THEORY OF METALLIFEROUS VEINS. 245 



Metallic Ores. — There seems no reason to doubt, then, that, in most 

 cases at least, vein-stuffs have been deposited from hot alkaline solu- 

 tions. Now, it is evident, from their intimate association with the vein- 

 stuffs, that the metallic ores must have been deposited from the same 

 solution. The exact nature of the solvent and the chemical reaction 

 is still very doubtful. We may imagine many by either of which the 

 deposit might take place: 1. Metallic sulphides are by far the most 

 common form of ore, and even when other forms exist we may in many 

 cases trace them to sulphide as their original form (p. 239, et seq.). But 

 metallic sulphides are slightly soluble in alkaline sulphides, and these 

 latter are often found associated with alkaline carbonates in hot springs 

 (solfataras), as in California and elsewhere. Such waters would hold 

 in solution silica, carbonates of lime, etc., and metallic sulphides, and, 

 coming up through fissures, would deposit them both by cooling and by 

 relief of pressure. Or, 2. Alkaline carbonate waters holding in solu- 

 tion silica and lime carbonate for vein-stone, and also containing alka- 

 line sulphide, meeting and mingling in the same fissure with other waters 

 containing metallic sulphates, by reaction would precipitate metallic 

 sulphides (NaS-|-MS0 4 =N"aS0 4 -fMS). This seems to be the reaction 

 by which the inky waters of some of the hot springs of the California 

 geysers are formed. Or, 3. The alkaline carbonates still remaining for 

 vein-stone, metallic sulphates, in solution in the same waters with 

 organic matter, would be reduced to the form of metallic sulphide, 

 which, being insoluble, would be deposited.* Or, 4. Alkaline sulphide 

 waters holding metallic sulphides and organic matters in solution — the 

 acids of organic decomposition (humus acids) would neutralize the 

 alkalinity and deposit the metallic sulphide. For greater clearness we 

 annex a table expressing these processes : 



l.Alk.S + MS in sol n deposit MS by cooling. 



2. Alk.S + MS0 4 meeting " MS " reaction. 



3. MS0 4 + org°mat r " MS " reduction. 



4. Alk.S + MS + org c mat r in sol" deposit MShj neutralization. 



Alk.OOs + HC0 3 + 



There are many difficulties in the way of every attempt to place 

 these reactions in a clear and distinct form, but in spite of these diffi- 

 culties there seems little reason to doubt that great fissures have been 

 filled by deposit from hot alkaline waters holding various mineral sub- 

 stances in solution. The more insoluble substances are deposited in 

 the vein, while the more soluble reach the surface as mineral springs. 



* It might at first seem that there is a chemical difficulty in this case — that metallic 

 sulphate can not coexist in solution with alkaline carbonate, but would be precipitated as 

 metallic carbonate. But it is evident that this reaction would not take place in a weak 

 metallic solution, in the presence of excess of carbonic acid, since in this case the metallic 

 carbonate is soluble. 



