108 DESCRIPTIVE GEOLOGY. 



mica-gneiss and hornblendic rocks characterize the ridge. The purely 

 mica-bearing beds show a close resemblance to the white gneissic rocks 

 ah'eady described as occurring near the mouth of French Creek and on the 

 summit of Deer Mountain. A specimen taken from the north end of the 

 ridge is a white or silver-gray gneiss, made up of translucent colorless 

 quartz, bright pearl-colored feldspars somewhat altered, and a small 

 amount of brown mica disseminated through the rock, but also segregated 

 in bunches and nodules. These beds also contain minute red grains of 

 garnet. 



On the eastern and northern slopes, hornblendic beds prevail, usually 

 close-grained, with the hornblende and feldspars sometimes intimately 

 blended, at others separated by layers of either mineral more or less pure. 

 These layers or bands vary in width : they are frequently lenticular beds 

 of irregular shape, which give the hills and escarpments a highly-altered, 

 metamorphosed appearance. South of the Laramie River, rocks of the same 

 general character prevail. At Sheep Mountain, the rocks are distinctly 

 bedded, and inclined toward the river. They are perhaps somewhat more 

 coarsely grained than those to the north. At the river, near the crossing 

 of the Laramie and North Park road, those of the high steep slopes seem 

 generally less compact, decomposing more readily, and in physical habit 

 seem to approach nearer to granite than the rocks already described. 



In addition to the larger rock-masses which constitute the great mount- 

 ain features, the Medicine Bow Range is characterized by the presence of 

 certain accessory minerals, which are of importance in comparing the series 

 of Archsean Rocks under consideration with those of the Colorado and 

 other ranges; and, although but little was accomplished in this line of 

 investigation, such facts as were noted should be mentioned. 



Narrow veins of quartz penetrate the mica and hornblendic schists and 

 gneisses. They are especially marked along the western foot-hills in the 

 neighborhood of Brush and Cottonwood Creeks. The quartz is usually 

 quite pure, accompanied occasionally by fine particles of magnetite, and 

 on Deer Mountain a vein was observed carrying small amounts of pyrites. 



Epidote occurs massive, associated with quartz veins, in the interior 

 of the range, in the neighborhood of the Laramie and North Park road. 



