Finlayson. — Scheehte of Otago. 115 



It is significant that all the reefs in this district outcrop 

 on a single plane in the schist. This indicates that the horizontal 

 shearing movement which localised the reefs followed a particular 

 zone in the rock, although it is quite likely that there may be 

 one or more zones or levels of lode- formation beneath the one 

 now exposed. 



(3.) The Scheelite. 



This mineral occurs, firstly, in segregated masses of varying 

 size, typically seen at Macrae's. These generally cling to the 

 foot-wall, and frequently pass right into the country rock, the 

 foot-wall being then obscured. These comparatively pure masses 

 grade off into highly quartzose ore scattered through the gangue. 

 Secondly, it occurs in irregular veins in the quartz leaders and 

 stringers, as well as in the larger quartz seams. It may con- 

 stitute a whole vein, or it may have a broad or narrow selvage 

 of quartz on either side. 



The hand-specimen, which always contains some quartz, 

 has a specific gravity of 5-12, that of the pure mineral being 

 5-9 to 6-1 (Dana). It is yellowish-white in colour, brittle and 

 friable, with an irregular fracture. It shows interrupted cleavage- 

 surfaces, and is massive in habit, no crystals being found, as far 

 as my observations showed. 



Microscopic Characters. 



In section (Plate XVI, a, b), the mineral is dark-brown, with 

 a high refractive index. In isotropic sections a faint positive 

 uniaxial figure may be seen. The interference colours are more 

 usually yellow and red of the first order. The individuals are 

 large, with sharp boundaries and pointed or pyramidal termi- 

 nations. Two interrupted sets of cleavage-traces crossing at 

 40° are seen in suitable sections, these being the characteristic 

 cleavages, p (111) and e (101). The cleavage-lines are frequently 

 crossed by irregular fractures, along which the mineral is dark 

 and clouded. A faint lamellar structure is occasionally seen, 

 resembling polysynthetic twinning. The lamellae, however, are 

 alternately broad and narrow, and can be distinguished, though 

 with difficulty, in ordinary fight. The appearance is probably 

 a strain-effect. 



Chemical Composition. 



The following analysis indicates the average composition of 

 Otago scheehte. Quartz is always present in intimate associa- 

 tion, as shown in Plate XVI, a and b ; in the analysis this con- 

 stituent was eliminated, and the figures recalculated to 100 per 

 cent. 



