GRANITE. 47 



biotite is absent it is lUie to a later alteration of the rock. In color tliey 

 are gray or very frequently of a reddish tinge. The red color is sometimes 

 very marked, and certain varieties ai"e fully as fine in color as the famous 

 Aberdeen granites. As an exceptional color is also found a reddish-yellow, 

 due apparently to hydrated oxides of iron. 



Those which it has been thought might be of eruptive origin are gen- 

 erally fine-grained, of gray color, and contain an abundance of biotite, 

 whereas those which are distinctly metamorphic are generally coarse- 

 grained, often red in color, and have a porphyritic structure owing to the 

 prevalence of large twin crystals of orthoclase. Surfaces of the latter type 

 often show such parallelism and rectangularity in the disposition of the long 

 narrow prisms of orthoclase as to present a superficial resemblance to the 

 so-called graphic granites. These coarse-grained metamorphic granites, 

 especially when found in the immediate vicinity of the overlying sediment- 

 aries, have sometimes a foliated structure approaching that of gneiss, but 

 the direct passage of granite beds into gneisses was not observed. As typ- 

 ical granites of the former or eruptive class, may be mentioned that found 

 in the Platte Valley, north of Mount Lincoln; in Democrat Mountain, at the 

 head of Buckskin gulch; and along the western slope of the main crest, 

 opposite the head of Mosquito gulch. 



Of the second class typical forms are found at Bartlett Mountain and 

 along the Arkansas Valley, which are distinguished from the former by the 

 development of orthoclase in tabular twins, following the Carlsbad law, 

 porphyritically distributed throughout the rock. That found at Leadville, 

 generally in large erratic bowlders, and which has been considerably used 

 as foundation stone, is a remarkably beautiful rock, the orthoclase having 

 a delicate flesh-red tinge, while the groundmass, if such it may be termed, 

 is a bright, clear-gray mass, rich in dark mica. 



The finer-grained granites of a deep blood-red color were observed on 

 the ridge between Empire and Weston gulches, and also in the valley of 

 Eagle River, opposite Tennessee pass. Yellow granite was found also in 

 the last-mentioned locality and on the summit of Weston's pass. 



In addition to the above are masses of secondary origin, which occur 

 in the form of huge white veins of extremely irregular outline, to which, in 



