ON THE BARRIER RANGES SILVER FIELD. 183 



bismuth, together with the carbonates of lime, and iron, and 

 silicates of magnesia, and iron, oxides of iron, alumina, and 

 manganese, soda, lime, and iron garnets, &c, many of these 

 being no doubt introduced subsequently to the original tilling of 

 the veins. The presence of feldspathic and hornblendic minerals 

 in these veins shows that the chemical conditions under which veins 

 of the first class have been formed, were, to a limited extent, still 

 active, while the presence of metallic and other salts found but 

 sparingly concentrated in the former, but largely represented in 

 those of later date, point to a change having taken place in the 

 solutions from which they were derived. It is not necessary to 

 assume the absence of these metallic elements from the rocks in 

 which the earlier veins were formed (since certain zones pierced by 

 them appear to have been very rich both in silver, lead, and copper), 

 but rather, that conditions were at that time unsuitable for their 

 free elimination from the rocks and their concentration in veins 

 then forming. In this connection the experiments of Ordway 

 on the solubility of metallic salts in solutions of alkaline silicates 

 is important in throwing light on the probable formation of 

 such metallic combinations as cassiterite, columbite, tourmaline^ 

 ilmenite, magnetite, and compounds of manganese and cobalt 

 found in these veins. (Vide Amr. Jr. Sc, 1861, xxxn., 338). 



The changed conditions of the latter solutions may be due partly 

 to the exhaustion of the alkalies (especially potash) by the fixation 

 of alkaline silicates, but more largely to the introduction of 

 atmospheric waters from the upper rocks bringing with them the 

 carbonates, sulphates, and chlorides of soda, lime, and magnesia, 

 whose action on the alkaline silicates would result in silicates of 

 these bases, and alkaline carbonates, sulphides, and chlorides. 

 These as is well known, have not only the property of dissolving, 

 but under slightly changed conditions, also of precipitating the 

 greater number of the metallic and other elements found in the 

 veins, and it only remains for us to study the most natural process 

 of such depositions. The solubility of quartz and many highly 

 silicated minerals, in the presence of alkaline carbonates, would 

 give rise to solutions consisting largely of silica, while the resulting 

 carbonic acid, acting upon the rocks, would convert silicates of 

 lime, soda, and probably magnesia, into soluble carbonates, at the 

 same time regenerating alkaline silicates for the further continu- 

 ance of these reactions. The simultaneous action of alkaline 

 sulphides during their process of lixiviation through the rocks, 

 might remove therefrom the salts of manganese, iron, lead, zinc, 

 copper, silver, &c; while the actions which take place between 

 alkaline chlorides and hydro-silicates of lime and alumina, and 

 through the mutual decomposition and exchange of these rival 

 bases, also of magnesia, might produce solutions with a high degree 



