Page:Philosophical Transactions - Volume 145.djvu/129

Rh from Cairo to the sea in a direct line is 106 miles, and from Rosetta to Damietta, or the base of the triangle, in a direct line, eighty-two miles, but following the sinuosities of the coast about ninety miles. In earlier times, when the Nile flowed in the Valley of the Waterless River, and when a branch entered the sea at Pelusium, near the modern Tineh, the base of the Delta must have been about 170 miles, but the low land extends beyond each of these limits.

Lower Egypt is thus a vast plain of alluvial land, with scarcely any natural elevations except the sand-hills near the coast; it is furrowed in every direction by a multitude of natural and artificial canals. The central part is composed of the mud deposited by the Nile, and of sand brought down by the inundations, or blown from the desert on either side; and all around the plain the boundary of Lower Egypt is composed of quartzose sands, that are generally white on the east, and reddish-white on the west, and the ground which these sands cover is at a higher level than that of the Nile at its highest inundations.

Two great shallow lakes, Burlos and Menzaleh, occupy the greater part of the base of the modern Delta, besides smaller lakes, lagoons and swamps, behind the sand-hills that line the coast. These sand-hills rest upon a reef which forms a powerful dam against the encroachments of the sea, and which describes as being in a continual state of formation and waste; as being a calcareous stone of a dirty grey colour, composed of sand mixed with worn fragments of ordinary marine testacea, mingled with microscopic shells, many of the latter being of freshwater and land origin, brought down by the Nile, thrown up again by the sea and mingled with marine shells. In structure the stone is not usually very coherent, but in some places it is hard enough to be used for building, and in ancient times numerous catacombs were excavated in it, some of which are the so-called baths of Cleopatra.

At the island of Philæ, about five miles above Assouan, may properly be placed the first entrance of the Nile into Egypt. The mighty stream has here a breadth of nearly one mile, but soon after it is divided into several branches, by the rocks that rise up in its bed to form the most northern of the rapids, the First Cataract, of which many occur in the higher parts of its course. The breadth of the river is here contracted to about a third of a mile. From the junction of the Atbara in latitude 17⋅38 N. until it reaches the sea in latitude 31⋅25, or nearly fourteen degrees of latitude, the Nile does not receive a single tributary, with the exception of torrents after heavy rains in the lower parts of its course in Egypt, from, the hills on either side. Assouan, according to the barometrical measurements of, is 300 feet