GEOLOGICAL IMPORTANCE OF SEDIMENTATION 337 



another, an intercalation of subaerial and submarine strata. As an 

 illustration may be cited discussions regarding the Neopaleozoic 

 deposits of the eastern United States. The interpretation has usually 

 been that of river sediments distributed over the bottom of a shallow 

 interior sea, in which subsidence took place pari passu with sedimen- 

 tation, so that occasionally mud-flats or marshes became exposed; 

 but subaerial delta surfaces are hardly considered, and the region is 

 held to have been essentially a permanent sea, so long as sediments 

 of the time were formed. 



Of course, where marine fossils occur, there is no question of the 

 presence of ocean waters, and, on the other hand, at times of coal- 

 formation there is no question as to the presence of a delta swamp; 

 but in regard to the great volume of more or less completely unfos- 

 siliferous detrital deposits the interpretation has usually been one of 

 marine origin, without an adequate consideration of a possibly sub- 

 aerial delta nature ; and this sometimes in spite of the fact that mol- 

 lusca are found whose habitat seems indicative of non-marine waters, 

 as, for instance, Amnigenia of the Upper Devonian. 



The deposits of the present, as Walther has noted, are studied in 

 horizontal plan ; the deposits of the past are studied in section. It is 

 to indicate from present delta-building the vertical relations which are 

 to be expected in ancient deposits between the continental, littoral, 

 and marine portions of the delta, that a considerable discussion seems 

 necessary. 



RELATION OF DELTAS TO REGIONS OF SUBSIDENCE 



The ratio of these three portions to be anticipated in the strati- 

 graphic record will depend largely upon the conception of the part 

 which subsidence of delta regions plays, as a usual, or merely an ccca- 

 sional, accompaniment of the process of delta-building. As facts 

 bearing upon the question, may be cited the presence of fresh- water 

 deposits within a number of the present larger delta regions at levels 

 beneath the surface of the sea. Again, periodical flooding of the delta 

 surface is evidence of land deposition going forward at the present 

 time, .while rivers intrenched, and never overflowing the plains for- 

 merly built up by their agency, are evidence that aggradation has 

 ceased, or even that degradation has begun. Taking a general view 



