Page:Philosophical Transactions - Volume 145.djvu/134

118 found, whereas the finer and more argillaceous and calcareous portions are held in suspension and carried out laterally by the gently overspreading waters.

A transverse section of the valley often presents the following appearance.

Fig. 2.

In the middle we see the Nile, and on both sides of it elevations of the ground like two dams. These run parallel to the river and form its banks. Beyond these, the ground again sinks and forms depressions, which, for the most part, are deeper than the present bed of the river, so that it flows, as it were, on a great dam. The explanation of this is, that the Nile accumulates more alluvium in its immediate neighbourhood, and this chiefly consists, though not always, of gravel and sand, whereas in places more distant, to which the water never reaches except during the inundation, or is conducted by canals, less alluvium is deposited; but as the water remains long there in a tranquil state, it lets fall the more fertilizing mud, and thus the land near the desert is the most productive. But in some places the banks of the river consist of from 23 to 33 feet of pure mud, sometimes divided by layers of sand. In numerous places, beds of mud may be seen rising from the level of low-water to the summit of the bank, and in digging below the lower level, the mud is frequently found to be continued. found some banks exhibiting what he considered to be stratified annual layers, varying from an inch to a few lines in thickness, in the same situation, the upper part of each layer being usually of a lighter colour than the lower part, and each separable from. that immediately above or below it. But, as will hereafter be explained, this appearance was local and the effect of a secondary cause, and was not produced by the regular annual inundation.

The height of the banks of the river generally diminishes from Assouan to the Delta, and thence in a greater ratio to the mouths of the river, owing to the wider extent over which the mud-charged waters spread below the point of the Nile's bifurcation below Cairo. About the time of the medium rise of the river the banks below Thebes are usually from 20 to 30 feet above the surface of the water, at Cairo from 15 to 25 feet, at Rosetta from 3 to 12 feet.

The annual deposit is variable in thickness in different parts of Upper Egypt and in the Delta from a variety of causes and that both in the vicinity of the river and at a distance from it. In the vicinity of the river, at particular places, where the stream is retarded by the comparative flatness of the country, the deposition is greater than in other localities. The deposit of one year is in some places stripped off by the flood of the next, and the quantity of earthy matter held in suspension in the water is sometimes augmented by portions of the mud cliff falling into the river. M., writing from Tahtah on the 3rd of November 1850, says, "Le Nil est déjà