Boult. — Occurrence of Gold at Harbour Cone. 445 



resorption zones took place. When this action ceased, partly 

 from the disappearance of the hornblende crystals and partly 

 from the formation of a surrounding shell of resulting granular 

 products which prevented further action, the remainder of this 

 liquid solidified in its turn, forming the pyrites visible to the 

 unaided eye. Thus the action which could not proceed in the 

 earlier stages of consolidation was rendered possible by the 

 collection of certain constituents of the magma in the higher 

 portions of the intrusion. 



The complete absence of the pyrites from the sandstone is 

 most striking, but when the great porosity of this rock is con- 

 sidered, and the consequent free circulation of water through it, 

 a subsequent removal of the sulphide and gold and silver seems 

 probable. This sandstone lying along the surface of the syenite 

 as it was carried up would be the easiest channel for vadose 

 water to circulate, and the passage of a large body of water 

 for a comparatively short time has effected what the limited 

 amount passing through the comparatively impermeable syenite 

 and bostonite has not been able to do. Owing to this very 

 porosity it seems probable that the sandstone was once mine- 

 ralised to a much wider extent than the bostonite. The Tin- 

 stability of gold-bearing sulphides has been illustrated in every 

 lode which carries sulphides, where the " iron cap " is often 

 quite poor in precious metals the deeper portions of the reef 

 being sometimes extremely rich. 



The greater richness of the mineralised bostonite may be 

 due to a differentiation of the reacting portion of the magma, 

 comparable to Soret's classical experiment, when the more 

 basic substances crystallized at the colder end of the tube, the 

 " colder end " being the lower surface of the bostonite. A 

 similar cause may have resulted in the differentiation of the gold 

 and silver, the bostonite being so much richer in gold. 



From theoretical grounds it would appear improbable that 

 the auriferous belt in this rock extends to any great distance 

 below the surface. Portions of the syenite ejected by the Port 

 Chalmers breccia are in such a weathered state that all pyrites 

 has been removed from them even if it was ever present. A 

 general segregation of the aqueous solution of sulphides to the 

 upper portions of the intrusion seems more probable than a 

 general segregation throughout the mass, since no sign of any 

 marked small segregations is visible on the small portion of the 

 rock exposed. This aqueous solution was strongly alkaline as 

 well as sulphide-bearing, as its reaction with the hornblende 

 produced segerine. 



The occurrence of sulphides and gold in this syenite is a very 

 striking feature. Its general alkaline properties render it more 



