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 tbe 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 assump- 

 tion that the proximate cause of eruptions is a local increment of subter- 

 ranean 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 vol- 

 canic heat. But the most refractory metamoi'phic 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 quai'tzites, limestones, &c, are never erupted, be- 

 cause 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 rhyo- 

 lites, since these lavas bear no evidence of being heated beyond a tempera- 

 ture 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 supervision. 

 Thus, in the comparison of the two classes of rocks, we ma} r discard from 

 consideration those of simpler constitution, like quai'tzites, 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, indi- 

 cates in many ways and argues strongly for a common parentage. The 



