231 



'Dyke, east of creek, Horns Camp." This is a medium- 

 grained 1 to 15 mm., light-grey-green in colour, with occa- 

 sionally approximately cubic phenocrysts (2 mm.) of felspar. 

 Microscopically it is porphyritic with an ophitic to semi- 

 ophitic groundmass. The felspars are predominant and belong 

 to two generations. They are partly saussuritised epidote, 

 forming in the phenocrysts along two sets of planes, in- 

 clined towards the twinning plane (possibly the basal cleav- 

 age planes), but more commonly the epidote is in the twin- 

 ning plane, this latter method being universal in the tabu- 

 lar crystals of the base. In the phenocrysts twin lamellfe 

 were clearly seen, and an extinction angle of 15° was re- 

 corded. In the tabular crystals of the mesh, extinctions of 

 19° were obtained, and wherever measurable the refractive 

 index was distinctly less than that of Canada balsam. Sec- 

 ondary ( ?) felspar, which is water-clear, occurs in small 

 amount, in one instance at least optically continuous with 

 some partially-altered primary felspar. Scapolite is present 

 in some amount, and the epidote is colourless when fine- 

 grained, coloured and pleochroic (pistacite) when in large 

 masses. The augite is converted chiefly to chlorite, but a 

 small amount of uralite still remains. Ilmenite occurs in 

 large amount completely changed to leucoxene. 



GRANULITIC-DIABASES. 



In these rocks the structure is hardly the normal granu- 

 litic structure as defined by Judd "*> : rather is it a transition 

 between the semi-ophitic and gabbroid texture, in which the 

 felspars still retain to some extent the tabular meshwork, 

 but the interstices are occupied by a single angular grain not 

 in optical continuity with grains beyond the mesh. Such a 

 texture is exhibited by a specimen labelled "Dyke, 12 miles 

 east of Blinman." It is a rather fine-grained, dark-green 

 rock, containing grey felspar laths and large cream-coloured 

 felspar phenocrysts, 15 mm. in length. Microscopically the 

 predominant mineral is plagioclase considerably altered. 

 Specific determination is again difficult ; probably it is oligo- 

 clase. It is excessively full of secondary minerals, chiefly 

 epidote, in yellowish crystals, often comparatively coarsely 

 granular. Common also is the occurrence of epidote or zoi- 

 site with mica, in which the mica fibres extinguish parallel 

 to the original twinning plane of the felspar. In some cases 

 the felspar is entirely replaced by a mosaic of mica flakes 

 of high birefringence. Quartz is present in clear grains, 

 apparently primary. Pyroxene is completely changed, partly 



(4) IjOc. cit. supra. 



