87 



bable that a distinction may exist in this case be- 

 tween the larger tracts and the more limited col- 

 lections of strata. It is however undoubted, that, 

 in many instances, there is no such distinction; 

 but that even the largest masses or tracts occur in 

 an uncertain order. Thus, although the great 

 tracts of argillaceous schist are most commonly 

 found on the confines of the series of primary 

 strata, they sometimes also exist below quartz 

 rock and micaceous schist ; of which Scotland 

 furnishes examples. 



It must now also be remarked, that it is, in ge- 

 neral, only in the smaller tracts, or collections of 

 strata, that the fact of alternation, or the nature of 

 the relative position, can clearly be ascertained. 

 In the larger masses, the connections are often in- 

 visible or unassignable ; either from their dimen- 

 sions, and the great spaces which they cover, or 

 from the impossibility of ascertaining truly what 

 bed is uppermost where the inclinations of the 

 strata undergo a reversal, as they are found to do 

 among the primary rocks. To assign an order 

 in such cases, is beyond the reach of our powers. 



The occasional deficiency of certain classes of 



