292 Lindgren and Melville — Sodalite-Syenite 



pure orthoclase. The analcite which penetrates the feldspars 

 is the cause of the lowering of the specific gravity ; a mechan- 

 ical separation of the two minerals is hardly practicable. 



The hornblende occurs as slender black prisms bordered by 

 ocP and ocPoo ; terminal faces are not seen. It is idiomor- 

 phic both against the orthoclase and the sodalite. The color 

 in thin section is a very dark brown, so dark in fact that in slides 

 of ordinary thickness many crystals are only faintly translucent. 

 The pleochroism is very strong ; the rays vibrating parallel to 

 c and b are both very strongly absorbed, producing a dark 

 brown color, while those vibrating parallel to the axis of maxi- 

 mum elasticity are less absorbed with a yellowish brown color, 

 sometimes showing a tinge in green. The axis of minimum 

 elasticity is inclined to the principal axis at an angle not ex- 

 ceeding 13°. In some places the brown hornblende is under- 

 going a peripheral conversion iuto a green modification with 

 an extinction up to 25°.* The specific gravity of the horn- 

 blende was found to be 3 - 437. From the characteristics men- 

 tioned above the identity with Prof. Brogger's barkevikitef 

 seemed highly probable and to confirm this a quantity was 

 isolated and analyzed. 









II. Barkevik. 





T. 



Square Butte. 



Analyzed by G. Flink. 



H 2 (above 



100°) 0-24 





Si0 2 





... 38-41 



42'46§ 



A1„U 3 





--- 17'65t 



11-45 



Fe.O, 





... 3-75 



6-18 



FeO 





... 21-75 



19-93 



NiO 





. . trace 





MnO 





... 0-15 



0-75 



CaO 





... 10-52 



10-24 



MgO 





... 2-54 



1-11 



Na a O 





. .. 2-95 



6-08 



KO 





._. 1-95 



1-44 











99-91 99-64 



The mineral differs from barkevikite, an analysis of which 

 is given under II, in containing somewhat less silica and alka- 



* The conversion of brown compact hornblende into a green fibrous modification 

 was first observed by Mr. G. F. Becker, in his "Geology of the Coinstock Lode," 

 Monogr. Ill, U. S. G. S., p. 36, Washington, 1882. The same change has been 

 noted by Mr. F. Becke and Prof. G. H. Williams, Bull. 28, U. S. Geol. Survey, p. 

 45. 



f Z. f. Kryst, u. Min., xvi, p. 412. 



X The titanic acid in (I) is contained in the alumina. The rock carries - 29 

 per cent TiO« and contains 23 per cent, hornblende ; hence, there being no other 

 titanium minerals present, the Ti0 2 in the hornblende may be calculated at 1-26 

 per cent which would reduce the alumina to 1639 per cent. 



§ With some Ti0 2 . 



