140 HOLLAND: CHARNOCKITE SERIES. 



(b) Parallel to the vertical axis. 



(c) Parallel to the face of the unit rhombohedron and lying 



in the secondary planes of symmetry. 

 There may be other sets of needles in some cases, but it is 

 impossible to determine their crystallographic disposition in crystals 

 which are devoid of idiomorphic outlines. The needles lying in 

 isotropic (basal) sections show straight extinction ; but, being thinner 

 than the doubly refracting medium in which they are imbedded, 

 further details concerning their optical characters could not be 

 determined. 



The hair-like inclusions which occur in the garnets so frequently 

 found to be constituents of the charnockite series I have shown 

 before to be biaxial in their double refraction, exhibiting very wide 

 extinction angles. 1 



The potash-felspar is mostly in the form of microcline and 

 often presents the streiftge appearance due to regularly-arranged 

 intergrowths with a plagioclase to form the microperthitic structure 

 which has been so commonly recognised in the 

 Potas-espar. felspars of pyroxenic rocks similar to those of 



the charnockite series. " Quartz of corrosion w is frequently found 

 in the angular spaces between crystals of felspar. In some of the 

 coarse-grained " contemporaneous veins" traversing charnockite the 

 large crystals of potash-felspar resemble the well-known " moon- 

 stone " in presenting an opalescent appearance. 



The plagioclase present appears, from its narrow extinction 

 angles, to approach oligoclase in composition, 

 Plagioclase. ^^ fusiform bodies so frequently found in this 



mineral, and resembling at first sight those which produce the 

 microperthitic structure in orthoclase, possess a higher refractive 

 index and stronger double refraction than their host. Lacroix has 

 referred similar bodies to quartz, regarding their occurrence in the 

 oligoclase as a peculiar form of li quartz of corrosion." 2 



i Rec. Geol. Surv. Ind., Vol. XXIX (1896), p. 16. ■ 

 2 Rec. Geol. Surv. Ind., Vol. XXIV, p. 168. 



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