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 Isla, or almost on level ground, as near Comrie, 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 1 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 arc VOL. II. B B