220 



imperfect science, and to extend the boundaries 

 of our knowledge. 



It therefore follows from the preceding re- 

 marks, that in every distribution of rocks founded 

 on the present, or on any geological arrange- 

 ment, the same mineral compound will occasion- 

 ally be repeated in both divisions. Whatever 

 confusion or inconvenience may be supposed to 

 flow from this, it is evident that the circumstance 

 is in itself highly interesting, as forming part of 

 the natural history of rocks, and that it is an 

 important fact in geology. It is one of those 

 occurrences which prove that, in a practical 

 view, much greater inconvenience would follow 

 from an arrangement founded on mineral cha- 

 racters alone; as it would confound together 

 substances which it is most important to distin- 

 guish. In consequence of this occasional resem- 

 blance, or identity, between certain members of 

 the primary, and others of the secondary class, it 

 will assist the student to enumerate those rocks 

 appertaining to the former which have no re- 

 semblances in the latter ; and to point out those 



