118 PROCEEDINGS OF SECTION B. 



powder. A little original quai'tz, showing cubical cavities probably 

 i-esulting from the decomposition of iron pyrites occurs in the 

 gossan in isolated patches. 



The lumps of telluric bismuth enclosed in this gossan show on 

 freshly broken surfaces a general concentric structure, due to the 

 tetradymite having been incrusted with successive coats of decom- 

 position minerals. The outermost coat, from |- inch to ^ inch 

 thick is an earthy mineral the colour of yellow ochre, inclining 

 to orange in places, and showing cubical cavities. Next beneath 

 this telluri-bismuthic ochre is a layer of greenish yellow montanite, 

 having a somewhat streaky structure. In this second layer and 

 occupying former cubical cavities are spots of dark brownish 

 red montanite, from ^ inch to ^ inch in diameter. 



Next beneath the second layer and forming the nucleus of the 

 lump is a granular crystalline steel grey mass of tetradymite. 

 The minerals in these ores may therefore be classed as follows : — 



1. Tetradymite. 



9 M + ■+ / (**) Pale greenish yellow variety. 



• \ (b) Ferruginous, dark brownish red variety. 



3. Telluri-bismuthic ochre. 

 The two first minerals may be further described as follows : — 



1. Tetradymite, occurs in granular crystalline masses, of a 

 steel grey colour and bright metallic lustre. 



Under the microscope the crystals are seen to have a very 

 perfect basal cleavage so that the mineral splits readily into very 

 thin lamina?, over one hundred being visible in one crystal within 

 a space of ^V inch. Most of the crystals appear to be tabular. 

 The orientation of the crystals in each mass is tolerably uniform, 

 and the planes of basal cleavage lying consequently principally 

 in one line the mass has a tendency to split readily along these 

 planes, which accounts for the streaky appearance of the mon- 

 tanite surrounding the tetradymite. The hardness is about 1|, 

 the mineral being unaffected by talc, easily scratched with calcite, 

 and difticulty scratched with selenite. The streak is black and 

 shining, much like that of graphite. The lamina? are flexible. 



2. Montanite. (a) Pale greenish yelloio variety. 



This mineral encrusts the tetradymite, and does not show any 

 crystalline structure. Green tints are observable in this variety 

 wherever the particles of half decomposed tetradymite become 

 abundant, the green being due to the steel grey tetradymite 

 showing through the yellow coating of montanite. The lustre 

 varies from dull earthy to waxy. The hardness is about 1^. 

 (6) Ferruginous dark brownish red variety. 



This variety occurs in cubical pseudomorphs, single or aggi'e- 

 gated, and about ^ inch in diameter. 



The colour is dark brownish red excepting on thin edges where 

 the mineral is semi-translucent and of a deep claret colour. 



The streak is of a red ochreous colour. The mineral is brittle 

 and has an uneven irreafular fracture. 



