Page:Philosophical Transactions - Volume 145.djvu/133

From Wikisource
Jump to navigation Jump to search
This page has been proofread, but needs to be validated.
117
MR. HORNER ON THE ALLUVIAL LAND OF EGYPT.

When we consider, therefore, the large amount of earthy matter held suspended in the water, as will presently appear, and how much the volume of water is diminished before the Nile reaches the sea, it is evident that a vast amount of sediment must be annually left upon the land which the inundation overspreads, that a much larger proportion must be deposited in Upper Egypt than in the Delta, and, from the greater surface of the latter, that the depth of the annual accumulation there must be greatly less than in the more contracted part of the valley.

 

The solid matter conveyed by the Nile, to form its sedimentary deposits.

When the inundations commence the Nile comes down of a reddish colour, loaded with sand and mud. From the small amount of the fall between the cataract at Wadi-Halfa (the second) and that at Assouan (the first), a distance of 214 miles, and the difference of height between the two places being only 157 feet[1], thus making the average fall of the river not quite 9 inches in a mile, it is not to be expected that much coarse gravel can be carried forward, and that which arrives at the island of Philæ. must be much sifted and comminuted in its passage through the rocks forming the rapid of Assouan. Below that place, the fall of the land goes on diminishing, so that the transporting power of the stream is small. The greater proportion of the heavier detritus thus falls down in the higher parts of Upper Egypt, and from the very gentle slope of the Delta, it might be concluded that only a small amount of the solid matter suspended in the water can reach the mouths of the river. But very fine particles of earthy matter, as is well known, are long, in subsiding, and much is carried out to sea[2]. Herodotus notices that during the inundation the sea is rendered turbid, and Bruce observed the same thing. Newbold states, that he found the sea coloured at a distance of forty miles from the shore.

A modern traveller thus describes the appearance of the water in the Nile opposite to Thebes on the 7th of November:—"Ce sera curieux pour nous de revoir ce Nil lorsque l'eau en sera transparente, au lieu d'être, comme maintenant, de la couleur de café-au-lait très noir;" and he says of it three weeks before at Cairo, "Maintenant que l'eau est à son plus haut niveau, elle est d'une épaisseur inconcevable, presque comme du chocolat, et plus foncée[3]."

The sediment is slightly modified in character at various localities, according to the nature of the formation near to which the river flows. Its composition and texture are also subject to variation from its proximity to or distance from the main channel of the stream, where the coarser and heavier siliceous particles are usually

  1. See a paper by the author in the Edinburgh Philosophical Journal for July 1850.
  2. "The out-pouring of the Nile during the inundation is so powerful that fresh water may be skimmed off the surface of the sea at the distance of two or three miles out in the offing. During the full surcharge, potable water may be baled on the surface of the Mediterranean even out of sight of land."—The Mediterranean, by Admiral Smyth, P. 84 and 169.
  3. Melly, Lettres d'Egypte et de Nubie, printed for private circulation, 1852.