254 GNEISS. 



is either actually fissile, or else displays indica- 

 tions of a foliated structure. As that structure 

 becomes more perfect, it recedes further from 

 granite. 



In the schistose variety, the texture is com- 

 monly minute, while the position of the several 

 minerals above mentioned, is more accurately 

 parallel. Hence the rock is almost always very 

 readily fissile; and, in some instances indeed, 

 possesses this quality in such perfection as to be 

 applicable to the same purposes as argillaceous 

 schist. This variety passes into quartz rock, by 

 the loss of its mica, or hornblende, or, some- 

 times, of its felspar also ; and, in this case, its 

 structure is commonly more granular than when 

 it passes into micaceous schist. When it gra- 

 duates into the latter rock, by the loss of its 

 felspar, it is generally very distinctly laminar or 

 schistose. 



The two preceding varieties are the most 

 abundant. The laminar is rare, but it occurs in 

 several parts of Scotland, as, for example, in Sky 

 and in Ardgowar. In this variety, each con- 

 stituent mineral is disposed separately, in laminae 



