186 C. W. MARSH. — GEOLOGICAL NOTES 



at or near their junctions with it, where they gradually change 

 to siliceous manganic iron-stone similar to that of the main 

 out-crop. 



Besides these there are numerous branch veins and lenticular 

 masses. The former starting from the main or one of the loop 

 veins break through the rock for some distance, after which they 

 follow the strike of the strata, eventually disappearing, or more 

 rarely continuing as disconnected lenticular masses. The latter 

 apparently have no connection with the fissure openings, and 

 have been proved by underground explorations to be limited alike 

 in depth and horizontal extension, some of these found below not 

 appearing at the surface, and vice versa. None of these surface 

 out-crops are particularly inviting, though at lower depths they 

 have been more or less decomposed, and their removed material 

 invariably replaced by metallic ores. Those nearest the main or 

 the loop veins appear to have suffered the greatest change. This 

 wide spread process of decay and re-formation has not only 

 affected these veins and deposits, together with their wall-rocks, 

 but has more or less decomposed the feldspar and mica of the 

 gneiss rock to a distance of several hundred feet on both sides of 

 the main vein. That the zones of decomposition should have 

 been chiefly confined to these veins and their wall- rocks, dimin- 

 ishing in the surrounding strata with its increase of distance 

 from them, is what we should expect, considering that such 

 fissure lines must have been the battle-ground on which the 

 opposing upward alkaline and downward earthy and organic 

 solutions would meet and contend. This exchange -and inter- 

 change of bases would ultimately result in the survival of the 

 fittest, and carbonic acid would be cast off and abandoned. A 

 part of the acid being circulated laterally through the rocks by 

 the permeating waters may have been mainly instrumental in 

 the kaolinization of the feldspathic minerals by the removal of 

 their alkalies and lime as soluble carbonates, and the fixing of 

 their aluminous and magnesian bases as insoluble silicates. 



The occurrence of garnets, apparently a mixture of colophonite, 

 aplome, melanite, &c., in crystalline and granular aggregates, 

 forming irregular masses often of great size in these veins and 

 their decomposed and partly replaced wall-rocks serve to show the 

 parts taken by the oxides, lime, iron, and manganese, during the 

 fixation of the aluminous silicates ; while the occasional presence 

 in these garnetiferous masses and their surrounding altered rocks 

 of such minerals as pleonaste, and dysluite, in which silicates take 

 little or no part, show that stages in the process have been reached 

 where in the presence of magnesia and the oxides of iron and zinc, 

 the almost complete disassociation of alumina from silica has taken 

 place. It would serve no useful purpose to enter into a minute 



