464 



OVERLYING ROCKS. 



IT was already remarked, in describing the 

 general plan of this work, that it appeared neces- 

 sary to treat of the whole of the rocks included 

 under this comprehensive title, in one division. 

 Much consideration was bestowed on various 

 plans for separating them; but none of these 

 could be adopted, without producing inconve- 

 niences far greater than those which follow from 

 the present arrangement. If all the masses of 

 these rocks in nature, are not posterior to the 

 secondary strata, no correct and unexceptionable 

 rules can be given for distinguishing those which 

 are of an earlier date ; while, in a general sense, 

 they possess the universal common character of 

 being unstratified, and posterior to the rocks 

 with which they are connected. 



With respect also to their mineral composition 

 and structure, it must be observed, that every 



