Page:Philosophical Transactions - Volume 145.djvu/127

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MR. HORNER ON THE ALLUVIAL LAND OF EGYPT.

the river, nearly throughout their whole length. In the neighbourhood of Assouan the hills are little more than 200 feet above the Nile, but they go on increasing in height to the parallel of Thebes, where they attain an elevation of 1065 feet above the Nile; and from that point, northwards they have a gradual fall, but rise again in some parts of their northern extremity to nearly 1000 feet.

The Libyan and Arabian ranges are nearly identical in mineral composition. The lowest sedimentary rock exposed to day, that which comes in contact with the igneous rocks above Assouan, is the lower sandstone above described. It constitutes the chief composition of the hills on both sides of the valley as far north as the neighbourhood of Esneh, about eighty-five miles below Assouan. Here it is covered by a limestone which both Russegger and Newbold identify with the chalk of Europe, the former considering it to belong to the period of the upper-chalk. It occupies both sides of the valley as far as Siut, a distance of nearly 130 miles, in a direct line.

At Siut the chalk is covered by nummulite limestone, a part of that vast tertiary formation which extends through Southern Europe, Asia and Northern Africa. The hills on both sides of the valley from Siut to Cairo, a distance of more than 200 miles, are composed of it, and it extends from the Nile to the Red Sea, and from the left bank of the river into the Libyan desert.

In the latitude of Cairo the Arabian and Libyan ranges of hills no longer run parallel; the former terminating in a line from W.N.W. to E.S.E., between Cairo and Suez, so that near Cairo the hills on the right bank of the Nile appear to turn abruptly, nearly at right angles to the course they have held from Assouan northwards. Near Cairo the group of Gebel Mokattam rises to the height of 448 feet above the Mediterranean. The hills composing the northern part of the Libyan range, from the latitude of Cairo take a N.W. direction, being a continuation of the hills that extend from the Red Sea. They in no part rise to a greater elevation than about 320 feet above the Mediterranean.

North and east of the Mokattam, the different members of the tertiary nummulite limestone formation are covered by a sandstone, identical in character with the upper sandstone near Assouan. No fossils have been found in it hitherto, but its overlying the nurnmulite limestone clearly determines it to be at least a tertiary deposit. This sandstone is the prevailing rock throughout the Isthmus of Suez, wherever it rises above the desert sand or other alluvial covering.

The engineers who accompanied the French army in Egypt in 1799, having made a survey of the Isthmus of Suez, came to the conclusion that the level of the Red Sea was 9 metres(29 feet 5 inches) above that of the Mediterranean, and their high reputation gave currency in the scientific world to this result for many years. Circumstances, however, having in more recent times thrown doubts on the accuracy of the survey, which had been made in a very short time, and under almost every disadvantage, a private association of French, English and Austrian civil engineers