BLACK HILLS GEOLOGY. 225 



Crow Peak. 



Crow peak has been described by Newton as follows : " Crow 

 peak is a pustular outbreak of volcanic rock through the Red 

 Bed limestone twelve miles northeast from Terry peak. Though 

 it does not rank as one of the highest points of the hills it springs 

 so abruptly from its immediate surroundings as to make it a 

 very conspicious point. Its approximate height above the Red 

 Valley in its vicinity is 1500 feet. As seen from the east or 

 west, it appears to be composed of two peaks closely united ; 

 the southern one is the rhyolite core, while the northern con- 

 sists of the uplifted sedimentary strata, which are elevated higher 

 on that side. The rhyolite point is conical, with, however, a 

 larger development in a north and southern direction, so that 

 the summit is a ridge several hundred feet in length. Along 

 this barren ridge the rock outcrops prominently. It has a dis- 

 tinct cleavage lengthwise of the ridge, and is divided thereby 

 into plates, which in some places are quite thin. 



The steep slopes of the sides are masses of loose and sliding 

 fragments. 



The rock is a light gray, compact, tough rhyolite. It has 

 been forced through the sedimentary strata, which, from the 

 Potsdam to the Red Bed limestone, are exposed around the 

 base, and are all more or less disturbed. The Red Bed lime- 

 stone is least influenced and surrounds the peak in a gentle 

 slope or ledge, while a long low swell or ridge extends for 

 several miles northerly into the Red valley, diminishing and 

 finally dying out entirely. Within a few hundred feet of the 

 peak the Carboniferous is seen in a canon lying nearly horizontal, 

 while it laps up against the base at an angle 75 or 80 degrees. 



The Potsdam is exposed at several places in the canon, hav- 

 ing the usual character, while under the Carboniferous against 

 the peak it stands vertical. It has been more or less completely 

 metamorphosed into a hard quartzite, though none of the other 

 sedimentary rocks appear to be in the least changed by prox- 

 imity to the igneous mass." 



To this description the present writer can suggest no important 



