Page:Philosophical Transactions - Volume 145.djvu/126

110 west direction until it gradually sinks in the desert of Libya. This part of the range nowhere rises to a greater height than about 214 feet above the bed of the Nile, and 546 feet above the Mediterranean. Granite is the predominant rock, of different varieties, sometimes passing into gneiss, sometimes having an admixture of hornblende, when it gets the name of syenite, from its occurrence near the ancient town of Syene.

The granites and other unstratified rocks are associated in the district near Assouan, or the First Cataract, with two sedimentary rocks, both sandstones, and very similar in mineral structure, but very different in point of age; for the one belongs to the lower members of the cretaceous period, and the other covers in several places, farther north, a tertiary nummnulite limestone.

Through a labyrinth of these granites and sandstones extending from the island of Philæ to the neighbourhood of Assouan, the Nile enters Egypt in a succession of rapids having a descent of about 85 feet in a distance of about five miles and a half from Philæ to Assouan, forming what is called the First Cataract. There is no waterfall, as commonly understood by the term cataract, for and his companions were dragged up in a boat the whole distance in two hours, during the time of low water, that is, towards the end of January.

The valley of Upper Egypt is flanked by two parallel ranges of hills, the Arabian on the east, the Libyan on the west. At Assouan, the southern extremity of the valley, they each approach close to the Nile, the bed of which is strewed with rocky islands, the most northernly being the celebrated Elephantine. Both ranges are divided by rents of various magnitudes, forming valleys, some of them running north and south, others crossing the ranges from east to west. One of the great north and south valleys gave a passage to the waters of the Nile, in a somewhat tortuous course; the appearance of the boundaries on either side, and the very gentle fall of the land, from south to north, excluding all idea that the valley has been excavated by the action of running water. The Nile valley varies considerably in breadth, its widest part between Minieh and Benisuef being about eighteen miles, frequently contracting to two miles; and at Gebel Silsilis, about forty-five miles below Assouan, the hills approach so close to each other, that the river, for three-quarters of a mile, runs through a pass about 1200 feet wide, and there is scarcely a yard of alluvial deposit on either bank for a considerable distance. At the apex of the Delta the valley is six and a half miles wide.

The Libyan range falls with a slope towards the valley, the rocks of which it is composed appearing to extend under the valley, forming a solid basin covered with sand and detritus that had accumulated before the alluvial matter brought down by the Nile began to be deposited over it.

The Arabian hills, except where broken by transverse valleys, present cliffs towards