CHAPTER III. 



THE IGNEOUS ROCKS OF ELECTRIC PEAK AND SEPULCHRE 



MOUNTAIN. 



By Joseph Paxson Iddings. 



GEOLOGICAL SKETCH OF THE REGION. 



As already stated in Chapter II, the series of eruptions that broke 

 through the synclinal fissure in what is now the eastern part of Electric 

 Peak are so plainly related to the lavas that form the volcanic pile of 

 Sepulchre Mountain and the foothills at its southwestern base, and the 

 character of this relationship is of such petrographical importance, that 

 these rocks will be treated conjointly. 1 It has been shown that the erup- 

 tions that accompanied the synclinal folding and Assuring in the eastern 

 part of Electric Peak were subsequent to the intrusion of the sheets of 

 andesite-porphyry between the beds of shale and sandstone. 



The general character and form of Electric Peak are exhibited in the 

 accompanying map and illustrations. The Peak is the highest point in 

 the Gallatin Mountains, being 11,100 feet in altitude, and is situated upon 

 the northern boundary of the Yellowstone Park, the forty-fifth parallel of 

 latitude passing just south of the summit. For this reason it is not well 

 shown on either of the atlas sheets north or south of this parallel. The 

 accompanying map (PI. XVT) shows its relation to Sepulchre Mountain, as 

 well as its geological structure, which has been explained on pages 50 to 55, 

 where the character of the sedimentary formations and their position and 

 the nature of the intruded sheets of andesite-porphyry were described. 



The sharply pointed peak has broad, steep slopes on the west and 

 south, where streams have cut 3,000 feet below the summit of the mountain. 



• Ridings, J. P., The eruptive rocks of Electric Peak unci Sepulchre Mountain, Yellowstone 

 National Park: Twelfth Ann. Rept. U. S. Geol. Survey, 1892, pp. 569-664. 



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