58 PROCEEDINGS OF THE AMERICAN ACADEMY. 



a certain " damping of the fires " already accomplished. In basaltic 

 volcanoes assimilation of the normal, acid crust-rocks has evidently 

 not been important; at such vents the juvenile emanations are clearly 

 in control from beginning to end of each volcano's history. This 

 statement does not conflict with the fact that resurgent water, either 

 vadose or connate with sediments, is often responsible for the explo- 

 sions at basaltic and other volcanoes. The clearing-out of the explosion 

 funnel, which is always shallow and superficial, is not so vital to con- 

 tinued activity as the preservation of fluidity in the magma of the 

 conduit. 



Phases of Volcanic Action. 



According to the views expressed in modern text-books of geology, 

 the emission of incandescent matter at the earth's surface takes place 

 either in the form of fissure eruptions or in the form of central erup- 

 tions. The writer believes that a third method should be entertained 

 as a possibility, namely, by the partial or complete foundering of bath- 

 olithic roofs. The relation of each of these three phases to abyssal and 

 satellitic injection may now be sketched. 



Fissure Eruptions (Massive Eruptions, Plateau Eruptions). 



The regional or greater lava-floods known to have emanated from 

 simple fissures in their underlying terranes range in date from the 

 pre-Cambrian to the present. Without exception they are, chemically, 

 of basaltic composition. As we have seen, such magma must be exotic. 

 It is a type of lava (extrusive magma) to which a secondary origin 

 cannot be theoretically attributed. Its abundance, its occurrence in 

 every continental and oceanic area, and its field relations to the other 

 chemical t}^es of magma, all point to the derivation of the plateau 

 basalt from a general substratum. Each flow thus represents an eff"u- 

 sion from an abyssally injected body which has not been modified by 

 assimilation of the earth's acid shell or of the sedimentary veneer. But 

 the very low original slopes of the flows (very often inclined at less 

 than one degree to the horizontal plane) and their correlative great 

 lengths, show that the basalt of fissure eruptions is notably super- 

 heated. Such temperature is appropriate to assimilation. 



That solution of pre-Cambrian gneisses or of other rocks has gener- 

 ally not taken place in sensible amount during fissure eruption must 

 have either of two meanings. It may mean that the various abyssal 

 injections underlying such a lava-field are narrow, with widths to be 

 measured in meters or tens of meters, but not in thousands of meters ; 

 or the failure to assimilate may be due to special rapidity of injection 



