PT. iiT. OK POISONED BY VEGETABLE GROWTH? 175 



one direction. No marine current could make a single 

 channel sloping from a height to the sea ; still less the 

 myriads on myriads of dry upper valleys which ramify 

 in all directions, from all river valleys through and 

 to all sides of the tops of all elevations, whether higli 

 or low. 



But, in fact, the action of the sea impedes the 

 formation of valleys, instead of making them. The sea 

 often beats back, in the form of a bar, what the opera- 

 tion of rain on rivers forces into it ; and valleys often 

 form land in the sea, instead of the sea forming valleys 

 in the land. The delta of the Ganges stretches 220 

 miles into the sea, with a base of 200 miles. The 

 Mississippi has pushed a delta fifty miles out into the 

 sea, with an area of 14,000 square miles ; yet so far is 

 it from ' acting on lines ' in eroding its banks, that at 

 New Orleans it is less than half a mile wide : and it 

 may be said to act on lines in building its banks, for 

 it has raised its own banks above the land they pass 

 through, and increases the area of its alluvial plain at 

 the upper end and sides, as well as the depth of it. 



These effects are owing to the increase of the delta ; 

 and the same cause has produced the same effects in the 

 valley of the Nile : for the lengthening of the delta 

 lengthens the channel of the river with it ; but the sea 

 (leaving out the effect of the tide) tends to keep the 

 surface of the river always at a dead level. Now a 

 river flowing for 200 miles, or even fifty miles, or one 

 mile at a dead level, is very ill-calculated to discharge 



