PREFACE. 



IN the " Description of the Western Islands 

 of Scotland," and under the article which termi- 

 nates the account of the Gneiss Isles, will be found 

 the embryo of the following book. In examin- 

 ing many of the rocks which occurred in this 

 group, it was found that, although in a geological 

 sense, they belonged to the family of gneiss, they 

 did not conform to the mineralogical definition 

 of that rock commonly received among geologists ; 

 this term having been allotted to a foliated mix- 

 ture of quartz, felspar, and mica. Had these 

 varieties been limited, or of trifling importance, 

 the author might perhaps, like his predecessors, 

 have been contented to pass over the whole with 

 little notice, and to have infringed on the mine- 

 ralogical definition of gneiss without acknow- 

 ledgment. But such a practice appeared as per- 

 nicious as it is inconsistent with accuracy ; since 



