Page:The Journal of geology (IA journalofgeology11893univers).pdf/427

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GENETIC RELATIONSHIPS AMONG IGNEOUS ROCKS.
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Moreover, since the process of differentiation necessitates the coëxistance 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 subsequent 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 variability 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

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