11 



IV. Petrography. 



(1) PORPHYRITIC GRANITE. 



The type specimen was collected on Granite Island. The 

 anthor is ind^bted^ to Mr. Rnmbelow, of Encounter Bay, for 

 a specimen from West Island, and this is identical in all 

 respects with the type. 



The minerals visible megascopically comprise potdsli 

 felspar phenbcrysts, rather tabular in habit, and usually 

 showing irregular carlsbad twinning; quartz, bluish opalescent, 

 the opalescence being occasionally interrupted and at times 

 zonally disposed; hiotite and small felspar, occasionally 

 striated, in the interspaces, and at times a little pyrifen. 



Under the microscope are seen quartz, plagioclase, micro- 

 cline, and biotite, with subordinate muscovite, apatite, and 

 magnetite, and a little zircon and pyrites. The first three of 

 these, and possibly biotite as well, occur in two generations. 

 The rock texture is dominated by the larger crystals of micro- 

 cline, quartz, and plagioclase, with an interstitial filling of 

 the same* minerals and biotite. 



Microcline. — This mineral has been described in some 

 detail by H. W. Gartrell,**^^ who refers it to the auorthoclase 

 group, but a re-examination of some of tlie optical and other 

 physical properties points to the mineral being rather a 

 micropertliitic microcline. A section parallel to (010) gives 

 an extinction of 6°, and show^s streaks of another felspar, in 

 roughly parallel orientation, with a K.I. higher than that 

 of the host but lower than Canada Balsam. The D.R. 

 of these streaks is slisfhtlv stronorer than that of the host, and 

 an extinction angle of 19° is found. The direction of the 

 streaks makes an angle of 64° with the basal cleavage, which 

 would make them parallel to (100). Sections of the pheno- 

 crysts cut normal to (010) show the usual microcline twin- 

 ning, spindle-shaped rather than cross-hatched, and in addi- 

 tion have small discontinuous lenticular inclusions of 

 plagioclase arranged roughly normal to the pinacoidal 

 cleavage. There is* no doubt then that there is a micro- 

 pertliitic intergrowth of albite with microcline. 



Mr. L. A. Cotton, M.A., B.Sc, kindly made some rough 

 measurements of the optic axial angle of the mineral, with 

 the result that the approximate value of 76° was found 

 for 2V. This is much closer to the value for microcline than 

 to that for anorthoclase. 



Gartrell's analysis of the felspar as given in his paper 

 shows nothing unusual in the way of soda percentage for a 



'8) Loc. cif. sup. 



