14.6 



PHYSIOGRAPHY. 



[chap. 



now choked up, and the so called island is united by an allu- 

 vial tract with the mainland. But, as a rule, the rivers of this 

 country are not large enough, and have, comparatively, too 

 rapid a fall, to produce deltas. Moreover, in tidal rivers, the 

 regular to-and-fro motion of the water in its estuary hinders 



Fii;. .j7. — Deta of Lhe Mississippi. 



deposition of sediment. The scour of the ebb-tide co- 

 operates with the rapid flow of the river to sweep away any 

 sediment thrown down during the flood-tide, when the 

 downward current of the river is arrested. In some estuaries, 

 the tidal current is so charged with muddy matter that it is 



