50 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Electric Peak. The shales are cut by numerous intrusive sheets of 

 andesite-porphyry and by a few dikes, one of which cuts the 10,100- 

 foot peak already noted In the sandstone bed intercalated in the lower 

 part of the Benton shales the little oyster, Otstrea anomioides, occurs 

 abundantly. The beds here have a dip of 10° N., the strike being N. 

 75° E. The andesitic sheets noted on this ridge are continuous for long- 

 distances, their ledges being traceable along the slopes on either side of 

 the ridge. In general, the andesitic sheets, being less easily eroded than 

 the soft black shales, form the high points and mountainous summits of the 

 ridge, while the saddles are cut in the softer rocks. 



ELECTRIC PEAK. 



Electric Peak is the highest and most imposing summit of the Gallatin 

 Range. Its apex rises boldly above the adjacent mountains, and the long 

 ridges which form its foundations dominate the country for many miles. 

 The mountain is composed of sedimentary rocks of Cretaceous age, broken 

 through and in part largely altered by igneous rocks. The sedimentary 

 rocks only will be treated here, as these igneous rocks and their occurrence 

 are of such interest that a special chapter is devoted to them. 



The sedimentary rocks composing the mountain embrace the most 

 recent strata of the sedimentary series to be found in the Gallatin Range, 

 including a thickness of 4,300 feet of Cretaceous beds, whose uppermost 

 portion is coal-bearing and belongs to the Laramie. Complete sections 

 may be studied at two localities. One is the southeastern spur of the peak, 

 where the beds are sharply upturned against the Gallatin fault; the other 

 is the long northern ridge of the peak which terminates in that mass of 

 upturned and exposed strata known as Cinnabar Mountain. Although the 

 latter locality lies just outside the limits of the Park, the section there 

 exposed is typical for the region, and, combined with the sections already 

 given of the Teton, Ellis, and Dakota formations, it forms a complete sec- 

 tion of the Mesozoic strata of the rang-e. 



Section of beds exposed on southeast spur of Electric Peal: 



Num- 

 ber. Feet. 



f 22 Carbonaceous shale 40 



Andesite-porphyry 40 



500 



^ > Carbonaceous shale 100 



o Andesite-porphyry 20 



Carbonaceous shale 300 



