Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/742

 688

EG

formed has been filled by deposits of coarse and fine sand and gravel, containing numerous fragments of igneous rocks, which underlie the present Nile mud deposit, as in the Delta; and similarly, also, deep borings have reached no rock-floor .up to the present. At Beni Suef, with the Tertiary limestone plateaux of the desert 3 miles distant, 400 feet of sands and clays were penetrated without any sign of rock, and at Medinet el Fayum a boring of 450 feet gave the same result. Entering Egypt proper, a little to the north of the Second Cataract, the Nile flows through a valley in sandstone beds of Cretaceous age as far as latitude 25° N., and throughout this part of its course the valley is extremely narrow, rarely exceeding 2 miles in width. At two points, namely, Kalabsha and Assuan (First Cataract), its course is interrupted by outcrops of granites and other crystalline rocks, which have been uncovered by the erosion of the overlying sandstone, and to-day form the mass of islands, with numerous small rapids, which have been described not very accurately as cataracts; no good evidence exists in support of the view that they are the remains of a massive barrier, broken down and carried away by some sudden convulsion. From latitude 25° N. northwards for 518 miles the valley is of the “ rift-valley ” type, a level depression in the limestone plateau, enclosed usually by steep cliffs, except where the tributary valleys drained into the main valley in early times, when there was a larger rainfall, and which now carry off the occasional rainstorms that burst on the desert. The average width of the cultivation is about 10 miles, of which the greater part lies on the left bank of the river; and outside this is a belt, varying from a few hundred yards to 3 or 4 miles, of stony and sandy ground, reaching up to the foot of the limestone cliffs, which rise in places to as much as 1000 feet above the valley. This continues as far as latitude 29° N., after which the hills that close in the valley become lower, and the higher plateaux lie at a distance of 10 or 15 miles back in the desert. The fertile province of the Fayum, on the left bank of the Nile and separated from it by 6 miles of desert, seems to owe its existence to movements similar to those which determined the valley itself. Surrounded by Tertiary limestone strata on the north, west, and south, the boring above mentioned met with no representatives of these beds in the 450 feet which were penetrated. Lying in a basin sloping in a series of terraces from an altitude of 65 feet above sea-level in the east to about 140 feet below sealevel on the north-west, at the margin (if the Birket-el-Kerun, this province is wholly irrigated by a large canal, the Bahr Yusef, which, leaving the Nile at Beirut in Upper Egypt, follows the western margin of the cultivation in the Nile Valley, and at length enters the Fayum through a gap in the desert hills by the Xllth Dynasty pyramids of Lahun and Hawara. The Desert Plateaux.—Speaking generally, Egypt consists of a broad plateau of sedimentary rocks, lying on the western side of a band of crystalline rocks which occupy the southern part of the peninsula of Sinai and the western side of the Red Sea from latitude 29° N. southwards. In the north, where beds of Upper Tertiary age occur, the desert plateaux are comparatively low, but from Cairo southwards, as the Eocene limestones come in, they rise to 1000 and even 1500 feet above sea-level. Formed mostly of horizontal strata of varying hardness, they present a series of terraces of minor plateaux, rising one above the other, and intersected by small ravines worn by the occasional rainstorms which burst in their neighbourhood. The weathering of this desert area is probably fairly rapid, and the agents at work are principally the rapid heating and cooling of the rocks by day and night, and the erosive action of sand-laden wind on the softer layers; these, aided

P T

[geography and statistics.

by the occasional rain, are ceaselessly at work, and produce the successive plateaux, dotted with small isolated hills and cut up by valleys (wadis), which occasionally become deep ravines, thus forming the principal type of scenery of these deserts. From this it will be seen that the desert in Egypt is mainly a rock desert, where the surface is formed of disintegrated rock, the finer particles of which have been carried away by the wind; and east of the Nile this is almost exclusively the case. In the western desert, however, those large sand accumulations which are usually associated with a desert are met with. They occur as lines of dunes formed of rounded grains of quartz, and lie in the direction of the prevalent wind, usually being of small breadth as compared with their length; but in certain areas, such as that lying south-west and west of the Oases of Farafreh and Dakhleh, these lines of dunes, lying parallel to each other and about half a mile apart, cover immense areas, rendering them absolutely impassable except in a direction parallel to the lines themselves. East of the Oases of Baharieh and Farafreh is a very striking line of these sand dunes; rarely more than 3 miles wide, it extends almost continuously from Moghara in the north, passing along the west side of Khargeh Oasis to a point near the Nile in the neighbourhood of Abu Simbel in Nubia—having thus a length of nearly 550 miles. In the northern part of this desert the dunes lie about N.W.-S.E., but farther south incline more towards the meridian, becoming at last very nearly north and south. Oases.—In the western desert lie the five large oases of Egypt, namely, Siwah, Baharieh, Farafreh, Dakhleh, and Khargeh, occupying depressions in the plateau or, in the case of the last three, large indentations in the face of the escarpment formed by the Lower Eocene and Upper Cretaceous Limestones. Their fertility is due to a plentiful supply of water furnished by a sandstone bed 300 to 500 feet below the surface, whence the water rises through natural fissures or artificial boreholes to the surface, and sometimes to several feet above it. These oases were known and occupied by the Egyptians as early as 1600 b.c., and Khargeh rose to special importance at the time of the Persian occupation. The Mountainous Region.—The mountainous part of the country is that occupied by the crystalline rocks in the southern part of Sinai, and the belt occupying the western shore of the Gulf of Suez and Red Sea, where the principal peaks rise to heights of 6000 and 7000 feet. Owing-to the slight rainfall, and the rapid weathering of the rocks by the great range of temperature, these hills rise steeply from the valleys at their feet as almost bare rock, supporting hardly any vegetation. In some of the valleys, wells or rock-pools filled by rain occur, and furnish drinking-water to the few Arabs who wander in these hills. Farther south, where the rainfall is greater, the valleys are more fertile, and support considerable numbers of camels, sheep, and goats. The Egyptian Nile.—The Nile ((/.u) enters Egypt proper a little to the north of the Second Cataract, and between there and the First Cataract has a length of 200 miles and a slope of From this point to the Barrage at the apex of the Delta the length is 605 miles and the slope X1j «n> o', and thence to the sea, by either the Rosetta or Damietta branch, is 145 miles more. The Nile. is at its lowest at Assuan at the end of May, then rises slowly until the middle of July, and rapidly throughout August, reaching its maximum at the beginning of September ; it then falls slowly through October and November. At Cairo the lowest level is reached about the middle of June, after which the rise is slow in July and fairly rapid in August, reaching the maximum at the beginning of