H. S. Washington — Solvsbergite and Tinyuaite. 177 



of blue hornblende crystals. The constituents are alkali- 

 feldspar, quartz and blue hornblende, with accessory titanite, 

 cordierite, apatite, and an undetermined mineral. 



The feldspar and quartz form a typically microgranitic 

 intergrowth, the former being far more abundant than the 

 latter. Both are xenomorphic, only occasional roughly lath- 

 like feldspars being visible. The feldspar shows often between 

 crossed nicols the moire appearance common in the anortho- 

 clases. Carlsbad twins are rather common, and some multiple 

 twinning lamellae are to be observed. The feldspars are per- 

 fectly fresh and clear, and contain, like the quartz grains, 

 scarcely any inclusions. 



The hornblende, which is the most characteristic component, 

 is scattered fairly uniformly through the rock, though there 

 are areas relatively rich or poor in it. The arrangement in 

 streaks has been already noted. 



The hornblende grains do not show crystal planes and are 

 irregular in shape, not acicular, as is generally the case with 

 these species. Prismatic cleavage at about 124° is prominent. 

 The color in ordinary light varies with the direction of the 

 section ; those cut perpendicular to 6 are greenish yellow, while 

 those parallel are dark or lighter blue-gray. 



The extinction-angle is small, measurements with a Ber- 

 trand's ocular varying from 4°-7° ; probably the latter figure 

 comes near the truth. Whether the extinction was positive or 

 negative could not be determined owing to the absence of 

 crystallographic planes. Pleochroism is very intense ; parallel 

 to 6 dark (almost opaque) blue gray, parallel to b the same or a 

 little lighter, and parallel to a pale greenish yellow. The de- 

 termination of the axes of elasticity was rendered difficult by 

 the intense coloration of the mineral, but a large number of 

 observations both with the mica plate and quartz wedge seemed 

 to show that the axis of least elasticity c lay nearest the verti- 

 cal axis. The absorption formula then would be c \ b ^> a. 



This determination, if correct, would indicate that the horn- 

 blende is glaucophane, and this view is sustained, as will be 

 seen later, by the results of a chemical analysis of the rock, 

 from which it appears that the hornblende is a mixture of 

 ferrous glaucophane and riebeckite molecules in the ratio of 3 

 to 2. 



Titanite occurs in colorless grains of fair size, with the usual 

 characters, and calls for no special comment. 



There also occur some very small, highly irregular grains of 

 a clear, doubly refracting mineral, either colorless or very 

 pale violet, in the latter case pleochroicto a scarcely noticeable 

 extent. The refractive index is not high and the double 



