IGNEOUS ROCKS. 281 



The intrusions of trachyte in the slates usually form dome-shaped 

 peaks, having penetrated or forced to one side the superincumbent strata 

 of Potsdam and Carboniferous. Though in this region trachyte forms in 

 places quite extensive ridges connecting the peaks and covering large 

 areas, no narrow dikes were anywhere observed traversing the slates, and 

 this occurrence of eruptive rocks in dikes so common elsewhere in the 

 Rocky Mountains seems to be wanting in the Black Hills. 



The time of this eruption of igneous rocks appears to have been coeval 

 with the elevation of the Black Hills at the close of the Cretaceous period; 

 the intrusion of feldspathic porphyry, forming Crow Peak, has upturned all 

 the different formations from the Potsdam to the Jurassic; the Cretaceous, 

 having been removed by erosion, is not seen in the vicinity. These igneous 

 rocks are very varied in appearance and character, but may be generally 

 classed either as trachyte or feldspar-porphyry, though merging in all 

 gradations into each other, so that an exact classification is extremely dim- 

 cult, Greenstone occurs both as an olive-green rock without any traces of 

 crystallized minerals and breaking with a fracture-like jasper, and also with 

 small feldspar-crystals scattered through its mass forming a greenstone- 

 porphyry. The rock composing Terry Peak seems to be wholly com- 

 posed of white feldspar, an aggregation of small crystals of that mineral, 

 resembling a fine-grained granite, without quartz or mica Custer Peak is 

 formed by the intrusion of a peculiar massive gray rock, with an exceed- 

 ingly close and uniform texture, resembling a compact gray limestone in 

 appearance more than an igneous rock. Black Butte, a high peak on the 

 east side of Spearfish Canon near its mouth, is made up of a genuine 

 trachyte. 



Near Camp Terry, a few miles north of Custer Peak, the prevailing 

 rocks are garnetiferous mica-schists, resembling the rocks of the French 

 Creek district. This occurrence of an area of schists in the clay-slate 

 formation is not easily explained. Possibly the irregular line of contact 

 between the two divisions of the Archaean may here extend a greater dis- 

 tance to the east. Among these mica-schists on the head branches of the 

 east fork of Bear Butte Creek the soldiers found gold in the gravel 

 deposits of the side gulches of the stream, and a piece weighing about half 



