2 28 SEA AND LAND 



by rivers where they emerge through deltas to the sea, only 

 in one case the current is continuous in one direction and in 

 the other it flows alternately to and fro. Where a tide of 

 considerable height enters the mouth of a large river the 

 effects of the two actions are curiously commingled. The 

 most noticeable feature arising from their cooperation is the 

 occasional occurrence at such points of a singular great in- 

 rushino- wave, called the bore or eagre. The production of 

 this curious phenomenon appears to be due to the fact that 

 the swift current of the river for a time pushes against the 

 incoming tide, causing it to mount until it attains a height 

 sufficient to quickly reverse the direction of the stream, when 

 the resulting wave moves in with such speed, volume, and 

 height as often to produce disastrous effects. Although some- 

 thino- of this action is perceptible at the mouths of many 

 rivers which course swiftly through deltas to the sea and 

 there meet tides of great height, they are best shown at the 

 Amazon, the principal mouth of the Ganges, and the Tsientang 

 in China. In the case of the Tsientang the wave is said 

 sometimes to attain the height of thirty feet, and to move up 

 the stream for the distance of eighty miles with the speed of 

 twenty-five miles an hour. \Miere the bore is well developed 

 the effect of this action is gready to scour the channel and the 

 banks of the stream, and. as in the Amazon, to deliver very 

 large amounts of mud to be moved by the currents of the 

 river or those of the tide. It is probable that the peculiar 

 form of the Delta of the Amazon, especially the exceedingly 

 wide river mouth, is due, at least in part, to the intense scour- 

 ino- action which these singular waves produce. 



Where the tidal rise and fall is slight, as about the mouth 

 of the Mississippi, the movement in and out of the channel 



