Boult. — Occurrence of Gold at Harbour Cone. 443 



silver = 0-0056 per cent. ; giving, per ton — gold=8dwt. ll - 8gr., 

 silver = 1 oz. 16 dwt. 14-1 gr. 



No. 1. — Sample of mineralised bostonite. This rock is every- 

 where richlv impregnated with pyrites, the fractured surface 

 showing a brass-yellow colour. Mean of two assays : Gold 

 = 0-0022 per cent., silver = 0-00051 per cent. ; giving, per 

 ton — gold = 14 dwt. 8-9 gr., silver = 3 dwt. 3-8 gr. 



Assay of the micaceous sandstone gave no sign of gold or 

 silver. 



From the above it will be noticed that the amount of the pre- 

 cious metals in the rock becomes greater as the amount of py- 

 rites increases, and therefore it does not seem altogether illogical 

 to infer that the gold and silver exist in the pyrites, being the 

 so-called " sulphide gold." The proportion of gold to silver, 

 about 1 : 5, in the syenite is almost constant, but in the mine- 

 ralised bostonite the amount of gold suddenly increases, and the 

 proportion of gold to silver becomes about 4:1. 



The coarse crystallization of the syenite and its occurrence 

 in the Port Chalmers breccia render it indisputable that this 

 rock exists as a large intrusive mass under the district, and the 

 only question to settle is whether the precious metals and py- 

 rites were introduced into it subsequently to its consolidation 

 or whether they were an original constituent of the liquid magma 

 that was forced up. As mentioned under " General Geology," 

 the intrusion of the syenite was probably subsequent to the 

 outflow of the bostonite, and the former carried mechanically 

 upwards a portion of the sandstone beds. Another possible 

 solution for the presence of the sandstone is that the syenite 

 was originally a surface of the crust, and on it the Tertiary sand- 

 stone was deposited, the bostonite afterwards flowing out and 

 covering the sandstone : but this was probably not the case. 

 Supposing a pyrites-bearing alkaline rock such as the syenite 

 was exposed on the surface. The action of atmospheric water 

 would have weathered the rock to a great extent, and subjected 

 it to the well-known process of secondary enrichment. The 

 pyrites as well as most of the gold would have been dissolved 

 and redeposited in the deeper portion of the deposit, as has 

 occurred in almost every exposed lode. The pyrites, however, 

 in this deposit is unaltered at the surface ; and thus it is to be 

 safely concluded that it has never been exposed to the direct 

 action of a large amount of atmospheric water for any length 

 of time. 



From the presence of the resorption zones in the mass it 

 is evident that chemical action on the hornblende crystals has 

 taken place. This action may have occurred in three ways : 

 (1) Action on infra-telluric crystals of hornblende by the magma 



