Vol. 69.] PETROLOGY OF THE KALGOORLIE GOLDF1ELD. 657 



pericline-twinning is rare ; while Carlsbad twins have not been 

 seen. Zoning is occasional^ observed, but the zones do not always 

 show a regular progression of decreasing basicity from centre 

 to margin, since there is frequently an alternation between the 

 same two species of plagioclase ; nor is there any great difference 

 in composition between the zones, for the extinction-angles do not 

 differ by more than a few degrees. The centres of the crystals are 

 too highly sericitized for exact determination ; but there is generally 

 a clear zone on the exterior, which agrees in refractive index, ex- 

 tinction-angles, and optical character with albite. The sericitized 

 kernels are probably in some cases more basic, but in others they 

 agree prefectly in optical characters with the clear border of albite. 

 As undoubtedly secondary albite is present in some of the rocks, in 

 nests associated with epidote and calcite (fig. 3, p. 656), it is pro- 

 bable that the phenocrysts which now consist of albite are albitized 

 lime-soda felspars. The felspars of the groundmass are similar both 

 in habit and in composition to the phenocrysts, and are never 

 microlitic. Quartz is of rare occurrence as a phenocryst, in 

 rounded forms like those observed in quartz-porphyries. In the 

 diorite-porphyrite of the Power-House lleserve it occurs in a 

 micropegmatitic fringe to the felspars, and in addition as a mosaic 

 of secondary grains in the groundmass. 



The view that the porphyrites are partly (and, in some cases, 

 completely) albitized rocks is supported by the following analysis 

 of a specimen (G.S.M. 2943) from a shaft in the Water Reserve 

 No. 3398 E :— 



XXVIII. 



SiOo 63-05 



TiOo 0-97 



AI0O3 14-04 



Fe 2 3 — 



FeO 5-36 



MnO.... 0-02 Sp. gr. 2-75. Analyst— C. C. Williams, in 



MgO 1-58 E. S. Simpson, Bull. Geol. Surv. W. Austral. No. 6 



CaO 5-92 (1902) p. 74 



Na 2 6-74 



KoO 0-62 



HoO+ 0-26 



HoO- 0-04 



COo T47 



Total... 100-07 



In the description of this rock given by Mr. Simpson, it is stated 

 to be decomposed, from which we may infer that the felspars are 

 sericitized, while the presence of iron-ores and calcite is noted. The 

 analysis should, therefore, contain iron sesquioxide. If the potash 

 were present as orthoclase, the felspars would be, by calculation 

 after the American method : — 



Per cent/ 



Orthoclase 3'67 



Albite 56-96 



Anorthite 618 



Non-felspathizable lirne 4/67 



