CONDITIONS OF SEDIMENTARY DEPOSITION. 491 



Atlantic coast is fringed with estuaries which are carefully 

 mapped by the Coast Survey, but variations of deposit with 

 changes of current have apparently not been described. Writing 

 of the La Plata, an estuary 125 miles long, where the tide from 

 the Atlantic contends with the current of the rivers Parana and 

 Uruguay, Revy says: J 



"At this point, where the power of the tidal wave balances 

 that of the rivers, there will be no current ; the level of the estuary 

 will rise slowly like that of a lake receiving supply from all 

 round its border. It is here — where the rivers and the tidal wave 

 contend for supremacy, each trying to establish its own current, 

 and where for hours the power of either of them trembles in the 

 balance without any sensible movement in any direction — that 

 deposit copiously takes place ; matter, held in suspension by the 

 rivers as long as their currents are maintained agitating their 

 water, is dropped as soon as they come to rest. It is here, within 

 about 10 or 20 miles of the river's mouth that banks are most 

 rapidly growing and islands are forming, and the ultimate result 

 of these daily contests is invariably in favor of the rivers which 

 slowly but steadily encroach on the estuary and ultimately annex 

 its whole territory. The progress of the tidal wave is, however, 

 never checked an instant, the rivers only check the currents orig- 

 inating with the wave A tidal wave is never visible to 



the eye, and can only be conceived from observation, by a suc- 

 cessive measurement of its dimensions, which are very large. We 

 may, from an elevated position, see 10 or 15 miles, but a tidal 

 wave on the La Plata is about 258 miles long. .... 



". . . . During the second half of the tidal wave, viz., from 

 flood to ebb when the surface of the La Plata is falling, there is 

 much more uniformity in the directions of the currents, which 

 for a time will be the same for the whole estuary, all tending to 

 the Atlantic. The wave will again proceed faster in the deeper 

 than in the shallower portions of the estuary, and will accord- 

 ingly make the level fall a little faster in the deeper channels, and 



1 Op. cit. pp. 29-30. 



