190 IRVING. 



the geologist to limit his observations to the periphery of the 

 hills and to the more extensive Algonkian exposures to the 

 east, he would have no suspicion of its existence. 



Erosion has, however, exposed the same complex of erup- 

 tives as that seen in the more closely studied eastern regions, 

 and it seems probable that we have here an important link of 

 connection between the intruded masses of the northeastern 

 portion of the Hills and the great eruptive center of the Bear 

 Lodge mountains. 



The area with which this paper is chiefly concerned is in- 

 dicated by the black rectangle on the accompanying sketch 

 map (Fig. 5) situated at the northern and narrower extremity of 

 the main Algonkian ellipse. It embraces a portion of the older 

 metamorphic rocks and that part of the Cambrian formation 

 with which are associated the siliceous gold ores. A small 

 area of Carboniferous limestone is included in the northeast 

 corner dipping down below the later sediments of Centennial 

 Park. On the west the area contains a large portion of the 

 Carboniferous plateau which lies between the Algonkian of the 

 main nucleus and the Nigger Hill uplift. 



II. TOPOGRAPHY. 



The topography of the district is intimately qonnected with 

 the general geological character of the Black Hills. The drain- 

 age is toward the north and the east, the largest and most im- 

 portant stream being Spearfish creek. This stream follows a 

 winding course with a general northerly trend and has carved for 

 itself one of the deepest and most precipitous cafions in the hills. 

 It crosses the district in the shape of a bow, in the southern 

 arm of which it flows toward the northwest, and then with an 

 easy bend takes a more northerly course. The latter is main- 

 tained until the river emerges from its deep canon and enters 

 the broad and open red valley which surrounds the hills. Into 

 Spearfish creek, from the west flow Iron and Little Spearfish 

 and many other streams, all having carved precipitous gorges 



