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knowD. But, in many places, large tracts of one, 

 or of the other class, are found separately ; in 

 others, detached portions of the secondary are 

 found connected with extensive tracts of the pri- 

 mary, or insulated masses of the latter are found 

 among the former ; the relative positions of the 

 two being further, in each case, such, as either to 

 afford no evidence respecting the superiority of 

 the one or the inferiority of the other, or, some- 

 times, even indicating a relation which is dis- 

 covered, by an extensive set of analogies or com- 

 parisons, to be false. It is, moreover, often neces- 

 sary to decide on the class to which a single spe- 

 cimen should be referred, when detached from 

 its native connexions. The single character of 

 relative inferiority, is not therefore sufficient to 

 form a practical distinction for the primary class ; 

 although it is the essential one, and that on 

 which the class itself is founded. 



There is another character, however, by 

 which it has been supposed that the primary was 

 distinguishable from the secondary class of rocks ; 

 and on which, indeed, too much stress has fre- 

 quently been laid, as if it formed an essential dis- 



