ON THE BARRIER RANGES SILVER FIELD. 181 



rocks, though not a constant, is a prevailing feature, the more 

 siliceous being chiefly confined to the gneissic rocks, while the 

 mica-schist appears to have favoured the formation of those made 

 up of the more basic minerals. 



Besides the essential minerals before mentioned, the siliceous 

 veins often contain muscovite in bunches, and isolated crystals, 

 seldom uniformly scattered through the rock, and more rarely 

 hornblende, andalusite, staurolite, iron-alumina-garnet, ilmenite, 

 columbite, copper, tourmaline, and oxide of tin. The accessory 

 minerals accompanying the basic veins are, besides many of the 

 above, common to both, epidote, chlorite, lime-alumina-garnet, 

 magnetite, and rarely manganese and cobalt. The compounds of 

 such metals as copper, lead, silver, and bismuth, often met with 

 in each class of vein, are apparently due to subsequent chemical 

 action. 



By calling to our aid the beautiful and instructive experiments 

 of Daubree, Friedell and Sarrisin, De Lenarmont, Sterry Hunt, 

 and others, who have, under a wide range of conditions, succeeded 

 in forming, by synthesis, the greater part of the mineral species 

 found in these veins, and assuming these minerals to bear a direct 

 relation to the solutions from which they were deposited, we 

 may from chemical considerations form a reasonable basis on 

 which to construct a theory of such deposits, and under assumed 

 •conditions attempt to trace the mode of their formation through 

 the various stages of development with a high degree of probability. 



Taking the siliceous veins, for instance, they appear to have 

 been filled by the secretion of highly saturated solutions of 

 alkaline and aluminous silicates derived from the surrounding 

 rock. The predominance of either soda, potash, or silica, 

 above that required to combine with the whole of the silicate of 

 alumina present in solution, might determine the formation of 

 albite, orthoclase, or free-quartz, and by the exhaustion of the 

 silicate of soda to the lowest point of its equivalent combining 

 value, leaving an excess of alumina with potash in solution, these 

 might, by mutual decomposition, generate muscovite, a portion of 

 iron and manganese replacing alumina during the reaction. By 

 the further removal of the alkalies, stages in the process would be 

 marked by the formation of andalusite, and free-quartz, whereas 

 if the action took place in the presence of iron, iron-alumina- 

 garnet might be formed. 



The production of veins of the basic class, characterised by 

 such minerals as oligoclase and labradorite, with hornblende 

 or pyroxene, would require solutions with an excess of silicate of 

 alumina, in the presence of silicate of soda and the earthy basis. 

 The reactions of these latter upon ea,ch other, and upon the 

 double aluminous silicates, would give rise to such admixtures as 



