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Show GENERAL RESULTS OF COMPARISON. 125 only to the low percentage in the feldspar and augite, but also to an equally low percentage in the base. The high percentage in rhyolite and trachyte is due not only to the feldspar, but still more to the even higher percentage of silica in the base. If there has been segregation, it must, therefore, have affected not only the crystals, but the base even more than the crystals. Such a separation, therefore, does not seem explicable by supposing a precipitation of crystals. Gathering together now the threads of this comparison, we are led to the conclusion that the constitution of the eruptive rocks forbids the belief that the acid varieties, or even the intermediate varieties, can be primordial masses from vesicles which separated in a liquid condition from the original earthmass and remained liquid up to the time of their eruption. Chemical considerations of a cogent character lead up to the inference that primordial magma ought to possess a constitution similar to rocks of the basaltic group, though perhaps somewhat less ferruginous (?), and that it should be nearly homogeneous. And in general our inference from the nature and constitution of the volcanic rocks, from their great variety, from the localization of eruptive, phenomena, from the intermittent character of volcanic action, from the independence of the several vents, is that the lavas do not emanate from an earth-nucleus wholly liquid, nor from great subterranean reservoirs still left in a liquid condition "from the foundations of the world,"but from the secondary fusion of rocks, a part of which may have formed the primitive crust, while the remaining part consisted of deeply-buried and metamorphosed sedimentary strata. No doubt some cautious philosophers may regard this inference as specifying a little too minutely the locus of volcanic activityâ€"niore minutely than a rigorous deduction from known facts will permit us to regard as positively proven. But at all events there is one proposition which may be laid down with no small degree of confidence, and it is this: We must at least admit that the source of lavas is among segregated masses of heterogeneous materials. This arrangement would be well satisfied by a succession of metamorphic strata resting upon a supposed primitive crust of magma having a constitution approximating that of the basaltic .group of rocks. IT. The second general consideration has reference to the dynamical |
