226 PROCEEDINGS OF THE AMERICAN ACADEMY. 



found that deltas were not due necessarily to a large amount of sedi- 

 ment ; that they are not explained by the greater or less velocity of 

 current ; that their presence is not determined by a deep or shallow sea 

 in front of the mouth of the river ; that they are not explained by the 

 presence or absence of an offshore bar ; that they occur even where tides 

 "are strong ; that " the presence of a controlling ocean current (Tiber, 

 Rhone) does not alone suffice to prevent the formation of deltas " ; and, 

 finally, that deltas are not prevented by the wind. He then goes on 

 to show that delta growth is aided by slow elevation and hindered or 

 prevented by gradual depression. He concludes that relative elevation of 

 the land with respect to the water is the controlling cause of delta growth. 



Such slow elevation of the land is surely an aid to delta extension, but 

 it is only one of the factors which work together in the determination of 

 delta growth ; and cannot be considered necessary, in the opinion of the 

 present writer, for the aggradation of a coast line by rivers. 



Further Study. — The subject of deltas offers a very attractive field for 

 investigation. The writer has not been able to make out nearly as much 

 as he had expected to in regard to the stages of development shown by 

 deltas. He is convinced that each delta goes through appropriate stages, 

 but the variables are so many, and vary between such wide limits, that 

 the laws of development are not clearly seen. Vigorous advancing 

 deltas are characteristic of maturity, following both uplift and depres- 

 sion. But the maximum of delta growth may be either before or after 

 this period, as shown above. Bay-deltas have been separated from the 

 rest, as they show characteristic stages following depression. When 

 better understood, other deltas will fall into their appropriate stages. 



Classification. — Deltas are here classified, not according to stage in 

 cycle, but according to ratio of activity between river and sea. The 

 examples of shore development by delta growth are arranged in the 

 following series. 



1. Lobate deltas : (a) unilobate ; (b) multilobate. 

 These show the river intention successful. 



2. Cuspate-lobate deltas. 



The river intention is in these deltas predominant, but the sea action 

 prevents typical lobes. 



3. Cuspate deltas. 



The river mouths at the point of intersection of two shore curves, 

 concave seaward. 



4. Rounded deltas. 



The shore currents prevent the cuspate extension. 



