106 DESCRIPTIVE GEOLOGY. 



of quartzite, is probably the continuation southward of the Medicine Peak 

 beds, which, if the observed strike of north 20° east is correct, would follow 

 the trend of the creek ; and, in passing down the caSon, we follow along 

 the strike of the rocks. Now the argillites, slates, and fine schists observed 

 at the foot of the quartzite mass, near the summit, may be closely related to 

 the quartzose and mica slates noted farther down the stream, while the 

 recorded strike indicates that such is probably the case. Still farther down 

 the stream, and before leaving the rougher portions of the canon, the heavy 

 beds of light-colored mica-gneisses, which occur along the western ridges 

 of the range, make their appearance, and continue down to the mouth of the 

 canon. 



Mill Peak, at the head of Mill Creek, is the culminating point of the 

 eastern ridge, and is situated from eight to ten miles, a little north of due 

 east, from Medicine Peak. Here the prevailing dip of beds is to the west- 

 ward, as if forming a broad synclinal fold with the Medicine Peak group ; 

 the synclinal valley being occupied by the broad plateau, which separates 

 the two ridges. 



The eastern slope of Mill Peak is covered with a dense growth of tim- 

 ber, and loose soil, which conceal nearly the entire geological structure. 

 The rocks appear, however, to be made up almost exclusively of reddish- 

 white vitreous quartzite, closely resembling in physical and lithological 

 habit the Medicine Peak strata, but less free from foreign impurities, more 

 discolored by the decomposition of iron secretions, and carrying beds hold- 

 ing more or less feldspathic material. 



Not far below the summit, and directly overlying the white quarzite, 

 rests a body of heavy, compact, red conglomerate. It varies considerably 

 in texture, color, and character of the groundmass, and any description of one 

 locality seems hardly applicable to other beds. The lower beds, however, are 

 made up of pebbles and irregular-shaped pieces of both opaque and opalescent 

 quartz, chert, and hornstone of various hues, held together by a siliceous fer- 

 ruginous cement, making a mass which is exceedingly firm, and breaks with 

 great difficulty under the hammer. Some of the quartz pieces are 3 or 4 inches 

 in length, but penetrated by narrow seams filled with red ferruginous mate- 

 rial. There are layers which contain much less iron in the cementing material, 



