of the Gold-fields of California. 325 



sidered an unfavourable indication with regard to its auriferous 

 character. 



The quartz considered by miners as most " favourable for 

 gold" is seamy, stained by oxide of iron arising from the decom- 

 position of pyrites, mottled, and somewhat marble-like in ap- 

 pearance. In addition to ordinary quartz in a more or less crys- 

 talline form, amorphous hydrated silica, or semi-opal, and chal- 

 cedony are occasionally met with : and in some instances the opal 

 is interfoliated between layers of true quartz, and is sufficiently 

 auriferous to repay the expenses of treatment. 



Generally the walls of auriferous veins are smooth and well 

 defined, often affording evidence of a considerable amount of 

 dynamic action; and in the case of the lead being divided into 

 bands by interfoliations of slate, these, to a less extent, are some- 

 times marked by groovings indicative of mechanical motion. 

 Between the vein and the enclosing rock there is sometimes, 

 but not always, a thin stratum of clay, which occasionally en- 

 closes small particles of gold. 



The metallic minerals enclosed in the gangue of auriferous 

 veins are ordinary iron pyrites, blende, and galena, and, less fre- 

 quently, arsenical pyrites, magnetic and copper pyrites, and cin- 

 nabar. These sulphides invariably contain gold ; and veins in 

 which some one or more of them does not occur in consider- 

 able amounts, are not regularly and lastingly productive. In the 

 earlier days of quartz-mining these sulphides were allowed to 

 escape, and the "free gold" was alone obtained; but at the pre- 

 sent time they are all carefully collected, and form an important 

 addition to the profits of the miner. 



Near the surface the iron pyrites and other sulphides become 

 decomposed by the action of air and the percolation of meteoric 

 water through the mass, staining the quartz of a red or brown 

 colour, and leaving the gold in a form highly favourable for 

 amalgamation. Under such circumstances numerous cubical 

 moulds of iron pyrites are found in the veinstone; and although 

 this mineral has been entirely removed by chemical action, the 

 cavities left contain finely divided gold, obviously liberated by 

 the decomposition of pyrites. 



Beneath the line of natural drainage of the country the 

 sulphides remain undecomposed, and the extraction of gold be- 

 comes more difficult ; but if "rock" containing crystals of pyrites 

 be placed in nitric acid and allowed to remain for a few hours 

 in a warm place, the sulphide becomes dissolved and finely di- 

 vided, or filiform gold will partially occupy the resulting cavities. 



In addition to the gold thus enclosed in the metallic sulphides, 

 grains and small plates of that metal are disseminated through- 

 out the veinstone; and this is particularly the case in the vici- 



