IX.] THE WORK OF RAIN AND RIVERS. 145 



catchment-basin, but exactly opposite in direction. In the 

 catchment-basin all the branches converge to the main 

 stream ; in the delta they all diverge from the trunk channel. 

 The difference between the catchment basin and the delta 

 is shown in Fig. 36. 



In many deltas, the alluvial land is swampy, or washed by 

 the sea at high tide; and the alluvium may, in some cases, be 

 traced beneath the level of the sea, in the form of shoals 

 and sand-banks, which are made up of the lighter particles 

 of detritus swept out beyond the true delta. The great 

 Indian rivers, the Ganges and Brahmaputra, form together 



Fig. 36. — Catchment-basin and delta of a river. 



a vast delta, consisting, for the most part, of marshy land 

 supporting a growth of mangroves and nipa palms. The 

 delta of the Mississippi (Fig. 37) is an enormous tract of 

 swampy ground in the Gulf of Mexico, furrowed by 

 numerous streams and lakes. Holland may be regarded as 

 an old delta, formed by the Rhine and the other rivers that 

 pass through it. And, on the coasts of this country, we 

 frequently find tracts of alluvial land, such as that forming 

 Romney Marsh. Occasionally, the estuaries of our rivers 

 become silted up, more or less completely, and thus impede 

 navigation. In the times of the Romans, the Isle of Thanet 

 was separated from the Kentish coast by a channel suffi- 

 ciently wide to admit the Roman fleet ; but this channel is 



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