24 Transactions. 



quartz, or " blows," carrying wolfram and native bismuth. 

 The " blows" when followed down develop into irregular pipe- 

 shaped masses surrounded on all sides by granite, which is 

 the country rock. When the quartz is extracted there remain 

 only empty pipes or vents. These pipe-like ore-bodies possess 

 a peculiar genetic interest. They appear to closely resemble 

 the siliceous pipes formed in rhyolite by the hot springs 

 in the Rotorua volcanic region, and the mushroom - shaped 

 quartz blows at Kuaotunu. 



There are near Waihi in New Zealand several massive 

 deposits of chalcedonic quartz which are stated by Rutley to 

 be replacements of the andesitic country rock. ::: 



A similar replacement of andesite by silica is described by 

 Spurr as occurring at Monte Cristo district in Washington.! 

 He mentions that the silicification has proceeded until most 

 of the rock is made up of quartz, which, he says, varies from 

 coarsely to very finely crystalline in structure, and contains 

 sulphides, chiefly blende, pyrites, and chalcopyrite. Spurr 

 continues, " Thus we have a complete and gradual transition 

 from andesite to a sulphide ore with quartz gangue, by the 

 progressive replacement of the original materials by silica and 

 metallic sulphides." 



In 1894 and 1896 I made an exhaustive examination of 

 the Hauraki andesites for gold and silver. The samples sub- 

 jected to examination were selected by myself in situ. The 

 analyses were conducted by the cyanide test, on samples 

 ranging from 21b. to 51b. in weight. The pulverised material 

 was leached in glass jars with a 0'3-per-cent. aqueous solution 

 of pure potassium-cyanide for seventy-two hours. The cyanide 

 solutions and washings were evaporated, fluxed with a little 

 pure litharge and borax, and the resulting button of lead 

 cupelled. Simultaneous tests were made so as to check the 

 purity of the litharge and fluxes. All the andesites examined 

 were found to contain gold at the rate of 1 gr. to 1-5 gr. per ton, 

 and silver varying from 3 gr. to 30 gr. per ton of rock. The 

 augite-andesite, at 3,000 ft. from the mouth of the Moanataiari 

 tunnel, contained 1^ gr. of gold and 3 gr. of silver to the ton ; 

 and the hypersthene-augite-andesite, from the waterfall in 

 Waiotahi Creek, near the Fame and Fortune Mine, ligr. of 

 gold and 30 gr. of silver.]: 



A penological examination^ of the rocks showed that the 



* J. Park and P. Rutley, '"Notes on Rhyolites of the Hauraki Gold- 

 fields," Quart. Jour. Geol. Soc, London, 55, 1899. 



f J. E. Spurr. U.S. Geol. Survey, Twenty-second Annual Report, p. 833. 



I J. Park, "The Geology and Veins of Hauraki Goldfields," Trans. 

 N.Z. Inst. Min. Eng., 1897, p. 52. 



§ J. Park, " Some Andesites from the Thames Goldfields," Trans. 

 N.Z. Inst., vol. xxxiv, p. 435. 



