MEDICINE BOW EANGE. 101 



From Brush Creek northward for 15 or 20 miles, the same light-colored 

 mica-gneisses and dark hornblende-schists, with occasional interstratified beds 

 of vitreous quartzite, prevail. They form the higher ridges, as well as tlie long, 

 gentle slopes that extend out toward the North Platte, until hidden under 

 the later Tertiary sandstones. The geological relations of the beds are 

 very complicated, varying greatly in both strike and dip, and but little could 

 be made out as to their structure. South of Brush Creek, the prevailing 

 strike appears to be northwest and southeast, with a southerly dip, while to 

 the north of the creek the strike remains the same, with a northerly dip, 

 thus indicating, in a general way, a broad anticlinal axis. The summit of 

 Deer Mountain, at the head of Cedar Creek, is made up of the very light- 

 colored gneiss, similar to that already described fi^om French Creek, except 

 that it contains even less mica than the former, and the feldspars are unu- 

 sually white and clear. Minute grains of red garnet were found in the rock, 

 100 feet from the summit. On the slopes of the mountain, hornblendic 

 beds occur, largely developed, with the dark green hornblende, and both 

 white orthoclase and plagioclase-feldspars, but they scarcely require mention 

 here, except to note the large number of accessory minerals revealed in a thin 

 section b}'' the microscope, which include titanite, zircon, apatite, and an 

 undetermined chlorite- like mineral. 



From Deer Mountain northward, for 6 or 8 miles, the long slopes and 

 ridges that jut out toward the valley afford excellent opportunitj^ for observ- 

 ing and following the hornblende-gneisses that form the western foot-hills. 

 A recorded strike gave north 40° west, with a dip of 60° southwesterly. They 

 vary from fine-grained up to heavy coarse beds, and from beds carrying but 

 little quartz to those where the groundmass is largely composed of siliceous 

 grains. The rock from the slope of Cedar Mountain bears a close resem- 

 blance in its external features to the diorite-dike already described from 

 French Creek, although the former is undoubtedly a metamorphic product. 

 It is a hard, compact rock, with no regularity in the arrangement of the 

 constituents. Unlike the diorite from French Creek, however, the ground- 

 mass is composed of quartz and dark-green hornblende, while the feldspar 

 is very minutey striated plagioclase. North of Cedar Mountain, our obser- 

 vations tend to show that the character of the rock changes, and the dark 



