34 GEOLOGY 



along the lowest level, and, repeating the same process, fills 

 up that depression, and forms a new bed again in another 

 part, thus continuing to raise new beds, for itself in the lowest 

 part of the land thi'ough which it flows. After a long cycle 

 of years it may come again to that channel which it fii'st left, 

 which wiU probably not have risen one foot, while the neigh- 

 bouring country may have risen twenty or thirty feet. 



Thus we see that rivers in alluvial soils, especially in 

 Deltas, have a tendency to raise their banks and confine 

 themselves to their beds for an indefinite time, until accident 

 bursts their bounds; so that any chronological conclusions 

 founded upon such data must be very fallacious, whether 

 derived from borings into the strata or from calculations based 

 upon mere superficial deposits. 



We may sum up our knowledge relating to the alluvial 

 deposit of the Nile, and the alterations which have taken place 

 in the bed of the river, under the following heads : — 



1. The land is slowly sinking in Lower Egypt. 



2. No very marked change has taken place in the bed of 

 the river in historical times. 



3. The river is always slightly shifting its bed. 



4. The Nile, within the historical period, has risen to a 

 different height at places from that to which it rises at the 

 present day. 



5. The Nile was formerly a more rapid river. 



6. All bones and shells found in the alluvium may be 

 referred to species now inhabiting the Nile-valley. 



7. No marine shells have been discovered in the alluvium 

 of the Nile. 



8. No chronological evidence can be drawn from the 

 thickness of the beds of alluvium. 



