624 COAST OF THE NETHEELANDS. 



cross-dikes, the quantity of solid matter wMcli would be conveyed 

 to a given portion of land during a single inundation would be 

 extremely small. Inundations of the Ehine occur but once or 

 twice a year, and higb water continues but a few days, or even 

 hours ; the flood-tide of the sea happens seven hundred times in 

 a year, and at the turn of the tide the water is brought to almost 

 absolute rest. Hence, small as is the proportion of suspended 

 matter in the tide-water, the deposit probably amounts to far 

 more in a year than would be let fall upon the same area by the 

 Rhine. 



This argument, except as to the comparison between river and 

 tide water, apphes to the Mississippi, the Po and most other great 

 rivers. Hence, until that distant day when man shall devise 

 means of extracting from rivers at flood, the whole volume of 

 their suspended material and of depositing it at the same time on 

 their banks, the system of cross-dikes and colinatage must be 

 limited to torrential streams transporting large proportions of sedi- 

 ment, and to the rivers of hot countries, like the Nile, where the 

 saturation of the soil with water, and the securing of a supply for 

 irrigation afterwards, are the main objects, while raising the levd 

 of the banks is a secondary consideration. 



