192 GEOLOGY OF THE BLACK HILLS. 



of the 49th parallel, describes the Sweet Grass Hills as having a similar 

 peaked structure, around which the sedimentary strata, the Lignitic Tertiary, 

 have a similar quaquaversal dip.* The rock also of these peaks, a " rhy- 

 olitic trachyte-porphyry", passing into " sanidin-trachyte", seems from the 

 descriptions to be similar to the igneous rocks of the Black Hills. 



The peaks have so little variety that it will not be profitable to describe 

 them individual^. A detailed account will be given only of the principal 

 ones, viz: Custer Peak, Terry Peak, Bear Peak, Crow Peak, Inyan Kara, 

 Sun Dance Hills, Warren Peaks, Bear Lodge, and the Little Missouri Buttes. 

 . Custer Peak stands at the eastern margin of the Carboniferous plateau, 

 just within the area of the slates. It is a symmetrical, conical mass, and 

 is a prominent landmark in the northern region of the Hills. Its height 

 above the valley at its base is 675 feet, and its elevation above the sea is 

 about 6,950 feet. It is very little higher than the neighboring edge of the 

 plateau. The mass of the peak is so covered with fragments of the rock 

 that neither its intimate structure nor its relation to the prevailing rocks 

 of the region could be well determined. The slopes of the sides show 

 only a loose mass of angular fragments, over which the ascent was not a 

 little difficult. On the extreme summit the rock is exposed in a Y-shaped 

 ridge, with the stem of the letter bearing north 35° west. The rock has so 

 perfect a vertical cleavage, following approximately the bearing of the 

 ridge, that it might readily be taken for a bedded deposit ; and in its close 

 texture, uniformity, and color it so closely resembles the limestone of the 

 Carboniferous, that it might almost be so mistaken on a superficial examina- 

 tion. Close inspection, however, reveals its igneous nature. It is a rhyo- 

 lite of a light bluish gray color, and a fine-grained and uniform structure, 

 containing occasional crystals of sanidin and bright black hexagonal tables 

 of biotite. 



To the east and northeast of Custer and within a short distance are 

 two small peaks which appear to consist of igneous rock, but they were 

 not examined closely. With this exception no other igneous points occur 

 until the Terry Peak region is reached, about eight miles to the northwest. 



* Geology and Kesources of the Eegion in the Vicinity of the 49th Parallel, by G. M. Dawson, p. 

 122 et seq. 



