■^"' DEPOSITS OF THE NILE. 603 



of the Nile are naturallj greater in Upper than in Lower Egypt. 

 Tliey are found to have raised the 8oil at Thebes about seven 

 feet within the last seventeen hundred years, and in the Delta the 

 rise has been certainly more than half as great. 



We shall, therefore, probably not exceed the truth if we sup- 

 pose the annually inundated surface of Egypt to have been ele- 

 vated, upon an average, ten feet,* within the last 5,000 years, or 

 twice and a half the period during which the history of the Po 

 is known to us.f 



* Fraas and Eyth maintain that we have no trustworthy data for calculating 

 the annual or secular elevation of the soil of Egypt by the sediment of the 

 Nile. The deposit, they say, is variable from irregularity of current, and 

 ■especially from the interference of man with the operations of nature, to a 

 degree which renders any probable computation of the amoimt quite impossi- 

 ble. — Fraas, Aus dern Orient, pp. 213, 213. 



The sedimentary matter transported by the Nile might doubtless be esti- 

 mated with approximate precision by careful observation of the proportion of 

 suspended slime and water at different stations and seasons for a few succes- 

 sive years. Figari Bey states that at low stages the water of the NUe contains 

 little or no sediment, and that the greatest proportion occurs about the end of 

 July, and, of course, while the river is still rising. Experiments at Khartoum 

 at that season showed solid matter in the proportion of one to a thousand by 

 weight. ■ The quantity is relatively greater at Cairo, a fact which shows that 

 the river receives more earth from the erosion of its banks than it deposits at 

 its own bottom, and it must consequently widen its channel unless we suppose 

 a secular depression of the coast at the mouth of the Nile which produces an 

 increased inclination of the bed of the river, and consequently an augmented 

 velocity of flow sufficient to sweep out earth from the bottom and mix it with 

 the current. 



Herschell states the Nile sediment at 1 in 633 by weight, and computes the 

 entire annual quantity at 140 millions of tons. — Physical GeograjyTiy, p. 231. 



The mean proportion of sedimentary material in the waters of the Missis- 

 sippi is calculated at 1 to 1,500 by weight, and 1 to 2,900 in volume, and the 

 total annual quantity at 812,500,000,000 pounds, which would cover one 

 square mile to the depth of 241 feet. — Humphreys and Abbot, Report, p. 

 149. 



f We are quite safe in supposing that the valley of the Nile has been occu- 

 pied by man at least 5,000 years. The dates of Egyptian chronology are un- 

 certain, but I believe no inquirer estimates the age of the great pyramids at 

 less than forty centuries, and the construction of such works implies an al- 

 ready ancient civilization. 



It is an interesting fact that the old Egyptian system of embankments and 

 canals is probably more ancient than the geological changes which have con- 

 verted the Mississippi from a limpid to a turbid stream, and occasioned the 

 formation of the vast delta at the mouth of that river. Humphreys and Abbot 



