158 ORIGIN AND MODE OF OCCURRENCE OF GOLD-BEARING VEINS. 



portions of a dyke, they are of the nature of impregnations, and 

 may either be contemporaneous with the rock itself, or afterwards 

 deposited there by infiltration of mineral waters. 



In the Com stock lode, not only has the country rock been 

 proved to contain in its minerals all the matter found in the 

 veins, but also the gold and silver are in the same proportion to 

 each other in the rock as in the veins. The decomposed portions 

 of the walls of the lode have not the same amount of gold and 

 silver in them as the undecomposed, and sufficient decomposition 

 of the walls is said to have taken place to account fully for as 

 much matter as is found in the veins, by supposing such to 

 have been derived from the decomposed parts. 



The intimate association of iron pyrites with gold has been 

 already referred to, and the fact that in the lower levels of our 

 gold mines, the larger proportion of the gold occurs in this 

 mineral has been shown. This will not appear so remarkable when 

 we consider that nearly all metals are found as sulphides in the 

 lower portions of metalliferous mines, in other words, in those 

 parts that are least altered or decomposed, and appear to have 

 retained to the greatest extent the original state in which they 

 were first formed. 



As to whether gold ever exists in a sulphide form in the pyrites 

 is not known, although some experiments seem to imply that 

 such is probable, but the sulphide of gold being a most unstable 

 compound, renders it exceedingly difficult to determine whether 

 it ever exists in .nature in that state. It is certain, however 

 that iron as a sulphide is the most usual associate of the precious 

 metal, and therefore, if these two, iron pyrites and gold, are 

 deposited from solution in the veins and lodes,, they must be 

 precipitated together by the same agent, or one is the precipitant 

 of the other. Experiments in the laboratory have proved 

 that sulphate and sulphide of iron will precipitate gold from a 

 solution of chloride of gold. Quartz also may be produced by a 

 heated solution of carbonic dioxide decomposing silicates and 

 depositing the silica on cooling. 



Noting such facts as these, and then taking into consideration 

 the intense heat, great pressure, aud other known and unknown 

 agencies that must be at work in the internal laboratory 

 of the earth, it seems that there are good grounds for believing 

 in the strong probability of most of our metalliferous or mineral 

 veins, and lodes, being deposited from mineral waters that obtain 

 their contained metals and minerals from the country rock through 

 which they percolate, by the strongly solvent powers they 

 possess under certain conditions ; conditions that are at present 

 only partly guessed at and may never be fully understood 

 practically, but always remain as theories, although based on 

 strong circumstantial evidence. 





