INTRUSIVE EOCKS IN ELECTRIC PEAK. 93 



composition, in some places rapidly and quite irregularly. Moreover, there 

 are abundant evidences of the successive eruption through the fissure or 

 conduit of different molten magmas. The porphyries also differ from the 

 main body of diorite in the character of their phenocrysts — that is, in those 

 minerals which were present in the magma when it came to rest. Most of 

 the porphyries contain phenocrysts of hornblende and biotite, but none 

 of pyroxene. In some of the diorites there was an early crystallization of 

 brown hornblende and of pyroxene, but none of biotite. In most of the 

 diorites there is no evidence of any development of phenocrysts. They 

 were magmas free from crystals at the time of their eruption. 



In order to understand the relation of the various dikes to the stock 

 rocks, let us consider the possible course of events that would follow a 

 synclinal Assuring of sedimentary strata when the dynamical action was 

 repeated and when igneous magmas were forced up through the cracks. 

 Evidently the first magma would penetrate all the small crevices connected 

 with the larger fissures and fill them with its material, which would solidify 

 rapidly as narrow dikes. The magma in the large fissure would remain 

 molten much longer, consolidation setting in along the sides and in the 

 narrow portions. A subsequent eruption would force out the molten por- 

 tion and replace it by other material. It would fill any new crevices made 

 at the time of its eruption. The number of such crevices would probably 

 be smaller toward the end of the series of eruptions than at the beginning-. 

 Hence the number of dikes of the later magmas would be smaller. The 

 magma that closed the conduit would in such a case be represented by 

 few dikes. If the final action was a violent explosion, the reverse would 

 be true. At Electric Peak the final eruption was comparatively weak, and 

 is represented by a small body of quartz-diorite-porphyry within the stock 

 and in six or eight narrow dikes trending southwest. 



These rocks form a very complex group, so intimately connected 

 geologically and exhibiting such gradual transitions in composition and 

 structure from one extreme to another that there appears to be no simple 

 method of describing- them or of discussing their various relationships. For 

 convenience of petrographical description they will be treated in the follow- 

 ing groups : 



I. The greater number of dike rocks and some of the contact forms of 

 the stock, probably older than the main body of the stock. 



