180 



pf the gradual variation of character, or else 

 of the ultimate loss of one, or more, of the in- 

 gredients which constitute the distinction. The 

 instance of gneiss, which, in losing its felspar, 

 becomes micaceous schist, and of shale, which, 

 by an increase of its hardness, becomes siliceous 

 schist, will explain the nature of these transitions 

 among the families of rocks. 



Where the transition is thus completed, as in 

 these two instances, there is no longer any diffi- 

 culty ; as the specimen can be referred to its 

 proper place. But, in the gradual change of 

 character which precedes the perfect transition, 

 it becomes often impossible to determine where 

 such a rock should be placed ; unless when de- 

 cided geological evidence can be obtained, to 

 confirm that which remained doubtful from its 

 mineral characters. 



This difficulty, however, is a part of the very 

 nature of the subject ; since no arrangement 

 can pretend to define and separate those objects 

 which the hand of nature has neither defined nor 

 separated. 



