110 DESCRIPTIVE GEOLOGY. 



of orthoclase-mica-granites, and gneisses, holding but little hornblende, 

 with the feldspars characterized by a prevailing reddish color. 



On the. other hand, the Medicine Bow Range is formed largely of 

 plagioclase-hornblende and plagioclase-mica-bearing rocks. The feldspars 

 of the granites, gneisses, and schists are either composed of triclinic species, 

 or else they are present in such large proportions as to characterize the 

 rocks in which they are found. In color, the feldspars are either white 

 or of very light tints. Hornblendic gneisses and schists, which occur all 

 through the Huronian series, predominate over large geographical areas in 

 the Medicine Bow Range. They are usually compact and heavily bedded, 

 many of them so free from orthoclase as to deserve the name of dioritic 

 gneisses. The mica-bearing rocks differ entirely from those of the Colorado 

 Range. They are very light in color, with a crumbling, sandy texture, 

 and the mica usually present in subordinate amounts. A striking charac- 

 teristic of this entire series of Archaean rocks, whether loose or compact in 

 texture, is the banded and laminated appearance of the constituent min- 

 erals. Beds as hard and firm as any crystalline mass seem to show the 

 same parallelism in arrangement as the typical gneisses,- and it is especially 

 noticeable in the alternate bands of dark green hornblende and white 

 plagioclase-feldspars, which form the rocks that have been designated dio- 

 ritic gneisses. 



The other rock formations of the Medicine Bow Range which overlie 

 the gneisses and schists are perhaps still more characteristic of the Hu- 

 ronian age as developed elsewhere. At the same time, they have no equiv- 

 alent in the area of the Colorado Range included within the limits of this 

 survey. They consist of the argillites, clay-slates, hornblendic irides- 

 cent schists, quartzites, conglomerates, and limestones, as described above 

 with considerable detail. 



Intrusive rocks of later age, if we exclude some of the granite bodies 

 in the region of Mount Clark, seem to have penetrated this Archaean series 

 only in narrow dikes. They do not appear to have produced any marked 

 physical or geological changes in the leading surface features of the range. 

 So far as observed, they are limited to dikes, with sharp well-defined walls, 



