BEAR BUTTE. , 195 



consists of the uplifted sedimentary strata, which are elevated higher on 

 that side. The rhyolitic point is conical, with, however, a larger develop- 

 ment in a north and south direction, so that the summit is a ridge several 

 hundred feet in length. Along this barren ridge the rock crops out promi- 

 nently. It has a distinct 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 limestone is least influenced and surrounds the peak in a 

 gentle slope or bulge, 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 of 75° or 80°. The Potsdam is exposed at several places in the 

 canon, having the usual character, while under the Carboniferous against 

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

 phosed into a hard quartzite, though none of the other sedimentary rocks 

 appear to be in the least changed by proximity to the igneous mass. 



Bear Butte rises from the level of the Plains on the northeastern side 

 of the Hills about six miles from the edge of the foothills, and though its 

 total elevation above the sea is but 4,570 feet, its abruptness and its height 

 above the surrounding plain — 1,200 feet — make it a conspicuous landmark. 

 In structure it resembles Crow Peak, having a similar conical outline, and 

 being an upburst of trachy tic rock through the sedimentary strata, some of 

 which are distinguishable all around its base. As seen from the south it 

 is a simple cone, but from the east and west the summit appears as a 

 ridge several hundred feet in length, with a trend about north 40° west. 

 The summit ridge is narrow and the rock, as in Crow Peak, has a vertical 

 cleavage running with the length of the ridge. About the ridge are several 

 bluffs which expose the rock in place nearly to the base, while the inter- 

 mediate spaces, each like a cul-de-sac, are filled with loose fragments. 

 These fragments are small, angular, and flat, and are so loosely aggre- 



