210 IRVING. 



level table-land below. On the northeast of the hill is a short 

 and quite deep gulch draining to the northwest and uniting with 

 Whitetail gulch just below the horseshoe loop of the Black 

 Hills and Fort Pierre R. R. 



The general shape of the hill is that of a rather flat cone, 

 with a sharp crest, steep on the north and west, but buried in 

 sediments, both east and south. As laccolites go, it is extremely 

 small, scarcely attaining a maximum diameter of three-quarters 

 of a mile. 



On the west side of Whitetail gulch the country falls gently 

 from the rather flat region south of the town of Terry, but is 

 interrupted some 50 feet from the bottom of the gulch by low 

 walls of phonolite, which have an irregular castellated scarp and 

 slightly increase the steepness of the descent into the bed of the 

 stream. This same slope is broken by the incision of two quite 

 prominent, parallel gulches, through the most southerly of 

 which passes a spur of the Deadwood Central R. R., entering 

 Whitetail just below the " Ruby Bell " mine and Stewart gulch, 

 slightly north. 



Geologically, Sugar Loaf hill is situated on the northern 

 border of the Cambrian escarpment. To the north is the broad 

 expanse of Algonkian slate, a long tongue of which formation 

 runs up the bed of Whitetail to the mouth of Stewart gulch. 

 Above this, and lying horizontally, are some 40 to 50 feet of 

 Cambrian quartzite and shales ; over this in turn is the phon- 

 olite of Sugar Loaf. The lower contact is best seen in the two 

 westwardly-heading gulches, and at a point a short distance be- 

 low the Union mine. If we now ascend the bed of White- 

 tail, the blocky columnar phonolite may be observed on both 

 sides of the gulch, extending uninterruptedly upward on the 

 east and north to the crest of the peak but passing on the op- 

 posite side beneath the overlying shales. The latter run out for 

 some distance on the tops of the little divides. Still further and 

 near the Union shaft the shales overlie the phonolite on both 

 sides of the stream, and the stream bed itself passes up into 

 that rock eight or nine hundred feet north of the bend in the 

 B. & M. R. R. From the east and south sides of the moun- 



