Boult. — Occurrence of Gold at Harbour Gone. 433 



with dilute hydrochloric acid on a microscope-slide, and then 

 slowly evaporated to dryness. Under the high-power objective 

 the presence of nepheline was indicated by the occurrence of 

 small cubes of sodium-chloride. 



Auriferous Syenite. 



Two writers have mentioned this rock. Professor Ulrich, 

 in the " Geology of Otago " (1875), by Hutton and Ulrich. 

 describes it as " a coarsely crystalline rock composed of triclinic 

 feldspar, hornblende, and some quartz, and being more or less 

 densely impregnated with pyrites." Professor Park* describes 

 the rock as a " grey porphyritic rock of plagioclase feldspar 

 and hornblende," and names it a " porphyritic diorite." As 

 will be seen below, there is no visible quartz in the rock, and 

 it is distinctly neither porphyritic nor yet a diorite. In hand- 

 specimens the rock is seen to be composed chiefly of feldspar 

 and a dark mineral resembling hornblende in habit. It is seen 

 to be impregnated sometimes rather freely with pyrites, show- 

 ing on the fractured surface small flakes of the mineral. Some 

 specimens, however, contain hardly any sign of pyrites. It 

 is extreme^ haid and tough, being very difficult to fracture 

 on account of the latter property. Under the microscope the 

 rock is seen to be holocrystalline and of course texture. The 

 bulk of the rock is composed of bypidiomorphic crystals of 

 orthoclase, sometimes twinned on the Carlsbad law. and triclinic 

 feldspar twinned lather coarsely on the albite law. These give 

 a maximum extinction on the albite plane of 12°, being there- 

 fore oligoclase. 



Throughout the body of the rock are very many small crystals, 

 with irregular outline, of green transparent aegerine pleochroic, and 

 sh owing the high birefringence characteristic of that mineral. Small 

 rounded grains and needles of segerine also occur ibundantly. 



Large and very decomposed crystals are present intergrown 

 with the feldspar, and where these are not entirely decomposed 

 they show the low birefringence and straight extinction of nephe- 

 line. In most cases, however, the mineral is entirely decom- 

 posed, and as it is not present in great abundance no absolute 

 determination is possible. 



Throughout the rock are large granular masses, in shape 

 roughly that of typical hornblende. In most instances these 

 are the same right through the mass, but in some cases a core 

 of brown pleochroic hornblende is seen (Plate IX, fig. 2). It 

 is possible to cut as many as ten sections without finding this 

 feature. Where it occurs the mass presents the following charac- 



*Rep. N.Z. Geological Survey, p. 34 (1888-89). 



