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to draw such conclusions respecting the first of 

 these ; as the peculiarities by which they are cha- 

 racterized, although distinguishable by a prac- 

 tised eye, can scarcely be described in words 

 with sufficient accuracy to convey a definite idea 

 of their differences. 



There are, further, some rocks in the primary 

 class, so nearly resembling others in the secon- 

 dary, that, in some specimens, no difference 

 whatever can be detected ; while 5 in the re- 

 mainder, the distinctions are so slight as scarcely 

 to be perceived, except by long and habitual 

 intercourse with all their modifications. In these 

 cases, the mineral character serves no purpose in 

 ascertaining the class to which a rock belongs ; 

 and recourse must then be had to the other cha- 

 racters, which comprise its geological connections. 

 Where these are obscure, or difficult of access, a 

 rock may thus, for a long time, remain doubtful, 

 as far as relates to its relative antiquity ; or erro- 

 neous conclusions may be drawn, to be rectified 

 only by careful subsequent investigations, or by 

 a fortuitous concurrence of circumstances. 



Lastly, the absence of organic remains has 



