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CHAPTEE XXII. 



- 



REPRODUCTIVE EFFECTS OF TIDES AND CURRENTS. 



DEPOSITING POWER OF TIDAL CURRENTS -SILTING UP OF ESTUARIES DOES 

 NOT COMPENSATE THE LOSS OF LAND ON THE BORDERS OF THE OCEAN- 

 BED OF THE GERMAN OCEAN— COMPOSITION AND EXTENT OF ITS SAND-BANES 



•STKVTA DEPOSITED BY CURRENTS IN THE ENGLISH CHANNEL- ™ 

 SHORES OF THE MEDITERRANEAN— AT THE MOUTHS OF THE AMAZONS, 

 ORINOCO, AND MISSISSIPPI— WIDE AREA OVER WHICH STRATA MAY BE FORMED 

 BY THIS CAUSE. 



ON THE 



>/ 



— From the facts enume- 



rated in the last chapter, it appears that on the borders of 

 the ocean, currents and tides co-operating with the waves of 

 the sea are most powerful instruments in the destruction and 

 transportation of rocks ; and as numerous tributaries dis- 

 charge their alluvial burden into the channel of one great 

 river, so we find that many rivers deliver their earthy con- 

 tents to one marine current, to be borne by it to a distance, 

 and deposited in some deep receptacle of the ocean. The 

 current, besides receiving this tribute of sedimentary matter 

 from streams draining the land, acts also itself on the coast, 

 as does a river on the cliffs which bound a valley. 

 waste of the cliffs by marine currents constitutes on the 

 whole a very insignificant portion of the denudation annually 

 effected by aqueous causes, as I shall point out in the sequel 



of this chapter (p. 571). 



In inland seas, where the tides are insensible, or on those 

 parts of the borders of the ocean where they are feeble, it is 

 scarcely possible to prevent a harbour at a river's mouth 

 from silting np ; for a bar of sand or mud is formed at points 

 where the velocity of the turbid river is checked by the sea, 



Yet the 



or where the river and a marine 



current neutralise each 



