316 MCMAHON : PETROLOGICAL NOTES ON SOME PERIDOTITES, ETC., 



The original rock appears to have been a holo-crystalline mixture 

 of olivine, monoclinic pyroxene, and labradorite, the olivine largely 

 predominating. The olivine has been almost wholly transformed into 

 serpentine, pale yellow in transmitted light. It exhibits the usual 

 mesh-structure in polarized light, and eyes of unaltered olivine are 

 visible here and there. Marginal deposits of magnetite are abundant 

 as in normal serpentine. 



The pyroxene and labradorite are both allo-triomorphic. The 

 pyroxene is very highly altered and steatitic. In only one slice, 

 No. 1994 of the Calcutta Geological Museum microscope slides taken 

 from No. 8-277, is any of the pyroxene at all fresh. The labradorite 

 exhibits multiple twinning, the maximum extinction being 28i°. 

 Some of the felspar is traversed by fine infiltration canals which have 

 penetrated into it from the serpentine. 



The hardness (4) and Sp. G. of the rock are both rather high for 

 serpentine. 



No. 8—<2g3. Serpentine, from the valley west of Sirsa La ; collected by 

 T. D. LaTouche; Sp. G. 2-638. 



The bulk of this rock is made up of serpentine after olivine. 

 The conversion of the latter mineral has been complete and no eyes 

 of olivine have been left in the serpentine. The thin slices examined 

 also contain remnants of .augite, a finely fibrous serpentine, picotite 

 and some magnetite. The pyroxene fragments are mere remnants 

 of large crystals, the major portions of which have been converted into 

 serpentine by aqueous agents. The augite exhibits a single cleavage, 

 and in some cases the angle of extinction measured from that 

 cleavage is as low as from 6° to n°. This may be due to partial 

 uralization. 



The only mineral that calls for any comment is the finely fibrous 

 serpentine. It is rather suggestive of bastite, but as it has evidently 

 been derived from augite and not from enstatite, it cannot be refer- 

 red to that mineral. It is almost colourless in thin sections, it has 

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