| OCR Text |
Show 120 GEOLOGY OF THE HIGH PLATEAUS. » tallogenic force, for it continues to polarize light. This is the case with typical porphyries and with many trachytes and rhyolites. In the extreme varieties all traces of crystalline arrangement in the base have disappeared, and the inclosing matter is very similar to common glass, while the inclosed crystals are sharply defined within it. But while there is a sufficiently close agreement between the eruptive rocks on the one hand and some of the metamorphics on the other, there are many metamorphics which have very little in common with the eruptives. Such rocks as quartzite, limestone, dolomite, and argillite are never found in the eruptive condition. Here it is necessary to anticipate, in part, the course of the argument. The hypothesis to be invoked will consist in the assumption that the proximate cause of eruptions is a local increment of subterranean temperature, whereby segregated masses of rocks, formerly solid, are liquefied. Since a state of fusion is necessary to an eruption, we may throw out of consideration all those materials which are so refractory that they cannot be liquefied by temperatures within the highest range of volcanic heat. But the most refractory metamorphic or sedimentary strata are the very ones which have no correlatives among the eruptives; and, conversely, those strata which are most fusible have rocks of correlative constitution among the eruptives. Hence we may in part clear the way for the proposition that quartzites, limestones, &c, are never erupted, because they are infusible at the highest volcanic temperature. We have not, indeed, the means of directly measuring volcanic heat, but we may infer that it is never in excess of that required to melt the most refractory rhyolites, since these lavas bear no evidence of being heated beyond a temperature just sufficient to liquefy them. Rhyolites and trachytes bear strong internal and external evidence that at the time of eruption they were just fused and no more, while basalts often betray evidence of superfusion. Thus, in the comparison of the two classes of rocks, we may discard from consideration those of simpler constitution, like quartzites, dolomites, argil-lites, limestones, &c, and confine our discussion to those more complex, stratified masses which alone are fusible and, therefore, alone eruptible. Our comparison of the metamorphic and igneous rocks, therefore, indicates in many ways and argues strongly for a common parentage. The |
