BLACK HILLS GEOLOGY. 219 



Terry Peak Laccolite. 



Terry Peak has been described by Newton as " the crowning 

 point of an igneous region of considerable extent, having a max- 

 imum development in a northwest and southeast direction . . . 

 Its sharply-pointed summit is the most conspic- 

 uous landmark in the northern region of the hills, and is visible 

 from the plains far to the north and east. Its altitude by mer- 

 curial barometer is 7,230 feet, and there are but two summits 

 in the hills that outrank it. It rises fully one thousand feet from 

 its base." 



If we stand at the summit of the mountain the topographic 

 features may be readily observed. The highest portion com- 

 prises a sharp, rather circumscribed, conical mass, falling ab- 

 ruptly for two hundred feet to a broader, more gently sloping 

 portion on the north and southwest, but connected by a steep 

 ridge to a separate and rather conspicuous ridge to the south- 

 west. The slopes of this conical crest are so thickly strewn 

 with talus that the actual contact with the Cambrian shales is 

 completely obscured. Toward the southeast runs a long 

 sloping ridge capped by rounded knolls, and which connects 

 the slope of the mountain, with Deer mountain to the south. 

 To the northwest an extension of the same ridge connects the 

 mountain with the phonolite peak southwest of Green moun- 

 tain. On the northeast the mountain falls with an even 

 slope in a series of parallel ridges to the comparatively flat 

 country of Ruby basin. On the southwest a broad ridge 

 connects the crest of the mountain with the limestone flats, 

 which separate the gulches of Raspberry and Lost Camp creeks, 

 and into whose headwaters is a very steep fall of eight hundred 

 feet. 



As one ascends the mountain from the north, prospect holes 

 have revealed, here and there, exposures of Cambrian shales 

 whose dip gradually increases as we approach the crest of the 

 mountain, until at a point of some two hundred feet or less 

 from the summit, immediately adjoining the talus and debris, it 

 has attained an angle of something over twenty degrees. From 

 this point on to the top of the mountain, and far out onto the 



