380 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



theory during the past half century may be made to the problem of the 

 relation of continental and marine deposits. The rise of geology in 

 western Europe, where mountain glaciers and river deltas though inter- 

 esting are relatively small and inconsequential factors at the present geo- 

 logic time, and where marine erosion on the other hand is an impressive 

 phenomenon, gave an initial trend to geology which underestimated the 

 possibilities of continental glaciation and continental sedimentation. A 

 trufer perspective regarding glaciation has been attained by the study of 

 the j)olar regions and of sedimentation by the study of other continents ; 

 but the records of delta deposits and their distinction from other forms 

 of sedimentation are not so clear as are the marks of glaciation, with the 

 result that the criteria for the recognition of delta deposits are still open 

 to discussion and further elaboration. 



But glaciers have their cycle of inception, growth, and retreat, and in 

 any refined apj^lication of the criteria of glaciation they must be dis- 

 cussed from the standpoint not only of the part of the glacier which is 

 concerned but of the stage in the glacial life. The erosion b)^ rivers like- 

 wise passes through its cycle of youth, maturity, and age, and the charac- 

 teristics of the river valley and river waste change with the distance 

 from the headwaters and with the progress of the erosion cycle. There 

 must also be a delta cycle, and it is to be expected that the size of the 

 delta and the character of its deposits will depend not only on the origi- 

 nal relation of the other physiographic elements of the continent, but on 

 tlie 23rogress of the cycle of erosion on the one hand and of tlie cycle ol' 

 (lej^osition on the other. 



Consequently the first part of this paper is largely deductive in its 

 mode of presentation and deals with the definitions of a delta and its 

 parts, followed by an analysis of the delta cycle, dependent on river ero- 

 sion and the changing level of the sea. The conception which it gives is 

 then applied to a certain sequence of formations as an example of its use 

 and tends to ascribe to delta-building a kind of deposition which has 

 heretofore been assigned to an estuarine origin. 



The second part of the paper is inductive in its method, and consists 

 of a revicAV of all those stratigraphic criteria which have been used within 

 recent years as evidence by which to separate continental and marine 

 formations. Some are concluded to be of negative value; others may 

 indicate either mode of origin, according to their character and domi- 

 nance. Still others are positive criteria for the parts of the formations 

 in which they occur. Although the method is inductive, yet in the 

 absence of observed facts in regard to some modern conditions of sedi- 



