﻿Vol. 66.] OEE-DEPOSITION IN LEAD AND ZINC VEINS. 319 



sulphides, then, has been : chalcopyrite, blende, galena. The dis- 

 tinct crystallization of chalcopyrite suggests that some time elapsed 

 between the arrival of this ore and of the other two, which are 

 more intimately related and sometimes intergrown. Venules of 

 galena penetrating blende are commonly seen in ore from Tre- 

 castel], Conway. Specimens from veins on Carrock Fell exhibit 

 crystals of chalcopyrite enclosed in and wrapped round by galena. 

 Specimens from the Collision copper-mine show chalcopyrite em- 

 bedded in quartz, while blende occurs deposited on both chalco- 

 pyrite and quartz, and finally arsenopyrite occurs as groups of 

 crystals deposited on cavities in the blende. The ' bluestone ' or 

 granular lead-zinc ore of Parys Mountain (Anglesey) and of Avoca 

 (Wicklow) consists of an intergrowth of galena and blende 

 enclosing earlier crystals of chalcopyrite. The general relations of 

 the chief sulphides appear to be fairly well established by these 

 observations. It should be noted that, while galena is generally 

 later than blende in primary ore, the relations may be reversed 

 where the ore has undergone secondary alteration. Thus, some 

 specimens from superficial parts of the Nenthead mine show galena 

 traversed by blende rilling irregular craeks and corrosion-cavities. 



The silver-content of galena was also investigated by observation 

 of polished ores. Specimens of galena taken from deep levels, 

 unaffected by secondary alteration, showed no silver or silver- 

 compound in any case, even under the highest powers. The 

 galena showed no heterogeneity on etching with various reagents, 

 and the cleavage-planes were not occupied by any extraneous ore. 

 It therefore appears that the silver in these ores exists either in an 

 excessively fine state of dissemination or as a sulphide in intimate 

 isomorphous combination with the galena, a combination which 

 would not be resolved by etching. Specimens of argentiferous 

 galena taken from higher levels, within reach of underground 

 waters, however, showed in several cases strings of greyish-white 

 native silver, running along cleavage-cracks in the galena. The silver 

 occurs generally in straight threads intersecting one another along 

 the cleavage-planes (figs. 10 & 11, p. 318), and stands up in slight 

 relief above the surrounding galena. A similar occurrence, it may 

 be noted, has been recorded by T. L. Phipson in galena from the 

 Phoenix Mine (Cornwall). 1 There is little doubt that this develop- 

 ment of native silver is the result of secondary concentration by 

 descending surface-waters, the silver being taken into solution 

 during oxidation of the galena near the surface, and redeposited in 

 the ore at lower levels. These observations appear to have some 

 bearing on ore-dressing problems. Thus, the occurrence of native 

 silver in secondary ore would explain the partial removal of the 

 silver by amalgamation in some experiments of M. lloswag, J and 

 would also account for the notable loss of silver in the dressing of 

 many ores, the fine silver going off in the slimes.^ On the other 



1 C. E. Acad. Sci. Paris, vol. Ixxviii (1874) p. 563. 



2 L. de Launay, ' L' Argent' Paris, 1896, p. 47. 



3 W. F. A. Thomae, ' The Zeehan & Dundas Silver-Field, Tasmania' Trans, 

 Inst. Min. & Met, vol. iv (1895-96) p. 60. 



Q. J. G. S. No. 20i. y 



