230 



OPHITIC-Dl ABASES. 



Specimen labelled "Dyke, Blinman South." Microscopi- 

 cally the texture is ophitic, the grainsize small (0'5 m.m.). 

 The felspar has largely gone into parallel flakes of mica,. 

 though twinning is still visible. Determinative extinction 

 angles are rarely obtainable, but such as there are, together 

 with the low index of refraction, suggest oligoclase. Augite- 

 is present in large amount, though subordinate to the plagio- 

 clase ; it is almost colourless (malacolite) and decomposes in 

 a variety of ways — either to brown-green pleochroic uralite, 

 in clear patches or much clouded with red-brown haematite- 

 dust ; or to green jDleochroic chlorite (clinochlore). Ilmenite 

 occurs in large crystals and plates slightly altered to leu- 

 coxene. Epidote is very common in large yellow pleochroic 

 grains, bordered by magnetite in a manner very similar tO' 

 the habit of olivine. It is in this tyjDe of occurrence gener- 

 ally surrounded by chloritic matter. Epidote may also occur 

 in small, rounded yellow grains scattered through the fel- 

 spar, or most commonly in exceedingly minute, colourless 

 grains in the felsj^ar, and derived from that mineral. 



"Dyke, west side, Blinman Mine." This is a dark-grey 

 granular rock of basaltic appearance, apparently somewhat 

 altered. Microscopically the fabric is ophitic. The predomi- 

 nant mineral is a plagioclase occurring in its typical lath 

 meshwork. It is partly altered into saussurite, composed of 

 very fine-grained clinozoisite or a more coarsely-grained 

 growth of epidote (and zoisite ?) in a poikilitic 

 groundmass of scapolite. The epidote and zoisite 

 are both colourless, but the grains suspected of being 

 zoisite have a much lower birefringence. The small 

 amount of felspar still remaining unchanged to saus- 

 surite seems to have been converted to gibbsite or mica,, 

 but the distinction of these from scapolite is not always satis- 

 factory. Hints of the originally twinned nature of the fel- 

 spar are occasionally obtainable. Scraps of actinolite and 

 chlorite are not uncommon in the saussurite. Augite entirely 

 altered to uralite is the next mineral in the order of abund- 

 ance. The uralite is a brownish-actinolite, rather than smar- 

 agdite. It contains also a number of fine grains of epidote 

 with limonite and leucoxene ( ?), rendering it still dustier. 

 It has been further changed into chlorite to some extent. 

 Ilmenite in irregular grains and crystals, with, the usual 

 triangular decomposition to leitcoxene, is present in abund- 

 ance. Quartz occurs interstitially. 



Xain e . — Saussuritic-diabase. 



