BY W. G. WOOLNOUGH. 517 



high. The double refraction is also noticeably greater than 

 usual for a felspar. Observations in convergent light are not 

 very satisfactory; they appear to indicate that in sections parallel 

 to (010) an optic axis emerges just outside the field. This is 

 what ought to happen for bytownite, but the figure is so hazy 

 that it is impossible to be quite certain. 



Augite phenocr3^sts ai-e fairly abundant, though much less so 

 than felspar. They are Ijounded in the vertical zone by <{ 100 )- 

 and <j 010 \- very strongly developed, <{ 110 [- just noticeable, 

 and are terminated by <J 111 [- . The ordinary (100) twinning 

 is present in a large number of cases. The prismatic cleavages 

 are fairly perfect, but the rather uncommon cleavage parallel to 

 (010) and perpendicular to the trace of the twinning plane is 

 even more marked in niany cases. The colour by transmitted 

 light is very pale greenish-yellow. Faint as the colour is, a 

 pleochroism is just observable. Its scheme is : 



jl = very light yellow. 

 b= „ ,, green. 

 r= ,, ,, brown. 



The absorption is : it<b<C. 



The average size of the crystals is about f mm. long by ^ mm. 

 thick. 



The refractive index and double refraction are quite normal. 



Many of the crystals are distinctly corroded, and show a 

 marked resorption rim in which is developed some secondary 

 magnetite. The inclusions in the augite are not very numerous; 

 magnetite grains and crystals are by far the most abundant, but 

 a few very small apatite crystals also occur. In addition to 

 these there are a few indeterminate greenish plates which, when 

 edge on, appear like threads. 



The augite is practically entirel}^ free from decomposition, not 

 even a trace of chloritic material being observed, unless the plates 

 just referred to are of this character. 

 33 



