128 Professor Bischof on the Origin of 



nishecl with the felspar. But we can never be at a loss in 

 the assumption of a period of time, however long, if we can 

 only indicate a process, however slow in its action, from 

 which compositions and decompositions I'esult. The im- 

 pregnation of the adjacent rock with silica, presents itself 

 not only in quartz veins, but also in many others peculiar 

 to certain vein formations. This impi*egnation, it is well 

 known, goes sometimes so far, that the characteristic colour 

 and original texture of the adjacent rock disappear, and it 

 makes'a transition into hornstone. V.Weissenbach* cites 

 several instances of this kind. Thus this phenomenon per- 

 vades, quite remarkably and generally, the tin vein forma- 

 tion of Altenberg. I shall take another opportunity of 

 treating of how this impregnation, passing into the thinnest 

 strings, appears to have a causal connection with the intro- 

 duction of the tin ore itself. Near many of the veins of the 

 Schneeberg district, the clay-slate is richer in silica than 

 elsewhere. In and near several of the veins of the Freiberg 

 district, the gneiss and mica slate appear so much impreg- 

 nated with silica, that the original striated structure can 

 scarcely be observed ; the micaceous particles are dull and 

 clayey, the felspar has disappeared, and there presents it- 

 self, at last, a hornstone, passing quite into the quartz, in 

 which the oinginal gneiss is scarcely recognisable. 



The same appearances present themselves here in gneiss, 

 therefore, as in Coi*nwall in granite : here, as there, the 

 quartz predominates, as felspar and mica disappear. We 

 may, therefore, assert pretty generally, that when watery 

 solutions of silica circulate among rocks containing felspar 

 and mica, or any alkaline silicates, the former is precipitated, 

 the latter dissolved and carried off. And so, in veins in clay- 

 slate, the walls of which are silicified, it may have been 

 alkaline silicates, micaceous particles, &c. &c., which effect- 

 ed the exchange. We thus find in veins the same relative 

 circumstances as 1 have mentioned in reference to mineral 

 springs, only with this difference, that the acid water changed 



* Ahhildungcnmcrhtvardiger Gangvcrhaltnisse aus dein Scichsischen Erzge- 

 lirgc; Leipzig, 1836, p. 31. 



