92 I'ransactions. 



Had denudation removed the mushrooni caps, the sulphide- 

 bearing necks would now be exposed at the surface. 



The well-known Martha Lode, at Waihi, consists of chal- 

 cedonic and crystalline quartz, apparently the result of hydro- 

 thermal activity, which at one time probably manifested itself 

 at the surface. There is no overhanging cap. 



At the outcrop the quartz is almost pure silica, containing 

 no sulphides excepting a trace of argentite associated with 

 free gold containing about one-third its weight of silver. Above 

 water-level the ore is clean, and free from oxidized products. 



In many places both above and below \vater-level the joints 

 in the veinstone are discoloured with films of manganese and 

 iron oxides, which appear to owe their origin to the infiltration 

 of meteoric water from the wall-rock, and not to the oxidation 

 of contained sulphides. 



Between the adit level and No. 1 level there began to appear 

 small limonite-crusted cavities in the thin veins of crystalline 

 quartz which traverse the main lode. At No. 1 level there 

 are detached branches of iron-pyrites in the quartz, and at 

 No. 2 level the sulphide ore forms a rib two or three feet 

 thick. 



The lode is being worked to a depth of 750 ft. below adit 

 level ; and although there has been an increase in the proportion 

 of iron-pyrites, there has been no decrease in the gold and silver 

 values. 



A gi'eater measure of denudation than the lode has already 

 suffered would have exposed the sulphide ore at the surface. 



In the study of vein-filling it is always well to bear in mind 

 that veins which outcrop at the surface may have been trun- 

 cated to a greater or less degree by denudation. 



After their formation, some veins, through movement of the 

 walls, have been brecciated and recemented by circulating 

 mineralised waters. Such waters, ascending through the crushed 

 vein- matter, would deposit their metallic contents as sulphides 

 through the reaction of primary sulphides contained in the 

 ore. 



In this way a secondary concentration of sulphide ore may be 

 effected by ascending waters. The common belief, however, 

 is that secondai-y enrichment is in the majority of cases the 

 result of the transference of material from the oxidized portions 

 of a vein to a lower level through the agency of descending 

 waters, from which the metallic contents are precipitated by the 

 reducing action of organic matter or primary sulphides. 



