ROCK SPECIMENS FROM CENTRAL AND WESTERN AUSTRALIA. 307 



All the pyroxenes are clearly posterior both to the olivine 

 and the felspar, embracing the latter in typically ophitic 

 fashion (Fig. 6, Plate XIV, and Text Fig. 1). In general 

 the hypersthene appears to be posterior to the monoclinic 

 pyroxenes, 1 but in some cases is seen in parallel growth 

 with them, and again is sometimes clearly intergrown with 

 them, forming a pyroxene-perthite (Fig. 5, Plate XIV). 

 The finer pyroxene-perthites are apparently homogeneous 

 augites in ordinary light, but between crossed nicols they 

 closely resemble micropegmatite. In the latter the inter- 

 growth is too fine to permit the determination of both 

 species of pyroxene, although one of them is often hyper- 

 sthene. 



Hornblende, a greenish variety, is confined to the more 

 acid rocks, and in these to the exterior of the pyroxene 

 grains. In Nos. 12 and 13, where it is fibrous and confined 

 to the exteriors of the hypersthene, it may confidently be 

 described as uralite. In Nos. 15 and 16, where it is com- 

 pact, brownish-green, and occurs as crystallographic out- 

 growths on the pyroxene, it is more probably original, but 

 in these also there is a subordinate amount of a paler and 

 somewhat fibrous hornblende which is probably uralitic. 2 



Biotite occurs in small quantity in almost all the rocks. 

 It is found only in ragged flakes, and in most cases surrounds 

 or lies alonside the iron ore, from which it appears to have 

 been derived by a partial resorption. 



The felspars, which are the most abundant minerals, are 

 for the most part glassy clear, but in some of the rocks the 

 plagioclase has a milky brown appearance due to the 



1 Cf. Elsden, J. V., The St. David's Head ' Kock-Series.' Q.J.G.S., 

 lxiv, (1908) pp. 286-8. 



2 For a more detailed discussion of the writer's views on the distinction 

 between uralite and primary hornblende, and the raison d'etre of the 

 latter's occurrence, cf. Thomson, J. A., Petrological Notes to Bulletin 33. 

 Geol. Surv, Western Australia, 1909, pp. 132-5. 



