GENETIC RELA TIONSHIPS AMONG IGNEO US ROCKS. 843 



Moreover, since the process of differentiation necessitates the 

 coexistence of differently constituted derived magmas in various 

 parts of the parent body or reservoir, the kind of magma drawn 

 off at an eruption will also depend upon the portion of the 

 reservoir drawn from. 



Third. If, in a given region of eruptive rocks, each body of 

 rock was the immediate solidification of the magma drawn 

 directly from one common reservoir, they would represent the 

 phases of differentiation in the parent magma at the time when 

 the eruptions took place. If, however, the magma drawn from 

 the reservoir did not solidify immediately, but remained in a 

 molten condition within the fissure or conduit, a still further 

 differentiation within this derived magma might take place 

 under conditions imposed by its new environment. In this 

 manner differentiation might proceed at quite different rates 

 and possibly with diverse results in the parent magma and 

 in the derived magma. Material, then, which, through sub- 

 sequent eruption, might come to a place where it could 

 solidify, might be derived from the parent magma or from 

 the derived magma, and would represent different phases 

 of differentiation. Either set of conditions of eruption may 

 exist in nature, and much more complex ones. The first may 

 very well be found in great fissure eruptions such as have taken 

 place in western America. The second are probably represented 

 by groups of volcanic vents. Both are simply modifications of 

 eruptive processes, and differ in no essential respect. 



The genetic relationship of rocks belonging to one center of 

 eruption, or to one group of centers, or to one petographical 

 province, makes plain the fortuitous character of so-called rock 

 types ; the constitution of any rock mass depending primarily 

 upon the phase of differentiation, and on the portion of the 

 reservoir let out. It proves the fundamental character of the vari- 

 ability in composition of such rocks, both as between different 

 bodies of rock and also within the mass of one continuous body 

 in many cases. The degree of homogeneity in a rock body will 

 depend upon the relation of its volume to that of the reservoir 



