648 DIALLAGE ROCK. 



and in which, either the diallage, or else the fel- 

 spar, are at times altogether absent, the one or 

 other mineral alone remaining. 



Diallage rock is either simple, or, essentially, 

 compounded of felspar and diallage ; the aspect 

 varying exceedingly according to the magnitude 

 of the parts. When these are very minute, it is 

 often difficult to recognize ; the peculiar charac- 

 ters of the diallage nearly disappearing^. The 

 colours of the diallage vary from very pale greyish 

 green to a brighter green, to light and dark grey, 

 purplish brown, arid black. The aspect, of the 

 felspar is very various. It is either platy and dis- 

 tinct, or else confusedly compacted, or very finely 

 granular, or, lastly, uniformly compact. The 

 colours are white, or greenish grey, or grey, or 

 purplish grey ; but, in Shetland, the paler vari- 

 eties predominate. From these several causes, 

 the general colour of this rock varies from light 

 grey or greenish grey, to dark grey, and brown. 



This rock passes into talcose and chlorite schists 

 by the intervention of mixtures of talc or chlorite 

 with the felspar ; there appearing to be transi- 

 tions from diallage to both of these minerals, f 



