GEOLOGY, NILE VALLEY, CRYSTALLINE ROCKS.

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the rock on which Egypt rests began; at Zaķâzîk in the Delta the borers were worked down to a depth of 345 feet, but the rock was not reached. The layer of mud and sand which forms the characteristic soil of Egypt came to an end at a depth of about 110 feet, and what was found below this depth consisted of coarse sand, clay, and shingle. The thickness of the mud soil of Egypt varies at different places. Thus at Bani Suwêf it is only about 36 feet deep, and at Sûhâk it is about 56 feet; both these places are in the Nile Valley proper. At Banhâ and at Ķalyûb it is 56 feet and 40 feet respectively; both these places are in the Delta. Up to the present the greatest depth of Nile mud has been found to be at Zakâzîk, and here, as said above, it is about 110 feet deep. To-day the Nile is depositing mud on its bed at the rate of nearly 4 inches in a century. This statement agrees with that of Capt. H. G. Lyons, who says: The resultant effect of this deposition during flood and erosion during the falling stage of the river has been to raise the river-bed between Aswân and Cairo at the average rate of about 10 centimetres per century during the last 2,000 or 3,000 years, and certainly for a much longer period.

The direction of the Nile Valley is generally in a north and south direction, and this is due to great earth movements which took place in Miocene times; and the long depression now occupied by the Central African Lakes, the lower area south of Abyssinia, the Red Sea, the Gulfs of Suez and ‘Aķabah, and the Jordan Valley, is due to extensive fracturing of the earth's crust. The line of this fracture can, in the opinion of Messrs. Willcocks and Lyons, be traced from the Mediterranean Sea nearly to the First Cataract. In late Miocene or early Pliocene times the sea made its way so far south as Asnâ, and in doing so it laid down thick deposits of sand and gravel, and the tributary streams, fed by a rainfall much heavier than that of to-day, brought down masses of broken stony matter from the limestone plateaux and piled them up along the margins of the valley. A rise of the area turned this arm of the sea into a river valley, and the deposit of Nile mud and the formation of cultivable land began.

The crystalline rocks begin in latitude 28° N., and form the southern portion of the Sinai Peninsula and the range of hills which border the Gulf of Suez and the Red Sea, and extend as far south as the northern boundary of Abyssinia.

In width they gradually increase, reaching two-thirds of the way to the Nile east of Ķanâ, while at Aswân, Kalâbshah, and Wâdî Ḥalfah, and at numerous points further south they occur in the Valley of the Nile, forming cataracts and gorges, though often still hidden over large areas east of the Nile by the Nubian sandstone. The crystalline rocks are at base a gneiss, which is overlaid by mica, talc, and chlorite schists, above which is a very thick volcanic series, and into this are intruded a grey hornblendic granite and also later a red granite. The best known of these rocks is the red hornblendic granite of Aswân, which was used by the Egyptians of all periods for obelisks, statues, stelae, and temples. Among the rocks of the volcanic series must be mentioned the famous porphyry, the quarries of which near the Red Sea were extensively worked in the Roman period. The three places in Egypt and Nubia where the old surface of the crystalline rocks lies nearest to the surface are Aswân, Kalâbshah, and Wâdî Ḥalfah, and here the Nile has made cataracts in forcing its way through them.

The layer of sandstone which lies on the crystalline rocks covers nearly the whole of Nubia, and extends so far north as Asnâ, where it is in turn covered over by the clays and limestones of Cretaceous age. It is yellow in colour, and at its base usually becomes a quartz conglomerate; it was quarried chiefly at Kartassi in Nubia and at Silsilah in Egypt, and most of the temples in the southern part of Egypt and throughout Nubia are built of it. Above the sandstone in many places lie a large series of green and grey clays, and thick beds of soft white limestone; and above these is a very thick layer of soft white limestone which forms the cliffs of the Nile Valley from Luxor to Cairo, and furnishes most of the stone used for building in Egypt.

Another kind of siliceous sandstone is found at Gabal Aḥmar, near Cairo ; this is, in reality, a shallow water deposit, which has been in many cases cemented into a hard refractory rock by silica; this stone was largely used in building temples in the Delta. On all the above strata thick deposits of sand and gravel were laid down by the sea which, as has already been said, ran up as far as Asnâ in prehistoric times, and subsequently, under the influence of climatic conditions which closely resemble those of our own time, river deposits of dark, sandy mud were laid down at levels which were considerably higher than the deposits of to-day. There is a complete

absence of fossils in the Nubian sandstone. From Abû Simbel northwards the Nile Valley is bounded on the west by a high limestone plateau called Sinn al-Kiddâb, which at this point is about 56 miles from the river, and it gradually approaches the stream until at Aswân it is only 25 miles distant, and at Gabalên it marches with the river. North of Aswân we find two interesting plains, which Sir W. Willcocks calls the " plain of Kom Ombo" and the "plain of Edfû”; these were once ancient Deltas of rivers coming down from the high ranges which skirt the Red Sea. The sands and clays of these belong to an age anterior to the Nile, and are covered with granite and porphyry pebbles brought down from the Red Sea range, and have no affinity with those met with at Aswân, Kalâbshah, and Wâdî Ḥalfah. About five miles to the north of the temple of Kôm Ombo is a good section which illustrates the relative positions and depths of the ancient sandy clay and sand deposits overlaid by the more recent Nile mud. Limestone is first met with at Ar-Raghâmah, a little to the south of Silsilah, and between this place and Victoria N'yanza there is no other limestone in the Nile Valley.

It has been generally supposed that the pass at Gabal Silsilah was an ancient cataract of the Nile, but though the present channel is narrow, yet it is only a branch of the river; the true channel is on the right of the hill in which the quarries are, and is at present buried under mud and silt. The word Silsilah, which has become the name of this place, means "chain." The word Silsilah is usually applied to a cataract on the Nile, but the common Arabic word for cataract is shallâl, the series of rocks being supposed to represent the hollows in the links of the chain. Gabal Silsilah can never have been a cataract, for the Nile deposits and certain shells are met with north and south of the pass at exactly the same level, and no change is experienced until we reach Gabalên, where there is a decided drop in the level of the ancient deposits. It is probable that a great cataract existed at Gabalên at a very remote period at least, this is what the up-turned and undermined hills at Gabalên suggest. Between Kanâ and Cairo the Nile flows between limestone hills; the Londinian formation extends to a point midway between Asyût and Minyâ, where the lower Parisian. strata appear on the tops of the plateaux. The upper Londinian strata disappear a little to the north of Minyâ, and

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the lower Parisian formation is now generally met with as far as Cairo.

The Fayyûm [for the description of the antiquities of the Fayyûm see p. 241ff.], which some have regarded as the first of the Oases in the Libyan Desert, is in reality a "deep depression scooped out of the Parisian limestone," the greater part of the bed of which is overlaid with thick belts of salted loams and marls, and upon this Nile mud has been laid down. In connection with the Fayyûm must be mentioned the Birkat al-Kurûn, i.e., "the Lake of the Horns," a long, narrow lake which lies to the north-west of the Fayyûm province. A great deal has been written about Birkat al-Kurûn, both by those who regard it as a part of the old Lake Moeris and by those who do not. Modern expert engineering opinion declares unhesitatingly that this lake, the water surface of which is about 130 feet below sea level, is all that remains of Lake Moeris, and it has, according to the authorities quoted by Sir W. Willcocks, been definitely proved that Lake Moeris never had a natural outlet towards the interior of the country, and that it was never connected in any way with the Wâdî Rayan, which it nearly touched.

According to Mr. Beadnell (Topography and Geology of the Fayum Province, Cairo, 1905, p. 26), the Fayyûm is a depression which in Pliocene times was occupied by the sea, which then extended for some distance up the Nile Valley. Later on, in Pleistocene times, when the drainage of North-Eastern Africa flowed down the Nile Valley at a considerably higher level than to-day, the Fayyûm depression became a lake communicating with the river. Later on, as the river eroded its bed, the depression was probably cut off from the Valley, until in early historic times the river bed had again risen sufficiently by deposition to render possible the diversion of part of its supply into the Fayyûm. From that time, by regulating the amount so diverted, it was possible to reclaim gradually almost the whole of the floor of this low-lying area for cultivation. Now all that remains of the former lake is an area of 233 square kilometres of brackish water, which is being reduced yearly, as the water which reaches it is less than that which is removed by evaporation.

One of the most extraordinary facts in connection with Lake Kurûn is that its waters are only slightly brackish; they are, moreover, quite drinkable, and fresh-water fish from the Nile are found in them in abundance. The cause of this is said to

be clefts and fissures in the bottom of the lake and the very considerable drainage which has gone on. The streams of water which flow from these subterranean passages travel towards the Marmarica coast between Alexandria and Derma. There, "owing to the tensile force inherent in all water at a high temperature, they are discharged at great depths below the level of the Mediterranean Sea." The effect of this constant drainage has been to lessen the quantity of salt in the lake, and to lower the level of its waters. In some places its depth is as much as 26 feet, and in others it is as little as 10 feet. As the Fayyûm basin is closed in on all sides by bluffs and hills of considerable height, had there been no subterranean drainage the salt in the waters of Lake Ķurûn must have increased, but the contrary is the fact, and the amount of salt in its waters at the present time bears no adequate proportion to that which the lowest estimate of experts entitles us to expect. support of the explanation of the relatively slight brackishness of the waters of Lake Ķurûn given above, Dr. Schweinfurth and Sir W. Willcocks mention the case of Lake Tchad in the Central Sûdân as exhibiting an example of subterranean drainage on a larger scale. The waters are perfectly sweet in spite of the absence of any apparent outlet. This lake is drained by active infiltration towards the north-east in low depressions, which are known as the Bahr al-Ghazâl.

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In connection with Birkat al-Ķurûn must be mentioned the famous Natron Lakes, which lie in the Natron Valley, to the north-west of Cairo. From these are obtained carbonate of soda and muriate of soda, both of which salts have been loosely classed as "natron"; these Birak or "Lakes are six or eight in number, and the valley in which they are situated is about 20 miles long, and varies in width from 1 to 5 miles. Dr. Sickenberger observed in 1892 that all the springs which gave birth to the "Lakes were situated on the eastern side of the valley, and this fact suggests that the "Lakes" are probably due to direct infiltrations from the Nile. [For a description of the Monasteries near the Natron Lakes, see p. 256-7.]

Along the northern coast of the Delta,* close to the Mediterranean Sea, are several large lagoons, of which the

* "Delta" is the name usually given to the triangular island which is often formed by the mouths of large rivers, e.g., the Indus and Nile, because it resembles in shape the fourth letter of the Greek alphabet, ▲. In the case of the Nile, the two sides are formed by the Rosetta and Damietta arms and the base by the Mediterranean Sea.