COLOEADO ARCH^AN BODY. 23 



still be considered as granites. The granite of the summit is a typical one. 

 It occurs inclined to the east at a low angle. It is coarse crystalline rock, 

 of a light gray color and somewhat friable texture. Quartz and ortho- 

 clase-feldspar are the prevailing minerals; tri clinic feldspars are rare and 

 quite small. The quartz is of a dirty grayish- white color, while the ortho- 

 clase is pearl-gray, with a highly vitreous lustre. The mica is not abun- 

 dant, but appears to be biotite in brilliant dark plates scattered through the 

 rock, occurring in small segregated bunches and in minute scales adhering 

 to the broad faces of the orthoclase crystals. Under the microscope, Pro- 

 fessor Zirkel has detected, both in the quartz and feldspar crystals, long 

 prismatic needles, which, from their crystallographic properties, he deter- 

 mined to be tremolite, although, to the unaided eye, no hornblende is visible. 

 Apatite was also observed microscopically, and the quartz crystals carry 

 fluid-inclusions. The same rock was examined chemically by Mr. R. W 

 Woodward with the following results: 



Sihca -. 73.40 



Alumina 14.53 



Ferrous oxide 2.25 



Lime 0.62 



Magnesia 0.56 



Soda 2.63 



Potassa 5.35 



Lithia trace 



Phosphoric acid , . . . . 0.22 



Water 0.53 . 



100.09 

 Specific gravity, 2.61, 2.77. 



This analysis is interesting, from the close resemblance to the analyses 

 of the Archaean granites of the Laramie Hills, which is the more striking, 

 as they differ from most of the granites in the ranges to the westward. The 

 detection of phosphoric acid in sufficient quantity for estimation would indi- 

 cate that the determination of microscopical apatite was a correct one, and 



