181 



As these transitions thus constitute an im- 

 portant part of the natural history of rocks, and, 

 more especially, of that branch of geological 

 science to which the remarks in this work are 

 limited, it will be useful to place in a brief 

 and comprehensive statement, the several rocks 

 among which they are most frequent, although 

 these circumstances are necessarily noticed in the 

 preliminary remarks to each family. The learner 

 will thus the more readily know where he may 

 expect to find these uncertainties, and cease to 

 suspect his own want of accuracy and discern- 

 ment, where neither care nor acuteness can avail 

 him. 



It will also be found, that this gradation of 

 character among the families of rocks, occurs, 

 especially, in those cases where a prevailing as- 

 sociation, either of sequence or of alternation, 

 exists among certain kinds, whether of stratified 

 or unstratified substances. 



For this reason, it is also proper to give a 

 general view of these prevailing associations : a 

 view otherwise necessary, as these affinities also, 

 constitute an interesting part of the natural his- 



