DISTRIBUTION OF POTSDAM. 83 



Wherever the formation is exposed around the margin of the Archaean 

 area it is found to dip outward from the axial area of the Hills. On the 

 western side the Potsdam underlying the broad plateau or table-land of 

 Carboniferous limestone shows little dip along its outcrop, but along the 

 western margin of the plateau the dip of the overlying and conforming 

 limestone is away from the axis of the Hills or toward the west. In the 

 southern, eastern, and western parts of the Hills the dip of the fossiliferous 

 strata is always well marked, varying from 10° to 25° or 30° from the hori- 

 zontal. But while the dip of the Potsdam may vary from point to point, it 

 sustains always the same relation to the underlying slates and schists and to 

 the overlying Carboniferous series. It ever rests unconformably upon the 

 former, and it is found always to be entirely and perfectly conformable with 

 the latter. 



The representatives of these two great systems, Silurian and Carbonif- 

 erous, which in other parts of our country are separated by many thousands 

 of feet of deposits, recording aeons of time, are thus found in the Black Hills 

 in direct and conformable contact. 



Around Crow Peak, where the strata have been turned up at a high 

 angle by the extrusion of the trachyte which composes the mass of the peak, 

 the Potsdam stands nearly vertical against the mass of volcanic rock, and 

 has been transformed by the latter into a hard white quartzite. The lime- 

 stone of the Carboniferous rests conformably against it. In a similar rela- 

 tion the upturned edges of the Potsdam are found about the volcanic uplift 

 six miles northeast of Inyan Kara. 



Warren Peaks, the prominent points of the Bear Lodge range, consist 

 of trachytic rocks, and the volcanic force by which they were ejected has 

 tilted and exposed the various members of the fossiliferous series with the 

 Potsdam sandstone at their base. The Potsdam is here in many instances 

 highly metamorphosed, but bears the same stratigraphical relations to the 

 accompanying rocks. 



By a reference to the geological map accompanying this report the 

 exposures of the Potsdam sandstone will be seen to form a very narrow 

 band surrounding the Archaean area, and even this is somewhat exaggerated 

 in width so that the formation may be distinctly represented. Near Terry 



