28 GEOLOGY OP THE BLACK HILLS 



several of the cliffs of Tertiary rock near the ridge are curiously capped 

 with a small thickness of conglomerate of granitic pebbles derived from 

 the ridge. By these facts it is shown that the elevation of the butte was 

 prior to the deposition of the Tertiary strata. 



The elevation appears to be a monoclinal ridge, such as would have 

 been formed along a line of faulting where one side of the dislocated series 

 was elevated considerably above the other, or such as is called by Major 

 Powell, in his classification of mountain structure, a "monoclinal ridge of 

 displacement."* 



At the springs or headwaters of the Niobrara, twelve miles north or 

 northeast from Rawhide Butte, there is an elevation exhibiting the older 

 schists, the Potsdam, and the Carboniferous, as shown in the annexed figure. 



The dip of the strata is 20° eastward, and the strike about the same 



/4§lll§5S|§^«^ -.,.—.^ as ^ na ^ a ^ R awm( ie Butte. The 



S>>^^ ^^ .^t^^^ . -^^^s^^^vv^ 1 lowest member seen is a mica- 



12 3 4 5 



Fig. 2.— Section of Uplift at Headwaters of the Niobrara. CeOUS gneiss 01' schist (1), COntahl- 



2. Coarse, somewhat metamorphosed sandstones (Silu- m g large quantities 01 quartz and 



3. Reddish and whitish sandstones (Silurian?); 125 feet, feldspar and seams of feldspatllic 



4. White or grayish limestone (Carboniferous?). , m , n . r ,-\ • • 



5. Tertiary sands and clays. quartz. llie dip of the gneiSSIC 



strata, though exceedingly variable, is high, the strike having an easterly 

 course. 



Above the gneiss lies a coarse, somewhat metamorphic sandstone 10 

 feet in thickness, then 2 feet of a highly ferruginous sandstone, and then 

 50 feet of a whitish and reddish quartzite containing much chalcedonic and 

 agatiferous quartz. This is capped by 75 feet of reddish sandstone, which 

 from its character and position is referred to the Silurian, though no fossils 

 were found in it. 



Resting conformably upon the sandstone series is a small thickness of 

 white limestone, doubtless of Carboniferous age, while surrounding the 

 elevation are the nearly horizontal beds of the Tertiary, which probably 

 once covered it as a cloak but have been worn through by erosion expos- 

 ing the upturned strata. 



Again, from the summit of the divide near the headwaters of Old 



* The Geology of the Uinta Mountains, by J. W. Powell; Washington, 1876; pp. 16 and 22. 



