412 WEED AND PIRSSON — HIGHWOOD MOUNTAINS OF MONTANA. 



they do not act on polarized light. From these facts we believe them to 

 be of glass. 



Sometimes the orthoclase is colored a pale brownish tone by a fine 

 dusty pigment. It shows in some places a slight tendency to kaoliniza- 

 tion and in some others is discolored by the alteration of its interpositions, 

 but usually it is quite fresh. The angle of the optic axes is variable, 

 generally small and sometimes nearly zero. 



It sometimes shows intergrow^n patches of a feldspar which has a 

 higher index of double refraction and is believed to be anorthoclase. 

 In a few cases a tendency for orthoclase laths to group themselves in 

 radial spherulitic forms starting from a common center were observed; 

 since the laths are broad and coarse it does not present a striking feature. 

 Again, in other places the patches of orthoclase filling adjoining areas 

 between augites and olivines have the same optical orientation over some 

 distance, thus presenting a rude poikilitic effect. 



Plagioclase. — A triclinic striated feldspar is also present, but in no con- 

 siderable amount. When the rock powder is placed in the mercuric- 

 iodide solution, and the ferro-magnesian minerals, the magnetite and 

 apatite have fallen out, no feldspathic materials are deposited until a 

 specific gravity of 2.60-2.61 is reached. At this point a very small pre- 

 cipitate is obtained of a feldspar insoluble in HCl. Subjected to qualita- 

 tive analysis it is found to be free from lime and gives abundant reaction 

 for soda. It is therefore albite, which agrees with the optical character 

 of the minerial in thin-sections, the extinction on either side of the albite 

 twinning plane reaching a maximum of about 15 degrees. The study 

 of this striated feldspar has shown that certain crystals possess remark- 

 able properties. Thus the twinning lamellae, which are very narrow, 

 can be seen in many cases very distinctly in ordinary light without 

 using the analyzer, some of them possessing a higher refraction than 

 others. Between crossed nicols it is seen that crystals possessing this 

 peculiarity have no position of equal illumination, but the lamellae can 

 be seen in all positions. It must be, therefore, that these lamellae possess 

 a different chemical composition from those adjoining them, and since 

 lime is excluded they must represent intergrowths of albite and anor- 

 thoclase of varying composition, joined after this singular manner. 



Recently Federoff* has called attention to similar intergrowths of 

 twin lamellae of different composition in the lime-soda feldspars and the 

 same phenomenon had been studied and noted previously by Michel- 

 Levy.t 



Nepheline. — The presence of this mineral is only indicated by the fact 

 that the powders falling between the specific gravities of 2.55 and 2.60 



*Zeit. fur Kryst : vol. 24, Heft 1 and 2, 1894, p. 130. 

 t Mineraux des Roches : Paris, 1888, p. 84. 



