DEPOSITS OF THE NILE. 521 
or twice and a half the period during which the history of the 
Po is known to us.* 
As I have observed, the area of cultivated soil is much less 
extensive now than under the dynasties of the Pharaohs and 
the Ptolemies; for—though, in consequence of the elevation 
of the river-bed, the inundations now have a wider natural 
spread—the industry of the ancient Egyptians conducted the 
Nile water over a great surface which it does not now reach. 
Had the Nile been banked in, like the Po, all this deposit, 
except that contained in the water diverted by canals or other- 
wise drawn from the river for irrigation and other purposes, 
July, and, of course, while the river is still rising. Experiments at Khartum 
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 Geography, 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. 
* We are quite safe in supposing that the valley of the Nile has been 
occupied by man at least 5,000 years. The dates of Egyptian chronology are 
uncertain, 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 
conclude that the delta of the Mississippi began its encroachments on the 
Gulf of Mexico not more than 4,400 years ago, before which period they sup- 
pose the Mississippi to have been ‘‘a comparatively clear stream,” conveying 
very little sediment to the sea. The present rate of advance of the delta is 
262 feet a year, and there are reasons for thinking that the amount of deposit 
has long been approximately constant.—eport, pp. 485, 436. 
