X PREFACE. 



it must have served to increase the confusion al- 

 ready prevailing in the characters of this rock, 

 precisely in proportion to the number of new 

 varieties that were observed. 



No expedient could be adopted for naming 

 these varieties, without framing new terms ; while 

 they were too numerous and important to be 

 passed over in silence. But such an attempt ap- 

 peared no less difficult than inconvenient ; and, 

 to have made use of the general name of gneiss 

 without explanation, would have been to produce 

 the confusion which has invariably resulted from 

 neglecting to define the sense in which such 

 terms are used. Hence originated the Synopsis 

 of Gneiss introduced into that work. 



It was intended to have followed the same 

 plan with respect to some other rocks described 

 in those volumes, where their varying compo- 

 sition did not appear to have been thoroughly 

 understood ; but it was soon perceived, that the 

 bulk of the work would have been inconveniently 

 increased ; and that an imperfect attempt toward 



