198 GEOLOGY OF THE BLACK HILLS. 



6,600 feet above the sea, and has an elevation of 1,300 feet above the bed 

 of Inyan Kara Creek near by. The igneous mass of the peak occupies the 

 center of what in form resembles a crater, for separated from it by an annular 

 valley there is an encircling ridge or rim whose top is 500 feet below the 

 summit of the peak. This rim is formed by the Red Bed limestone rising 

 up from under the surrounding red ckiys at an angle of about 40° and 

 completely encircling the peak except at a narrow break on the northeast 

 side where the drainage escapes. The limestone wraps around the outer 

 slope of the peak like a cloak, conforming to all local changes in dip and 

 bending without fracture. The upper red clays of the Red Beds lap up 

 against this limestone, which appears as a breakwater raised against the 

 turbulence of the billowy sea of the Red Valley. On the inside of the rim 

 is the annular valley, surrounding the igneous nucleus and having a width, 

 from rim to center of peak, of from one-half to three-fourths of a mile. It 

 has evidently been formed by the denudation of the easily eroded strata 

 beneath the limestone. 



From the midst of this crater-like depression the peak rises so abruptly 

 that there is but one side with an easy slope for climbing. The summit is 

 a broad but very irregular area, whose larger dimension has a bearing 

 about 30° west of north and upon which the rock is well exposed. It is a 

 hard, highly feldspathic trachyte, and on weathered surfaces large and 

 well formed crystals of feldspar were seen in great abundance, giving the 

 weathered mass a porphyritic appearance. In mass it is notably mag- 

 netic. The rock shows well marked cleavage or jointing planes, nearly 

 vertical, in two series. The first runs toward the northwest and the second 

 toward the west, dividing the rock into prisms and producing a quasi-colum- 

 nar structure. 



Though on a much larger scale, the entire upheaval being probably 

 two or three miles in diameter, the peak has essentially the structure already 

 described in Crow Peak and Sun Dance Hills. Among the uplifted beds 

 surrounding Inyah Kara no strata were recognized excepting the Red Bed 

 limestone and the underlying, impure, reddish sandstone, and beyond the 

 immediate base of the outer slope no disturbance was indicated. Indeed, 



