13 



of a rock be made the basis of arrangement, it is 

 equally plain, that under the granitic, the por- 

 phyritic, or the schistose, cognate substances will 

 be widely separated, and those which are entirely 

 different in other more essential circumstances, 

 be associated. 



/ 



As the very purpose of a mineralogical arrange- 

 ment is to describe rocks considered as indepen- 

 dent of their connexions in the great order of na- 

 ture, or to produce accurate definitions of speci- 

 mens, it necessarily renders of equal value those 

 which are the most and those which are the least 

 important in the history of the earth. Accidental or 

 rare modifications thus claim as great a degree of 

 attention as those which are the most steady and 

 of the most general occurrence; while differences, 

 extremely important in the geological history of 

 rocks, but not of a nature to excite much attention 

 in a specimen, or not presenting mineralogical 

 differences of a conspicuous nature, pass with 

 little notice. 



Hence, although the affinities on which a 



