OF EGYPT. 33 



is at present, and would consequently have swept nearly all 

 the silt below the first impediment in the river into the sea, 

 so that the extension of the Delta must have been more rapid 

 then than in the present day. As the Delta increased, the 

 rapidity of the stream decreased, so that much of the mud, 

 instead of being cast into the sea, would be deposited along 

 the river-bed, the sluggish stream having lost the power to 

 drive it forward ; and in this manner the bed of the river has 

 gradually been raised, so that at Memphis it is now sixty feet 

 above the level of the sea. This variation in the level of the 

 river-bed, and consequent change in the force of the stream, 

 must have distributed the sediment very unevenly over the 

 Delta. But there is a more powerful i-eason for its uneven 

 distribution, which arises from the manner in which Deltas 

 are formed*. 



On coming into contact with the sea the running water of 

 the river is checked, and the earthy matter it contains settles 

 down to the bottom, the larger particles falling near the river- 

 mouth, while the finer ones, which take longer to subside, are 

 carried fui-ther out to sea ; and thus, in course of time, a mud- 

 bank of a certain extent is raised, until it becomes almost dry 

 at low tide. Through this the river shapes its bed, gradually 

 embanking itself; for, as it overflows its channel it comes 

 into contact with the still sheet of water which covers this low 

 alluvial flat, and the running water thus checked is forced to 

 deposit its silt along the junction with the still water. By 

 slowly depositing mud along its course the river gradually 

 raises its bed, until the body of its waters is higher than the 

 neighbouring swamp ; it then bursts its banks, and flows 



* See Hugh Falconer on the formation of Deltas, ' Quarterly Journal 

 of the Geological Society,' 1865, vol. xxi. p. 372. 



D 



