ON THE ALLUVIAL DEPOSITS. 369 



of fragments and the accumulations of these below, 

 are sufficient evidences of these alluvia and of their 

 cause ; but, in other cases, it must be sought in other 

 indications less obvious. Such is that permanence of 

 trap veins noticed in treating of denudations; while as 

 these appearances occur on very gentle declivities, as 

 in Cumbray and Isia, or almost on level ground, as 

 near Cornrie, here is a demonstrated cause, of even 

 transportation, which geology has overlooked. That 

 even " boulders" may have been gradually moved, in 

 this manner, to great distances from the parent rock, 

 is abundantly obvious. And I may here say, once for 

 all, that whatever volumes may have been bestowed on 

 these " travelled blocks," there is no reason for sepa- 

 rating them from the several classes of alluvia to which 

 they belong. 



Alluvia of Rivers. 



A due attention to the two preceding classes of al- 

 luvia, will disengage the question of " diluvian" de- 

 posits from much of the obscurity in which it has been 

 involved ; but the greater confusion on this subject 

 has arisen from those deposits by rivers^ to which this 

 term is more strictly applicable. 



Though the conclusions respecting the alluvia of 

 rivers must be modified by recollecting that these may 

 have sometimes transported the materials of more an- 

 tient deposits, so as to have simply transferred what 

 they found, without destroying the rocks, it is seldom 

 easy to separate the two sources, nor is it often neces- 

 sary. I must here therefore consider the alluvia of 

 rivers as being their own produce; having thus pro- 

 vided against an error, which is, in the present discus- 

 sion, of no moment; since that which has been placed 

 by a river, is, in reality, its alluvium. And as these are 



VOL. II. B B 



