"WARREN. — ALKALI-GRAXITES AND PORPHYRIES. 219 



brownish often with a greenish shade. Absorption; 7<a>^. Sec- 

 tions over 0.03 mm. in thickness are practically opaque for the deeper 

 rays. Sections intermediate in position between the pinacoids often 

 show peculiar dull, bluish-gray or drab tones difficult to describe. 

 To the writer the most characteristic thing about the appearance of 

 this hornblende is the color for the ray very near the vertical axis 

 and the ray vibrating across the cleavage direction in (010) sections. 

 With high powers it can generally be seen that the distribution of 

 color for the a ray is not uniform. The strongest and purest blue 

 appears in the outer parts of the crystal, along cleavage or other cracks, 

 or in thin lamellae lying parallel to the c' axis. The remainder of the 

 crystal is green but assumes a bluish shade as the purer blue parts are 

 approached. But very slight, if any, non-homogeneity can be detected 

 for the other two rays. 



The other variety of hornblende present, particularly in the granite 

 from the western part of the granite area, where it appears to be nearly 

 always relatively abundant sometimes almost to the exclusion of the 

 blue type, seems to be closely related to the cataphorites of Brogger 

 and particularly to the sodic hornblende described by Pirsson ^^ as 

 occurring in the nephelite syenite of Red Hill, New Hampshire. 

 The pleochrism is strong and somewhat variable : a, light yellow-brown 

 often with a greenish tint; /3, dull green to almost black; 7, deep 

 olive-green or less commonly olive-lirown. The absorption is very 

 strong, /3>7> a. The angle, 7 A c' is large and is variable even in 

 crystallographically continuous crystals. This angle has been ob- 

 served to vary in a single grain from 20° to 32°, and these figures 

 represent variations commonly ol)served in the run of sections, al- 

 though extinctions as high as 37° have been noted. The variation 

 in the extinction angle is accompanied by a more or less marked 

 variation in the color of the ray. While in some sections the change 

 is marked l)y a fairly well defined zonal structure, the variations are 

 often distributed over ill defined areas and are often distinctly grada- 

 tional. Distinct crystals of the blue and green hornblendes seem to 

 occur in the same rock, but the two are commonly grown together in 

 parallel position (except in cases where the riebeckite is clearly 

 secondary) the riebeckitic or blue type being de^'eloped marginally. 

 The general mode of occurrence of the riebeckitic type in the granite 

 and its presence alone in the pegmatitic facies of the granite, seem to 

 point to the conclusion that it is the variety which develops when pneu- 



16 American Journal of Science, 23, p. 268 (1907). 



