ELECTRIC PEAK. 55 



end of this spur, the rocks being exposed in a small hill on the south- 

 western side of the stream, the channel having probably been deflected at 

 this place by the accumulation of glacial drift tilling the old valley 

 between Sepulchre Mountain and Little Quadrant. A detailed section 

 of the beds exposed on the crest of this ridge has been given. The 

 strike of the beds varies somewhat in ascending the spur, and the dip 

 also changes. The Dakota ledge, which crosses the ridge about 500 feet 

 above the river, has a strike of N. 32° E., showing a considerable change 

 in direction between the exposures here and those in the river bed. The 

 axis of this synclinal fold and the fault plane are found high up on this 

 southeastern spur, where the Colorado shales have a vertical dip. The 

 exact position of the fault plane can not be determined, and the extent of 

 displacement is not known. That the intrusive sheets of igneous rock 

 antedated the folding is clearly shown in the crushing and slight dynamic 

 metamorphism observed here. Slickensides are found within the porphyry, 

 and dragging planes are observable between the hard eruptive rock and 

 the soft shales. A cone-in-cone structure of the latter rocks is also 

 observed. On the other hand, the dikes of igneous rock which cut this 

 spur of the mountain show no sign of disturbance attributable to the 

 synclinal folding. They intersect sedimentary beds, and also the inter- 

 calated sheets of porphyry, at various angles. Where the shales and 

 intrusive sheets are on edge, the dikes are often parallel to them and are 

 easily confused with the intrusive sheets, which they closely resemble in 

 petrographical character. These dikes are connected with a large body 

 of igneous rock, mostly a diorite, which occupies a position on the line of 

 faulting and is situated in the deep gulch cut in the eastern summit of 

 Electric Peak. The sedimentary beds in the neighborhood of this mass 

 of igneous rock are extensively metamorphosed. 



At the eastern base of Electric Peak a profound fault separates the 

 mountain mass from the complex body of volcanic tuff-breccias and massive 

 igneous rocks to the east. This fault is the northern continuation of the 

 Gallatin fault, which has given rise to the abrupt escarpment faces on the 

 east side of the Gallatin Range. 



On account of the special importance of the relations existing between 

 the igneous rocks of Electric Peak and those forming Sepulchre Mountain 

 to the east of the fault, a detailed description of the geology of this locality 

 is given in Chapter III, in which the petrology also will be fully discussed. 



