216 



J. A. Thomson — Rocks of Western Australia. 



the State, is not a well-foliated rock, but has both the mineral 

 composition and the structure of a typical crystalline schist. The 

 minerals noted are garnet, biotite, sphene, apatite, a plagioclase near 

 andesine, orthoclase, and quartz. The garnet is not distinctly 

 euhedral, and is full of inclusions of the other minerals. Sphene 

 occurs in small characteristic acute prisms. The chief structural 

 peculiarities are the frequent inclusion of quartz in rounded plates 

 within the orthoclase, and the presence of intergrowths of quartz and 

 plagioclase (Fig. 1). 



These intergrowths have the gridiron shapes characteristic of 

 'myrmekite', rather than the script-like forms that have given rise 



Fig. 1. Gneiss showing the inclusion of quartz in orthoclase and a myrmekitic 

 intergrowth of quartz and andesine. Magnification, 42 diameters. 

 Ap. = apatite ; Bi. = biotite ; PL = plagioclase ; Or. = orthoclase ; 

 Q. = quartz. 



to the term ' graphic '. They are strictly confined to the plagioclase. 

 Myrmekite in Sweden is considered a sign of extreme metamorphisra 

 and great age in granite.' 



Two rocks collected by the writer on the beach between Albany 

 and the Pier resemble the last-described rock in both these structural 

 features, viz. the inclusion of quartz in orthoclase and the myrmekitic 

 intergrowth of quartz and a plagioclase near oligoclase in composition. 

 They are hornblende-biotite gneisses, with hornblende in equal or 

 greater amount than biotite. A further structural peculiarity is the 

 occasional presence of myrmekitic intergrowths of quartz and biotite 

 (Fig. 2) and quartz and hornblende. 



^ J. P. Holmquist, " Studien iiber die Granite von Schweden " : Bull. Geol. 

 Inst. Upsala, vol. vii, p. 116, 1904-5. 



