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logue would answer his purposes ; were it pos- 

 sible for him to acquire that accurate knowledge 

 of a few, without a familiarity with those general 

 principles of distinction which would equally 

 enable him to master the whole of his subject. 

 To prove the necessity of this knowledge, it is 

 sufficient to point out Augit rock and Hypers- 

 thene rock ; hitherto confounded with Green- 

 stones, for want of attention to those characters 

 by which the minerals whence they derive their 

 names are distinguished from Hornblende. 

 Even now, when the existence of these rocks has 

 been ascertained, the student has no means of 

 recognizing them except by the distinctive cha- 

 racters of the two minerals in question. In 

 every case of mixed rocks, indeed, he must have 

 recourse to his mineralogical knowledge ; as it 

 is only by an enumeration of the ingredients 

 that it is possible to describe them. 



Mineralogists have sometimes divided rocks 

 into two great classes ; the simple and the com- 

 pound. According to the geological principles 

 on which the present arrangement is founded, 

 such a distinction is seldom applicable, as already 

 shown, to any purpose but that of denoting 



M 2 



