J 24 ^V. M. Janner : 



Further, in weathered roeks. ferric oxide always predominates ovei 

 -ferrous oxide, whilst in the Queenstown rock the reverse is the case. 

 Petrology. — All specimens examined (some of them from the 

 -Caledonia mine coming from greater depths than 200 feet) showed 

 considera])le alteration, resulting in the formation of such secon- 

 dary minerals as chlorite, epidote, sericite, carbonates, leucoxene, 

 -pyrrhotite, and iron pyrites. In hand-specimen, the rock is even- 

 grained, and consists of about equal quantities of salic and femic 

 minerals, chiefly hornblende and felspar. Quartz is not visible 

 macroscopically. Microscopically it is a holocrystalline, medium 

 and even-grained rock, consisting essentially of brown hornblende, 

 felspar (both orthoclase and plagioclase), quartz, ilmenite apatite, 

 and the above-mentioned secondary minerals. 



The hornblende is the brown variety, which is so cliaiacteristic 

 'of the igneous rocks of the Walhalla, Wood's Point gold belt. In 

 most sections, little of it remains, as it has been extensively changed 

 to chlorite and epidote. It is usually hypidiomorphic and often 

 includes ilmenite and felspar, showing that it consolidated later 

 than these minerals. Tremolitic outgrowths in optical continuity 

 with the brown hornblende are occasionally present. Twinning, 

 -with twin and composition plane parallel to the orthopinacoid, is 

 not uncounnon. It is markedly pleochroic, exhibiting the following 

 ■colour scheme : — 



X light yellow brown. 



Y fairly dark brown. 



Z very dark brown. 



and Z>Y>X as usual. 



Plagioclase (Recurs as beautifully zoned crystals, sliowing the 

 usual albite twinning, and occasionally twinned accoiding to the 

 ("arlsbad and Pericline laws. Certain sections of zoned plagioclase 

 show no signs of the albite lamellae, but exhiliit basal cleavage, and 

 are therefore cut approximately parallel to 010. One such section 

 gave an extinction angle of-16o from the 001 cleavage for the 

 central core, and -t- 21^ from the same cleavage fur tlie clear outer 

 zone, indicating a kernel of labradorite (Ab An ), and an outer 

 zone of albite. Almost invariably the refractive index of the clear 

 exterior zone of the felspar is less than that of (}uartz. jiroving it 

 to be albite. Symmetrical extinction angles, from the all)ite 

 lamellae, range as high as 430 for the cores, showing phigioclase 

 near Bytownite. A few phenocrysts of untwinned oi' sini])ly twinned 

 kaolinised felspai-, having a refractive index less than that of 



