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logical classification, the same rock, or mineral 

 compound, must be described twice if it occurs 

 under two distinct general divisions ; which, in 

 some instances, it is known to do. This incon- 

 venience is trifling ; even were it necessary to 

 describe a second time that to which the student 

 may as easily be referred in its first situation. 



But this apparent inconvenience is, in fact, an 

 advantage. It is an important part of the geolo- 

 gical history of a rock, to note that it is repeated 

 under a different association : it is most essential 

 for the student to know, that many of the com- 

 pounds which form granite, for example, are not 

 necessarily inferior to the primary strata, but may 

 occur even above the secondary. 



It is objected further, that the system of a 

 geological arrangement is hypothetical and diffi- 

 cult of application. 



It may be retorted on this ground, to the ri- 

 val system, that it is not hypothetical, only be- 

 cause it includes the much greater defect of en- 

 tirely neglecting the natural affinities of rocks ; 

 and that it is not merely difficult of application, 



