probable. Some lamellae however have nearly straight 

 extinction. In some slides, undubitable multiple twinning 

 of diallage is present, and in others single twinning of the 

 augite appears to occur though it is not quite definite. 

 Occasionally the diallage is noticably pleochroic, the colours 

 varying between a pinkish and a greenish-brown. 



In the augites sometimes schiller inclusions are present 

 parallel to 100, giving a slightly diallagic appearance, 

 especially when viewed in a direction parallel to the ver- 

 ;he augites the extinction angle c a c rises 





ii!tr;il. .1- : r- rocks, thee 



colourless pyroxene, that occurs 



amotion angle is between 40°and 41 



This diopside has sometimes a slightly developed diallagic 

 parting parallel to (100). 



Rhombic Pyroxenes.— Both hypersthene and enstatite 

 occur, the former in the more acid gabbros, the latter in 

 the peridotites. Hypersthene forms roughly rectangular 

 grains when cut parallel to the basal plane; in other 

 positions it is more irregular in outline. It is often strik- 

 ingly pleochroic in tints of pale green and pink to purplish- 

 pink. Enstatite occurs in the peridotites in colourless 

 rounded grains. 



Alterations.— The alterations of the diallage are varied. 

 The origin of schillerisation cannot as yet be definitely 

 referred to as a secondary process ; its effect having already 

 been discussed, it will not be further considered here. The 



formation of Ion 



rregular cavities has been 



already described; they are particularly common, forming 

 along the boundaries of the pyroxene, the planes separating 

 11 '• grains from adjacent grains being an easy passage for 

 solutions. The cavities are generally filled with opaque 

 oxides. It is in the transformations of the pyroxene that 

 the greatest difficulty arises in the study of these rocks, 

 tallage changes into an aggregate of matted flaky or 



