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CHAP. XII. 



REMARKS ON THE ASSOCIATIONS AND TRANSI- 

 TIONS WHICH OCCUR IN NATURE BETWEEN 

 SEVERAL DIFFERENT SPECIES, OR FAMILIES, 

 OF ROCKS. 



BEFORE proceeding to describe the general 

 characters of the families of rocks, which is the 

 subject of the ensuing chapter, it is necessary to 

 call the reader's attention to a circumstance 

 which is a frequent source of uncertainty in 

 attempting to assign the names to the individuals, 

 or varieties, ranked under them. It has equally 

 been a source of difficulty in arranging these in 

 the several synopses, and has led to the inevitable 

 repetition of many of them under different titles. 

 The circumstance in question, is the tran- 

 sition, which, in the mixed or compound rocks, 

 in particular, so often occurs, not only between 

 the several varieties of each family, but even be- 

 tween the families themselves ; in consequence 

 N 2 



