38 Transactions. 



and concentration of the valuable contents to a lower depth,, 

 forming zones of secondary enrichment, are the work of 

 chemical dissolution and electro-chemical precipitation in 

 which the primary sulphides probably play an important 

 part. 



The power possessed by clays, silica, and porous mineral 

 substances to absorb or extract metals from dilute aqueous 

 solutions may play a more important part in the formation of 

 ore-deposits than generally supposed. 



W. Skey, :;: as far back as 1869, proved experimentally that 

 finely pulverised massive quartz, rock-crystal, and silica pos- 

 sess the power of absorbing or extracting the oxide of iron 

 from its acetate solution. He also found that prepared silica 

 especially manifests this property if ignited at a low tempera- 

 ture, and., besides, takes oxides of copper and chromium from 

 their acetate solutions. The more finely divided the silica the 

 more apparent is the absorption. 



In 1871 Skeyf found that when a weak ammoniacal 

 solution of copper containing a little caustic potash is poured 

 upon a filter of Swedish paper (cellulose), the liquid which 

 passes through the paper is quite or nearly colourless, and the 

 filter is found to have retained all, or nearly all, the copper of 

 such solution. 



In 1874 he} showed that clay possessed the property of 

 absorbing and fixing natural petroleum in such a way as to 

 form a substance resembling natural oil-shale, the oil being 

 chemically combined with the clay. He does not appear to 

 have tried to ascertain the absorptive power of clay upon solu- 

 tions of the metals, but his discovery that silica and porous 

 substances such as cellulose possess the property of absorbing 

 metals from their solutions has an important bearing upon 

 the chemistry of ore-formation. 



E. Kohler,§ m 1903, experimenting on the line followed by 

 Skey in 1869, showed that clays and porous substances such 

 as gelatinous silica, carbonaceous and colloidal substances, 

 possess the power of extracting metals from their dilute solu- 

 tions. 



In this property of clay, silica, and porous substances we 

 may have found the key to the concentration of gold in the 



* W. Skey, " On the Absorptive Properties of Silica, and its Direct 

 Hydration in Contact with Water," Trans. N.Z. Inst., vol. ii, p. 151, 

 Wellington, N.Z., 1869. 



f W. Skey, "Absorption of Copper from its Ammoniacal Solution 

 by Cellulose in Presence of Caustic Potash," Trans. N.Z. Inst., vol. iv, 

 1871, p. 332. 



{ W. Skey, " Notes on the Formation and Constitution of Torbanite 

 and similar Minerals," Trans. N.Z. Inst, vol. vii, 1874, p. 387. 



§ E. Kohler, " Zeitscbrift fiir Praktisihe Geologie," 1903, p. 49. 



