498 CLARK AND BIBBINS 
clay indicate rapid deposition in shallow waters and the general 
continuity of the beds points to wider and deeper water areas. 
The close of Raritan sedimentation by the depression of the 
continent border and the gradual transgression of the deposits 
of the marine Cretaceous southward is a phenomenon of unusual 
interest. 
The line of unconformity which separates the Potomac group 
from the later Cretaceous formations is so poorly defined in 
local sections that absolute unconformity could hardly be proved 
except by the evidence of this gradual southward transgression 
which finally cuts off the entire Raritan formation on the banks 
of the Potomac River. 
This review of the conditions of sedimentation during the 
Potomac period and of the relations which the individual for- 
mations hold to one another and to the rocks above and below 
them points to the probability of the existence of extended areas 
of fresh and brackish waters along the eastern border of the 
North American continent during Potomac time. Just how these 
conditions could have been produced in all instances is not clear 
and speculation regarding them seems hardly warranted by the 
facts which are at present before us. 
Correlation.— The correlation of the formations composing 
the Potomac group must rest largely upon the fossils which 
have been derived from them, although the physical history 
of the continent renders it possible to establish certain broad 
comparisons along the Atlantic and Gulf borders that are not 
without their value. The similarity of conditions during the 
deposition of the Tuscaloosa beds in the south is clearly indi- 
cated, and the corroborative evidence which paleontology 
affords in this connection will be presently mentioned. 
As was stated in the introduction to this article the tendency 
of most authors hitherto has been to regard the Potomac group 
as a single stratigraphic unit and the discovery of fossils of 
known affinities within its beds to be conclusive evidence of the 
age of the entire series of strata. FYrom what has been said in the 
preceding chapters it is evident that the Potomac group is made 
