THE EEPUBLIC TROUGn. 553 



necessary to believe that concentration either lias preceded the foldin<»- or 

 went on contemporaneously with it. In the case also of those deposits 

 which are traversed by the plane of division between the upper and lower 

 series, and so lie partly in both, the part of the deposit in the lower seines 

 may have been partly concentrated at the time of the Upper Marquette 

 transgression, while that in the upper series may be partly fine debris 

 derived from the underlying- body. In all the deposits the })rocesses of 

 enrichment have doubtless proceeded continuously through all subsequent 

 time. 



