172 Mr. D. Forbes' s Researches in 



When examining this district in May 1867, the attention of 

 the author was directed by Mr. Ezekiel Williamson to the pre- 

 sence of small particles of a mineral occurring disseminated in 

 the lead ores of this mine, and which he supposed to be the va- 

 riety of tetrahedrite found in Wermland in Sweden, called by 

 Svanberg aphthonite. For comparison, Svanberg's analysis* is 

 here extracted : — 



Copper . 



. . 32-910 



Zinc . 



.- . 6-408 



Silver . . 



. . 3094 



Iron . . 



. . . 1-313 



Cobalt . 



. . 0-491 



Lead . . 



. . 0-043 



Antimony 



. . 24-770 



Sulphur 



. . 30-049 



Arsenic . 



. . 1-292 



Rock 



. . . trace 



100-370 



On comparing the Tyddynglwadis mineral, however, with a 

 specimen of the aphthonite from Wermland, the external charac- 

 ters differed so considerably as to lead to the inference that it 

 was not this mineral, but some other variety of polytelite. 



Unfortunately it was found quite impossible to obtain the 

 mineral sufficiently free from the associated galena to enable 

 confidence to be placed either in the determination of its specific 

 gravity or exact chemical composition. The fact, however, of its 

 being a true polytelite was satisfactorily proved by the assays 

 of Mr. Williamson, which showed that the percentage of silver in 

 the mineral, as free from extraneous matter as possible, amounted 

 to 11*25 per cent. 



The minerals which have been discovered occurring in this iode 

 associated with the polytelite are as follows : — Native gold, native 



a percentage of silver as to cause metallurgists to regard them as silver ores 

 properly so termed, when distinguishing them from the so-called argenti- 

 ferous copper ores, I have done so in accordance with the views expressed 

 by Dana, in his paper " On System in Mineralogical Nomenclature" (Ame- 

 rican Journal of Science and Arts, vol. xliv. p. 145, September 1867), and 

 because I further consider this name a convenient one for distinguishing these 

 varieties of the mineral species tetrahedrite, for which w r e have no other 

 distinctive English name, — since the expression "argentiferous grey copper 

 ore or tetrahedrite " merely intimates the presence of silver (which is very 

 rarely, if ever, altogether absent from any tetrahedrite), but does not in- 

 dicate that this metal enters as an important constituent into the compo- 

 sition of the mineral itself. 

 * OJvers. Kongl. Vet. Akad. Forhandl. 1847, vol. iv. p. 87. 



