OVERLYING ROCKS. 465 



modification included under this title, occurs as 

 a recent secondary rock, and that they all, with 

 exceptions that do not affect the present question, 

 pass into each other in an imperceptible manner. 

 There is therefore no distinction either of a 

 mineral, or of a geological nature, capable of 

 guiding* us in any attempt to separate them into 

 distinct families ; much less, in dividing them 

 between the two classes of primary and secondary. 

 To illustrate all these circumstances in a manner 

 as ample as the case deserves, would require a 

 geological history of the whole, which it has been 

 found necessary to exclude from this work, and 

 which, in this instance in particular, would lead 

 to a great length of discussion, Hereafter, when 

 additional experience shall have added to our 

 present imperfect knowledge, it may become 

 comparatively easy to separate them under dif- 

 ferent heads, and thus to remove an inconve- 

 nience of which the writer is fully sensible, but 

 which he confesses himself at present unable to 

 remedy, without producing others still greater. 



The several rocks of this family, form tracts 

 of very various extent in different countries . 



H H 



