BY W. G. WOOLNOUGH. 535 



The internal portions of the felspar give, in sections from the 

 zone perpendicular to (010), a maximum extinction angle of 

 32" for the inner zones and 26^ for the outer zones, that is 

 labradorite of varying composition. No sections parallel to 

 (010) were available to check this result. 



Augite of the first generation is not nearly so abundant as 

 felspar. Its distribution in the rock is somewhat irregular, as it 

 forms nests in some parts whilst some slices are almost without it. 

 In the zone of the vertical axis the sections are neatly bounded by 

 the usual faces, but at the ends of this axis they are irregular or 

 bounded by pyramid faces; their length in proportion to their 

 breadth is not great, so that the}^ form stumpy prisms. The 

 colour is light in greenish and j^ellowish tints, but is darker than 

 the augites hitherto described. The mineral is perfect]}^ free 

 from decomposition. Pleochroism is noticeable but faint, the 

 scheme being : a = yellow. 



b = yellowish-green. 



C ^ somewhat bluish-green. 



The difference of absorption of the rays is very slight. In 

 polarised light the usual (010) twinning is noticeable, though 

 far from common. The extinction in (010) is 46'' forwards. 

 Double refraction is more powerful than in the colourless augite 

 of the other andesites. 



The augite phenocrysts have suffered somewhat from corrosion 

 by the groundmass, though not to the same extent as that of 

 felspar. No difference in the character of the base in the imme- 

 diate neighbourhood of these corroded augites can be detected. 

 Interpositions with a marked tendency to zonal arrangement are 

 abundant, and include magnetite and apatite, and liquid-cavities 

 with moving bubbles; the smaller cavities have the form of 

 negative crystals. It is worthy of note that olivine does not 

 occur included in the augite phenocrysts. 



Porphyritic crystals of olivine of relatively small size are 

 somewhat sparsely distributed. Many of the crystals are beauti- 

 fully idiomorphic, but some have been more or less damaged or 

 even reduced to the condition of grains. It is perfectly colourless. 



