140 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Table XV.— Order of eruption of the rocks at Electric Peak and Sepulchre Mountain. 



Succession of eruptions at Electric Peak. 



Succession of eruptions at Sepulchre Mountain. 



A. Intrusion of sheets of andesite-por- 



A. Extravasation of andesitic breccia from 



phyry from the southwest. 



some Arehean area. 



B. Intrusion of dike and stock rocks in 



B. Eruption of andesitic breccias and 



the following order: 



dikes in the following order: 



Pyroxene-porphyries, grading into 



Pyroxeue-andesites, breccia, and flows, 





passing into 



pyroxene- and hornbleude-diorites 



pyroxeue-hornblonde-andesites, brec- 



with biotite of final crystallization, 



cia, and flows, with dikes of similar 



with dikes of pyroxene- and horn- 



andesites, grading into 



blende-porphyries, grading into 





hornblende-biotite diorites with bio- 



hornblende-biotite-andesites, in dikes, 



tite of early crystallization, with 



grading into 



dikes of hornblende-biotite-porphy- 





nes j 

 quartz - biotite-diorite-porphyry with 



dacites with phenocrysts of quartz, 



some hornblende, with dikes of 



biotite, and some hornblende. 



quartz-biotite-porphyry. 





The igneous rocks that formed the breccias and lava flows of Sepulchre 

 Mountain, with their dikes and larger intruded bodies, constitute a series of 

 andesites, basalts, and dacites, which reach a degree of crystallization that 

 places part of them among the porphyries. They commenced with an 

 andesitic breccia that is filled with Arehean fragments, which must have 

 been thrown from some neighboring center of eruption located in an 

 Arehean area. Such a center exists a few miles to the north. This was 

 followed by a series of magmas that were at first somewhat basic and became 

 more and more siliceous. The series is represented in the right-hand column 

 of Table XV. From this it is seen that the succession of eruptions in each 

 locality was the same after the first period, A, in which the magmas evi- 

 dently came from different sources. Each series of the second period began 

 with basic magmas and ended with acid ones. Their division in the table 

 into four groups is not intended to convey the idea that they belong to four 

 distinct periods of eruption; the whole series in each case is, rather, a 

 single irregularly interrupted succession of outbursts of magma that grad- 

 ually changed its composition and character. Upon comparing the rocks 

 which have resulted from the corresponding phases of these series of erup- 

 tions, the similarity of the porphyrinic forms is immediately recognized. 



