PLUTONIC EOCKS OP GAKABAL HILL AKD MEALL BEEAC. 109 



grain of colourless olivine a small patch which is rendered cloudy 

 by the presence of innumerable, minute, dust-like particles, thickly 

 scattered through the clear olivine-substance. Olivine has been 

 found only in the highly basic rocks. Some of the serpentines have 

 been formed by the alteration of rocks composed almost entirely 

 of this mineral. Inclusions of olivine occur in the other ferro- 

 magnesian constituents. Iron ore is the only mineral found in 

 the olivine itself. 



Enstatite. — This mineral has been observed only in one or two of 

 the ultra-basic rocks. It is fresh and for the most part free from 

 inclusions. The characteristic pleochroism is well seen, but the 

 colours are not strong. The mineral is allotriomorphic with 

 respect to olivine, but idiomorphic with respect to brown horn- 

 blende and biotite. 



Augite and Diallage, — In the ultra-basic rocks this mineral is pale 

 brown in thin section. Prismatic and sometimes also pinacoidal 

 cleavages are well developed. Parallel rows of minute rod-like 

 inclusions are present in some sections. Cross-sections, giving the 

 prismatic cleavages and showing the characteristic interference- 

 figure, are sometimes seen to contain two rows of inclusions making 

 angles of 45° with the cleavages and at right-angles to each other. 



In addition to the pale brown pyroxene of the ultra-basic rocks 

 we find also a pale green mineral in the augite-diorites. This 

 variety is generally bordered irregularly by green hornblende. 

 The augite occurs usually in allotriomorphic grains. Twinning is 

 not uncommon, and sometimes indications of idiomorphism have 

 been observed. 



Hornhlende. — Two varieties are represented, brown hornblende 

 in the ultra-basic rocks and green hornblende in the intermediate 

 and acid rocks. 



The brown hornblende is generally allotriomorphic, especially 

 with reference to olivine and pyroxene. It is sometimes seen to 

 be idiomorphic with respect to plagioclase. 



Pleochroism.^ 



a (Dp) pale brown. 



/3 (n„) brown, 



y (iig) rich brown. 



Green hornblende occurs as allotriomorphic grains and also as 

 crystals. Perfect idiomorphism is very rare and has been observed 

 only in certain basic inclusions in the granite ; but traces of the 

 forms {110} and {010} are not uncommon in the tonalite. 

 Twinning of the normal type is fairly common. 



Pleochroism. 



a (Cp) pale green or brown. 

 )3 (nm) brownish green, 

 y ip-g) green. 



Iron ores, apatite, zircon, and sphene occur as inclusions. 



^ a, /3, and y stand respectively for the greatest, mean, and least axes of 

 elasticity. The notation in parentheses is that now used by Prench authors. 



