CORRELATION OF THE EOCKS. 141 



The nature and distribution of tlie phenociysts in the different varieties of 

 andesite and dacite, which determine their megascopical habit, have their 

 exact counterpart in the different varieties of porphyries. The microscopical 

 characters of the phenociysts in the corresponding' varieties of porphyries 

 and of the intruded andesites and dacites are identical. The character of 

 the various groundmasses, however, is different in the two groups, being 

 more highly crystalline in the porphyries — many of the andesites being 

 glassy. Many of the finer-grained diorites have a habit, derived from the 

 distribution of the ferromagnesian silicates and larger feldspars, which 

 resembles that of some of the andesites and dacites that correspond to them 

 chemically. 



Finally, the study of the chemical composition of the intrusive rocks 

 of Electric Peak and of the volcanic rocks of Sepulchre Mountain proves 

 that these two groups of rocks have identical chemical compositions, for 

 the varieties that have been analyzed are but a few of the many miner- 

 alogical and structural modifications assumed by these magmas on cooling. 

 The analyses serve as indications of the range of the chemical variability 

 of these magmas 



From the geological structure of the region, then; from the corre- 

 spondence between the order of eruption of the two series of rocks; from 

 the resemblance of a large part of the rocks of both series, megascopically 

 and microscopically, and from the chemical identity of all the rocks of 

 both groups, it is conclusively demonstrated that: 



I. The volcanic rocks of Sepulchre Mountain and the intrusive rocks 

 of Electric Peak were originally continuous geological bodies. 



II. The former were forced through the conduit at Electric Peak 

 during a series of more or less interrupted eruptions. 



III. The great amount of heat imparted to the surrounding rocks was 

 due to the frequent passage of molten lava through this conduit. 



We have, then, in this region the remnant of a volcano, which has 

 been fractured across its conduit, faulted, and considerably eroded, and 

 which presents for investigation, on the one hand, the lower portion of its 

 accumulated debris of lavas, with a part of the upper end of the conduit 

 filled with the final intrusions, and on the other hand, a section of the 

 conduit within the sedimentary strata upon which the volcano was built. 



