DALY. — THE NATURE OF VOLCANIC ACTION. 



51 



erally, a minimum depth of many kilometers. Upon its back each of 

 these acid continental plateaus carries a sedimentary pellicle, locally 

 increased to the imposing thickness of geosynclinal prisms. (See 

 Figure 1.) 



Figure 1. Diagrammatic section of the earth-shells, showing abyssal injec- 

 tion and accompanying central eruption. The broken lines below the cone 

 represent a geosynclinal prism merging into sediments of more usual thickness. 

 Solid black represents solidified matter of the basaltic substratum, the broad 

 horizontal band indicating the part of the .substratum crystallized since an 

 early pre-Cambrian period. Heavy dots represent fluid matter of the substra- 

 tum, in i)o.sition after the aby.ssal injection has i)artly crv.stallized (along its 

 walls). Scale given by height of cone, assumed as projecting o kilometers 

 above the plain of sediments. 



Mo.st volcanic rock is of basaltic or andesitic composition, and it is 

 clear that such material has not been derived from the mere li(juefac- 

 tion of the basement terrane, or of the sedimentary veneer, or of both 

 together. The great bulk of the world's lava is exotic. It has peue- 

 trtiteil the ba.sement terrane and the .sedimentary .shell without absorb- 

 ing these in large proportion. We thus seem driven to believe in 

 ahj/ssiil/iasure.i, opened in the earth's outermost shells for the pas.sage 

 of the molten rock. 



By analogy such abyssal injection may be assumed as the preliminary 

 stage of Pacific vulcanism, although there is much to be said for the 

 view that the primary acid earth-shell is lacking, or else is very thin, 



