PARK RANGE. 137 



especially hornblende and quartz, whicli are always present in varying quan- 

 tities,, in irregular-shaped bodies, are of common occurrence. 



Along the trail which leads up Grand Encampment Creek, across the 

 mountains to the westward, are found many excellent exposures of the 

 Archaean rocks. At the mouth of the creek occurs a gneiss, the mass of 

 which is largely quartz and feldspar, with the laminated structure very 

 irregularly developed. The mica, which is mostly biotite, seems to occur 

 in lenticular masses. The rock has little of the schistose structure of most 

 gneisses, and passes into a coarse-grained rock, in which large masses of 

 quartz and feldspar are enclosed in this gneissic material in a somewhat 

 similar manner to the segregations in the beds on Mount Zirkel. Above 

 this is a compact, dark-graj'' hornblendic rock, containing a slight admix- 

 ture of white feldspar, which gives it a porphyritic appearance. Still 

 farther up the creek occurs a large, interesting body of granite. It resem- 

 bles, in its mineralogical habit, many of the features of the granites in 

 the Colorado Range. It varies in texture and compactness, and is char- 

 acterized by reddish orthoclase and quartz, with but little plagioclase. or 

 mica, and probably no hornblende ; at least none was observed. The coarse- 

 grained granites are one of the most characteristic rocks of this region, and 

 show macroscopically what is usually only revealed by the microscope, 

 that the feldspar crystals enclose masses of quartz filling the fissures, which 

 often contain other feldspars, as well as well-developed crystals of mica. 

 In these coarse-grained beds, the crystals of flesh-colored orthoclase often 

 attain a size of several inches. The coarse rocks pass again into the opposite 

 extreme of granites, of which a specimen collected is a compact pinkish rock, 

 corhposed chiefly of flesh-colored orthoclase and translucent quartz, with 

 but little mica. Frequently, the mica is entirely wanting, or at least can- 

 not be detected by the unaided eye ; when present, it shows a tendency to 

 gneissic lamination, its crystals almost always lying in parallel planes, even 

 when irregularly disseminated. Zirkel has shown, in his examination of a 

 thin section under the microscope, that the red color of the feldspar is in a 

 measure due to fine laminae of oxide of iron in microscopical fissures in the 

 crystals; and that the mica is accompanied by a dirty green, strongly 

 dichroitic, chloritic-like mineral. The quartz of all these granites is very 



