The Paragenetic Relations of Minerals. 93 



the Freiberg furnaces, formed at a much lower temperature, 

 consist only of neutral and basic silicates, or, if there is a 

 large quantity of silica, of bisilicates. It is further remark- 

 able that quartz is never met with in rocks which have un- 

 doubtedly been formed by igneous fusion. However, the 

 mica in granite is not only fusible at a comparatively low 

 temperature, but is likewise in most cases a neutral silicate. 



While, then, there is little difficulty in regarding rocks, 

 consisting solely of silicates, as products of melted masses, 

 the case is very different with rocks containing both quartz 

 and neutral silicates. Some kind of hydrated pasty condition 

 is perhaps more easily conceivable with regard to them, and 

 at the same time their geognostic relations and transition 

 into gneiss, mica-slate, &c, must not be overlooked. Granite 

 is most frequently found to penetrate these rocks. More 

 rarely it forms beds conformable with mica-slate ; and al- 

 though these are declared to be the result of injection, there 

 is sometimes difficulty in perceiving from whence the granite 

 has been derived, — as, for instance, between Penig and Wol- 

 kenburg (Saxony), where there is no granite in the immediate 

 vicinity of the mica-slate. Further, lavas and basalt contain 

 no quartz, and the felsites met with in them are seldom or 

 ever of the same species as in granite. Pegmatolite has 

 never been found in basalt phonolite or lava, nor sanidine or 

 ryacolite in gneiss. 



It must likewise be remembered, that it still remains pro- 

 blematical whether the rocks possessing a schistose structure 

 are to be considered as the most ancient. There is, indeed, 

 some considerable probability that they are not of sedimen- 

 tary origin, for it may be supposed they were formed by the 

 solidification and scaling of the primitive liquid or pasty mass 

 of the earth at the surface, and that granite was formed more 

 slowly in the less agitated layers at a greater depth. Du- 

 rocher, however, says that it is in Scandinavia that we might 

 expect to find among the rocks of the greatest antiquity the 

 true primitive granite, which perhaps formed the original 

 solid surface of our planet. But gneiss is nowhere found 

 resting upon granite which we can venture to regard as more 

 ancient than itself. 



