LAEAMIE HILLS 13 



The following analysis was made by Mr. R. W. Woodward. 



Specific gravity, 2.6. 



Silica --- 71.54 



Alamina 15.04 



Ferric oxide . 2.45 



Lime • 1.40 



Magnesia 0.27 



Soda 3.15 



Potassa 5.27 



Lithia trace 



Phosphoric acid 0.07 



Loss by ignition 0.90 



100.04 



The main chemical differences between this variety from L-on Mountain, 

 and those previously given, is that the former stands a little lower in silica, 

 with a corresponding increase in bases, and the presence of some magnesia, 

 due to the biotite. 



Among the highly crystalline rocks of this Archaean series is an inter- 

 esting variety of gabbro, a rock the esssential ingredients of which are 

 labradorite and diallage. Its presence in the Laramie Hills, so far as 

 observed, is limited to the region east of Iron Mountain and of Chugwater 

 and Horse Creeks, where it forms Idw, irregular-shaped domes and knolls, 

 protruding through the granitoid rocks, which here show considerable variety 

 in lithological habit. The gabbro is of a bluish-gray color derived from 

 the prevailing labradorite crystals, which form by far the largest part of the 

 mass, much of it appearing like a coarse granular feldspar, but for the 

 finely-disseminated diallage. Much of the labradorite occurs in large broad 

 crystals, two or three inches in length, the cleavage-surfaces presenting a 

 highly vitreous lustre. The play of color belonging to labradorite is quite 

 brilliant, and the characteristic striae of triclinic feldspars frequently very 

 well developed. The larger and well-defined crystals show a tendency to 

 arrange themselves in parallel layers. Yellowish-green diallage is scattered 

 through the feldspar, but it occasionally forms thin layers and seams. As 



