Vol. 4] Lawson. — The Robinson Mining District. 



345 



are grains of a blue, pleochroic mineral with an enormous re- 

 fractive power and a moderately weak double refraction. Its 

 maximum polarization tints range from yellow of the first order 

 to blue of the second order, while the orthoclase for the same 

 thickness shows gray blue of the first order. The grains show 

 extinction parallel to the direction of elongation where such 

 elongation can be recognized. The sapphire blue color is not uni- 

 form but patchy in its distribution, and the pleochroism is from 

 blue to colorless. The blue ray corresponds to the axis of less 

 elasticity. Surrounding the grains of this mineral is usually a 

 border of white micaceous mineral, evidently secondary, and 

 derived in part from the grains which it envelops. These char- 

 acters are those of corundum, and it thus appears that we have 

 here to deal with a corundiferous minette, a fact which is of 

 course suggestive of the possibility of finding a facies of the rock 

 which might yield sapphires of value. Those revealed in the 

 study of the thin sections are of microscopic dimensions ; but 

 inasmuch as sapphires have been found in similar rocks in Mon- 

 tana,* their discovery here is a hint which may some clay be of 

 service in the search for gems. 



Besides the minerals above described there is considerable 

 pyrite scattered through the rock in isolated crystals. 



• Chemical Composition. — The corundiferous minette, consti- 

 tuting as it does an important feature of the mine as a wall or 

 barrier limiting the distribution of the actually developed ore 

 body on the north, was subjected to analysis for the writer by 

 Mr. Ross with the object of settling as definitely as possible its 

 petrographieal character and its possible relations to the ore- 

 bearing porphyry. The results of the analysis are as follows : 



* U. S. G. S. 20th Ann. Ept., Pt. Ill, p. 554 et seq. 



