MEDICINE BOW RANGE. 103 



varying from north 45° east to north 70° east, with a dip always to the 

 north, at angles frequently as high as 75° and 80°. 



The rock is chiefly made up of gray quartz, with both monoclinic and 

 triclinic feldspars, but so fine-grained that it is difficult to form any correct 

 opinion as to their relative proportions. There are also present what appear 

 to the naked eye as exceedingly fine particles of dark brilliant mica, but 

 which, under the microscope, Professor Zirkel considers as a chlorite-like 

 mineral. It is this mineral which gives the banded gneissic structure; and 

 although there is frequently but a small amount present, it is so evenly dis- 

 seminated that it gives a decided character to the strata. Many of the finer- 

 grained beds show a vitreous aspect, and where thickly bedded, or where 

 the bedding-lines are somewhat obliterated, suggest at first sight an impure 

 quartzite. 



In the specimens collected, hornblende does not appear to be present. 

 One taken from the summit near the geodetic station, a dark, hard quartz- 

 ose-looking rock, analyzed by Mr. R. W. Woodward, gave the following 

 result : 



Specific gravity, 2.6. 



Silica 72.72 



Alumina 15.16 



Ferrous oxide -.-.... 2.00 



Lime 2.44 



Magnesia :: 0.34 



Soda 4.03 



Potassa 2.43 



Lithia trace 



Ignition 0.73 



99.85 



This shows a marked difference from the analyses of the Archaean 

 series of i;he Colorado Range, and would seem to indicate that the feldspars 

 are mainly triclinic. The percentage of lime present is also much higher 

 than in the eastern range. * 



Medicine Peak is a mass of pure white quartzite, rising nearly 2,000 



