PARK RANGE. 141 



Chlorite was detected in the red orthoclase-granites, with an association and 

 habit similar to that observed in the granites of the Laramie Hills. 



Minute apatite crystals occur in the hornblende-gneisses of Davis Peak. 



The red orthoclase, structureless granites, poor in mica, of the Park 

 Range, bear a close analogy to the Colorado Range granites. The overly- 

 ing mica-gneisses are, in habit, also more closely allied to those of the Col- 

 orado Range than to the Medicine Bow series. They are characterized in 

 general by a somewhat compact texture, gray color, with broad mica flakes ; 

 while the Medicine Bow series have a much more friable and crumbling 

 nature, are white or silver-gray in color, and the feldspars more decomposed. 

 The mica is present usually in minute, thin plates. 



Hornblendic beds, however, recall the Medicine Bow series. They 

 are, indeed, the only rocks that indicate any very close analogy, and even 

 these have some points of difference. In the Park Range occur beds of 

 very fine-grained hornblendic gneiss, carrying considerable quartz, which 

 resemble the beds in the Laramie Hills, in the Sybille Canon, while the 

 finely-laminated rocks, made up of fibrous hornblende and bluish-white 

 plagioclase, which so characterize portions of the Medicine Bow Range, do 

 not appear to form a marked feature of the Park Range. On the other 

 hand, the massive white quartzites, argillites, iridescent slates, red con- 

 glomerates, and limestones are wanting in the Park Range. 



Intrusive rocks later than the Archaean series do not appear to have 

 penetrated the Park Range. At least, dikes or outbursts of such material 

 were not observed. This, however, is not remarkable, as they play a very 

 insignificant part in the other Archaean uplifts to the eastward. The later 

 Tertiary volcanic rocks may also be said to be absent from the interior of 

 the range, although, as already mentioned, they occur all along the west 

 flank. 



