?126 lY. E. Junner: 



preference for areas of ihnenite and undoubtedly replacing the 

 latter mineral in many cases. The sulphide is easily recognised as 

 pyrrhotite by its lighter colour than ordinary pyrites in reflected 

 light, and by its irregular outlines. It is proved without doubt 

 by extracting the magnetic minerals from the powdered rock, and 

 either examining the separated material under a microscope, or by 

 adding a little hydrochloric acid to the separated grains, when 

 stilphuretted hydrogen is readily detected. A very little iron 

 pyrit-is also accompanied the pyrrhotite thus separated. Pyrrhotite 

 chough present in practically all sections, is most common in peg- 

 matitic veins intersecting the diorite. 



Acid Df/A-es in Diorite. — Numerous small aplitic and pegmatitic 



•dykes intersect the diorite. The pegmatites, in general, consist 

 cliiefly of large hornblende prisms and colourless felspar, whilst 

 in the aplite dykes, hornblende is rare or absent, and microscopic 

 sections show that orthoclase predominates over plagioclase. Speci- 

 men No. 120, Caledonia Mine, is typical of these aplites. The hand 

 specimen of the rock is of a light yellowish colour. Abundant 

 stibnite and pyrites are visil)le, disseminated throughout the speci- 

 men. Microscopically, it is a fairly even-grained, holocrystalline 

 rock, with granitoid texture, and composed of the following 

 minerals : — Quartz, felspar, muscovite. iron and arsenical pyrites, 

 stibnite, carbonate and sericite. No femic minerals are present, 

 and quartz is in slight excess of the total felspar. Orthoclase, 

 generally untwinned and greatly kaolinised, preponderates over 

 plagioclase, which is also much weathered. Sections of plagioclase 

 occasionally show intersecting lamellar twinning according to the 

 albite and pericline laws. Zoning is rare, and in nearly all sec- 

 tions, the mean refractive index of the plagioclase, determined by 

 the Becke method, is less than that of quartz, indicating oligoclase 



■ or albite. Maximum extinction angles of 2()0 from the all)ite twin 

 planes, prove albite. A few rectangular sections of muscovite with 

 the usual separated leucoxene or rutile. iei»iesent, without doubt. 

 hornblende or biotite in the original rock. Most of the mica, how- 

 ever, is of the sericitic type and occius in the usual aggregates of 

 foils, sometimes arranged i-adially and often leplacing felspar. 

 Veinlets of carbonate and quartz cai-rying stibnite. travei-se the 

 rock. Acicular crystals of stibnite, together with a little ordinary 

 and arsenical pyrites, also occur promiscuously distiilmted 

 throughout the section. Certain miscroscopic veins of quaitz, which 

 intersect plagioclase crystals, are seoi to grade insensilily into the 



