OF THE SECONDARY CLASS. 383 



by no means unusual for the secondary strata to 

 follow the primary in parallel order. Even 

 among the primary as well as the secondary 

 strata, the reverse or unconformable position 

 sometimes occurs. It is plain also, that, as a 

 practical distinction, this rule is of little value, 

 w r ere it even more constant ; because it can only 

 be made use of at the boundary of the two classes, 

 and perhaps not often with great safety even there, 

 unless the very junction also is visible. 



As the unconformable position, however, 

 occurs in a sufficient number of instances to be 

 a remarkable circumstance in the history of the 

 secondary strata, it has materially assisted, in 

 conjunction with other characters, to fix the 

 boundary of the class in which they are included, 

 and, consequently, to distinguish the whole of 

 the strata which enter into it. Whatever rock, 

 last in the order of succession downwards, has 

 been found to present this distinction in the pre- 

 dominant cases, especially if that circumstance 

 is united to some other of the characters of the 

 secondary class, it is plain that every rock which 

 follows it in the order upwards, must belong to 



