BLACK HILLS GEOLOGY. 213 



can be fully appreciated. In going upward from the west, north 

 or ea.st, one passes over gently sloping strata of Carboniferous 

 limestone, which have an increasingly steep dip, until within 

 two hundred and fifty feet of the summit. A rounded bluff of 

 phonolite is then encountered, over whose crest one may 

 readily climb, and proceed up a decreasingly steep rise to the flat 

 top of the hill. From this point the mass presents a somewhat 

 unique topographic appearance, for it comprises two almost dis- 

 tinct, roughly triangular masses of phonolite. The broader 

 and flatter of the two lies to the east and is connected with the 

 more precipitous western mass by a narrow and almost dike- 

 like ridge of the same rock. From both of these masses in- 

 curving tongues of phonolite run out to the south circling to- 

 ward one another, so as to include and almost surround a large 

 southwardly inclined amphitheatre. The more westerly of the 

 two is the more pronounced. 



The most conspicuous feature of this enclosure is that its in- 

 terior boundary is exceedingly precipitous, and that the inner 

 cliff, notably at the western end, extends around so far as to run 

 in a direction almost parallel to itself It is broken away at the 

 center, and thus affords egress to springs that rise amongst the 

 the thick forest of Jack pines, which grow from the great 

 mass of sloping talus, and debris within. The steepness is not 

 confined to the interior of the basin, for the western arm forms, 

 on the outside, an exceedingly abrupt bluff, which rises almost 

 two hundred and fifty feet from the bed of Calamity gulch. 

 The great rough irregular columns of phonolite stand out 

 sharply in picturesque and rugged beauty against the sky. (See 

 Plate X.) 



Plate XL, from a photograph taken from the top of Elk 

 mountain to the southeast, will give an excellent idea of the 

 crater-like depression and the peripheral valley which surrounds 

 the hill on the south and east. 



If one now crosses Calamity creek to the top of the limestone 

 bench, at a point a little to the east of the Metallic Streak mine, 

 the slight, almost imperceptible westward dip of the limestone 

 may be seen to greatly increase as the phonolite is approached, 



