392 Eggleston — Eruptive Rocks at Cutting sville, Vt. 



Ilornblende-biotite Syenite (2, figs. 1 and 2). Horn- 

 blende-biotite syenite is of smaller total area than most 

 of the other plutonic types. Its principal exposures are 

 near the crest of the central part of Granite Hill, where a 

 small quarry was opened about 25 years ago. 



The type phase, taken from the quarry (Q 2, fig. 1), is 

 moderately coarse-grained, almost white in ground color, 

 and abundantly specked with black amphibole and mica. 

 Occasionally a slightly darker-colored, finer-grained, 

 indistinct patch or schlier, a few centimeters in dimen- 

 sions, appears in the hand specimen. 



In thin section the hornblende-biotite syenite shows a 

 hypidiomorphic, medium to coarse granular texture. 

 Eighty per cent or more is feldspar, of which the bulk is 

 plagioclase, in composition intermediate between basic 

 oligoclase and andesine. About one-third of the feld- 

 spar is orthoclase, some of which is intergrown with 

 plagioclase. 



After the feldspars, amphibole is most notable. It 

 appears in well-defined prisms, commonly twinned. The 

 pleochroism is distinctive, green to greenish brown in the 

 direction of the c axis; dark greenish brown to almost 

 black parallel to the b axis ; and pale yellow parallel to 

 the a axis. The absorption scheme is b > (nearly =) c 

 > a. The extinction angle on (110) cleavage pieces is 12°. 

 2E = 70°— 80° or less. A calculation of the angle of 

 extinction on (010) gave 8°. The specific gravity, by 

 pycnometer, at 20° C., is 3-518. These characteristics 

 taken together indicate a soda-amphibole near barke- 

 vikite. 



In order to check the optical determination of the 

 amphibole, a chemical analysis of the amphibole was 

 made by Professor C. D. Test of the Department of 

 Chemistry of the Colorado School of Mines. Good mate- 

 rial was obtained by the usual modes of separation 

 from the rock powder. The analysis resulted in the 

 values given in Column I of the following table, in which 

 those for related amphiboles are also entered: 





I 



II 



III 



IV 



V 



VI 



Si0 2 



38-04 



38-633 



4010 



42-46 



42-27 



38-41 



Ti0 2 



106 



5035 



4-35 



(In SiO.,) 



101 



1-26 



A1 2 3 



13-50 



11-974 



10-88 



11-45 



6-31 



16-39 



Fe 2 3 



6-21 



3-903 



7-81 



618 



6-62 



3-75 



FeO 



15-85 



11-523 



9-66 



19-93 



21-72 



21-75 



MnO 



1-21 



0-729 



015 



0-75 



113 



015 



