138 DESCRIPTIVE GEOLOGY. 



poor in liquid-inclusions. It is an interesting fact that these microscopical 

 observations hold equally well in the examination of the red granites from 

 the Laramie Hills. 



The slopes of Davis Peak, southeast from Encampment Meadows, pre- 

 sent a series of hornblendic and micaceous gneisses, very similar in petro- ' 

 graphical habit to those found on Mount Zirkel. The micaceous rocks are 

 a very fine-grained admixture of black mica-flakes, reddish-gray orthoclase, 

 and white, limpid quartz. Scattered through the gneiss are numerous small 

 garnets of a deep-red color, mostly about the size of a pin-head, sometimes, 

 however, as large as a pea. Under the microscope, no hornblende could 

 be detected, and the quartz was poor in liquid-inclusions. Near the summit 

 of the peak occurs a characteristic hornblende-gneiss, in which the lamina- 

 tion is very regular. It consists of alternating white layers made up mostly 

 of plagioclase, and black ones in which hornblende predominates. As in 

 the mica-gneiss no hornblende was detected, so in this rock no mica is 

 visible. Under the microscope, minute apatite crystals are seen, while the 

 quartz shows interesting double inclusions of liquid carbonic acid. In this 

 gneissic series is sometimes seen the same zonal structure with alternating 

 black and white bands, frequently an inch or more in thickness, as already 

 noticed in the Mount Zirkel series. The very summit of Grand Encamp- 

 ment Mountain is formed of a mass of hard, compact, dark-green horn- 

 blende, which is almost entirely free from other constituents, but which 

 carries interstratified a bed of white micaceous quartzite; it may be classed 

 as an amphibolite. 



On both the east and west flanks of the range, the attitude of the beds is 

 very similar, and in their larger general features resembles the rocks of the 

 range already described. A few especially interesting characteristics should, 

 however, be mention ed. At the north end of the Park Range, just north of the 

 upper end of Jack's Creek, where the first Archaean rocks are found emerging 

 from the horizontal Tertiary beds which form the divide, occurs, interbedded 

 in the hornblende rocks, a remarkable bed of pure white quartz, some 50 

 feet in thickness. Such is the dazzling whiteness of its mass that it was at 

 -first mistaken, at a little distance, for a snow-bank. It is a vitreous struct- 

 ureless mass, of a milky white, and at times almost limpid quartz. A thin 



