. Its lower part is 

 neither diatreme nor blowpiped hole, but represents the work of all the 

 agencies of magmatic assimilation in depth. This composite type of 



