256 GNK1SS. 



the ordinary varieties of gneiss, and also gra- 

 duates into them. It does not appear to have 

 been described by foreign geologists ; but it can- 

 not be considered as an accidental rock, as it 

 forms exterior tracts of country on the north- 

 west coast of Scotland. 



It must also here be observed, that there have 

 been introduced into the synopsis, several com- 

 pounds formed of other ingredients than those 

 now enumerated as the constituents of gneiss. 

 The reasons for this are, that these compounds 

 alternate with and pass into the more ordinary 

 varieties of that rock, and that no other place 

 exists in which they can be arranged. As yet, 

 they have either been overlooked by geologists, 

 or classed with those other rocks to which they 

 are also allied. In the present very imperfect 

 state of our knowledge, it seems impossible to 

 adopt a better arrangement ; but, to accommo- 

 date the student, such varieties, both here and 

 elsewhere, wherever they trespass on the charac- 

 ters of some other rock, or occur as parts of other 

 series, will be found enumerated in two places. 

 Such a superfluity appeared preferable to the 



