MEDICINE BOW EANGE. 99 



clase; the feldspars are nearly white, and the mica and hornblende black. 

 Under the microscope, apatite may be easily detected. 



Near the base of the same peak are found_ outcrops of coarse-grained 

 granite, which bear some resemblance to the granites of the Colorado Range, 

 and probably belong to the older series. They are so covered by morainal 

 detritus and soil, that their extent and relation to the surrounding rocks are 

 very obscure. They carry no hornblende, but small white mica flakes, 

 and are interesting for the large size and great activity of the liquid-inclu- 

 sions contained in the quartz. Near the last-described locality occur gran- 

 ites, also coarse-grained, with the feldspar in large masses, and the quartz 

 disseminated through it, producing the forms known as grapliic granite. 



North and west of the region just described, the rocks are almost 

 entirely mica and hornblendic schists and gneisses. Along the crest of the 

 ridge bordering the North Park, hornblendic rocks prevail, occurring both 

 coarse and fine-grained, with varying amounts of quartz and mica. Many 

 of them are characteristic diorite-schists. A typical rock, in the collection 

 from this lidge, is made up of dark green hornblende and bluish- white plagi- 

 oclase feldspars, many of them a quarter of an inch in length. A thin sec- 

 tion, examined under the microscope, gave no indication of any monoclinic 

 feldspar. Mica in black flakes and limpid quartz are present in very sub- 

 ordinate amounts. The mineral constituents are intimately mixed, and the 

 bedding-lines are only seen when viewed at a distance in the field. At 

 other localities in the same region, however, the green hornblendes never 

 show broad faces, but are always -present in long, narrow seams, or layers, 

 which give the rock a gneissic structure. The narrow, deep canon of the 

 North Platte, which extends for 30 miles along the western edge of the range, 

 everywhere exposes a series of dark hornblendic schists, with but little variety 

 in constituent minerals, but great difi'erences in the proportions of horn- 

 blende and feldspars present, the exposures occasionally showing beds carry- 

 ing but little of the iron mineral. The beds are inclined at varying angles, 

 most of those noted, however, standing very steeply. On the east side of 

 the canon, the prevailing dip appears to be easterly into the range. 



Near the mouth of French Creek, the canon of the Platte terminates, 

 and the east side of the river presents a low, undulating country, rising 



