BLACK HILLS GEOLOGY. 233 



Hill. (See Plate IX.) It is elliptical in outline, having its lon- 

 ger development in an east and west direction, and rises about 

 400 feet from its base. On the east and west it slopes quite 

 gradually down to the Carboniferous limestone, but on the south 

 by a somewhat steeper descent passes into the head of Elk 

 canon. These relations can be best understood from the model. 

 (Plate XIV.) On the top of the mountain is a ragged expo- 

 sure of quartz-aegirite -porphyry from which talus has fallen 

 and covered the upper slopes of the hill. Prospect holes have 

 revealed the limestone on both sides of the mountain at a point 

 170 feet lower in elevation than the summit. At various other 

 points shafts have been sunk, but all have been in the limestone, 

 and with the exception of one on the northern slope, and at a 

 very considerable distance from the top, have encountered noth- 

 ing but limestone. In this shaft two sheets of from 10 to 20 

 feet in thickness and with irregular contacts were struck at a 

 depth of about 75 feet. 



On the east of the ridge previously mentioned, and in the 

 bottom of Long Valley, extending out across the road, is an 

 irregular exposure of a rock similar in appearance to that on 

 the top of the mountain. It was not, however, examined under 

 the microscope. The only other exposure of porphyry near 

 the mountain is "that near the bottom of Calamity gulch, and 

 this is probably connected with the Ragged Top upheaval. 



On the east slope of the hill, at a point 300 feet below the 

 summit, quite extensive tunnels have been run. In them the 

 limestone is seen dipping at an angle of 20 degrees toward the 

 northeast. Excepting in this tunnel, no disturbance can be de- 

 tected in the strata that compose the mountain. 



It will at once appear that Elk mountain is not to be consid- 

 ered a typical laccolite. It is a comparatively thin capping of 

 porphyry (170 feet) on a mountain composed almost entirely of 

 limestone — a mountain which owes its existence to the protec- 

 tion afforded by the hard rock above, rather than to elevatory 

 forces acting from below. 



The sheets mentioned as occurring in Calamity gulch, on the 



Annals N. Y. Acad. Sci., XII, Nov. 25, 1899 — 15. 



