GULLIVER. — SHORELINE TOPOGRAPHY. 225 



lobate delta, for the activity of the sea is also a variable, and it may 

 happen that the ratio between the two activities is more strongly in 

 favor of the river at some time before or at some time after its period of 

 maximum activity. For example, if the time of maximum activity ot 

 the sea on a given shore occurs at a later stage than the time of the 

 maximum activity of a certain river, the largest ratio in favor of the 

 river will probably occur considerably before its maximum ; while if the 

 sea's maximum occurs at an earlier stage of shore development, and is 

 decreasing at a more rapid rate than the river's activity, the ratio in 

 favor of the river will be greater after its maximum is passed. 



It may be that the sea action is so strong off any river's mouth that 

 the river never is able to carry out its intention. Indeed, this seems to 

 be the case in a large proportion of the rivers of the world. The sea is 

 relatively stronger than the river in all cases except where the volume 

 of the river is exceptionally great, as in the Mississippi, or where the 

 mouth is protected from the stronger sea attack, as is the case of the Po 

 at the head of the comparatively narrow Adriatic. 



Delta Stages. — In the initial stage deltas do not exist. At any time 

 after the beginning of a cycle, a delta may be built, whose size will 

 depend far more on the volume and drainage area of the river than upon 

 the time since uplift. 



In the cycle following uplift a delta of a certain frontal outline may 

 occur at various stages, and the forms appropriate to successive stages 

 have not been worked out, because of the many complications of tlie 

 problem, some of whose factors are indeterminate. Deltas doubtless 

 follow a normal succession of forms under the various conditions. This 

 has been shown to be true in the case of bay-deltas. A delta foreland 

 of any considerable size would not be found projecting from an initial 

 coast, where the valleys of all the larger river systems had been sub- 

 merged. Until maturity of shore development has been reached, large 

 delta forelands would consequently not be expected upon depressed 

 regions. 



Credner. — Dr. Credner's monograph on Deltas* is today, nearly 

 twenty years after its publication, the most complete source of informa- 

 tion about the deltas of the world. While his descriptive portions are 

 classics, the theoretical conclusions of the text seem open to question. 

 Dr. Credner apparently looked at deltas as phenomena requiring some 

 common cause which would account for their presence or absence. He 



* Die Deltas, Pet. Geog. Mitt., Erg. XII., Nr. 56, 1878. 



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