List of Water Sources.

In the following list I have summarized all the water sources known to me in order of latitude from north to south. I believe the list contains all sources that are of importance, but it is doubtless far from being complete as regards the rock basins of the mountain areas for two reasons. Firstly, the Arabs are often reluctant to point out the sources, more especially in the more arid parts of the Ababda country; and secondly, as already remarked, since the rock basins depend for their supplies entirely on comparatively recent rainfall around them, they vary very much in yield in different years, so that basins which are of great use in one year may be totally dry the next, and vice-versa, and guides seldom take the trouble to point out a dry basin, even if in some other year it may have held a useful store of water.

With regard to the accuracy of the positions given in the list, it may be remarked that in the case of wells actually visited the position is given to seconds, and the localisation may be relied on within at most a few hundred metres; while for wells not actually seen but whose positions were pointed out by guides from some distance, the coordinates are usually only given to minutes, and these positions are uncertain by larger amounts, up to two or three kilometres in extreme cases.

It must not be assumed that water supplies can always be obtained at all the localities named. As already remarked, rock basins will usually only be full if rain has fallen in the district comparatively recently, large rock basins may furnish supplies for a year after rainfall, but the smaller ones last only a few months, weeks, or even days. The reserves are the wells, most of which never run dry except after a succession of rainless years. But when rock basins are full, many of the wells are filled with downwash, and digging must be resorted to in order to re-open them. A traveller arriving at a “well” sometimes finds no evidence whatever of its existence beyond the guides pointing to a spot on the ground with the statement “el bir hena” (the well is here). The explanation is that the well has been filled with alluvial matter washed down the valley by recent rain; but that same rainfall will have filled the rock basins in the hills, and the Arabs leave the digging out of the well until the basins are empty again. In other cases a well may be found to be dry through the water-level having sunk below the depth to which the well was last opened, and by deepening it a foot or so one may obtain a supply.

The notes in the column headed “Remarks,” on the quality of the water of the various sources, are mostly based on the observations of a single year, and must therefore only be taken as general guides. Usually, the shorter the interval since rain has fallen, the better are the supplies, both in quantity and quality.

List of Water Sources.[121]

Name.Latitude N.Longitude E.Remarks.
°′″°′″
Galt Um Tundeba245525344553Rock basin in ravine, 1kilometre from Wadi Um Tundeba. Gave good supply of pure water in1906.
Galt in Gebel Ghuel2454034390Large rock basins, yielding a good supplyin 1905-1906.
Bir Muelih245135335952In Wadi Muelih. Very salt, only drinkableby animals.
Hangalia well245027343540Modern well in Wadi Hangalia, sunk byminers. Supply small and of poor quality.
Galt Um Karaba (Migif)244848342755Large rock basins, yielding a good supplyin 1905-1906. Rather difficult of access for camels.
Bir Ghadir24482534470Well in Wadi Ghadir. Filled by downwashin 1906.
Megal el Harami244734343415Small spring in ravine on west side ofWadi Nugrus. Yields for some months after rainfall. Water verygood.
Bir Sibrit2443033580Well in Wadi Sibrit; water about 8 metresdown, of good quality. Supply stated to be constant.
Bir Murra2441034110Well in Wadi Shait. Water salt, onlydrinkable by camels.
Hamish mine244103460Water in mine shafts, that in thewesternmost of three shafts good in 1906. Well in Wadi Hamish,close to mines, 15 metres deep, dry in 1906.
Sikait, spring244050344710Tiny trickling spring of good water.Yields only about 50 litres per day, but supply of very goodquality and said to be constant.
Sikait, north well24401234460Modern well, sunk by miners. Was yieldingfair quantity in 1906, but of mediocre quality.
Sikait, south well243938344745Similar to last; both wells are in thefloor of Wadi Sikait.
Bir Um Gubur243703450Well in Wadi Shait. Water about 8 metresdown, good and plentiful in 1906.
Bir Abu Had2434034360Well in Wadi Abu Had, near its head, atjunction of two roads to Sikait. Water salty.
Bir Masur2431034130Several wells in floor of small wadi.Water good, about 10 metres down in 1906, but supplies cannot berelied on.
Bir el Ranga2426035130Very salt well, close to sea.
Galt Um Gerifat242255344020Rock basin in gorge off Wadi Huluz. Wasyielding good supply in 1906.
Bir Metawit2417034310Well in Wadi Metawit, on road between BirShadli and Bir Masur. Said to yield constant supply. Water about 8metres down, good and clear in 1906.
Bir Khashab2416034230Well in Wadi Khashab. Not to be dependedon. Dry in spring of 1906, but yielding a small supply of mediocrecharacter in the autumn of the same year.
Bir Shadli24123343755Spring or well near tomb of Sheikh.Perennial supply of variable quality and quantity. Favouritemeeting place of Ababda Arabs.
Bir Abu Hamamid2412034290Well sunk about 7 metres in floor of WadiAbu Hamamid. Yielding large supplies of rather salty and muddywater in 1906.
Amar Spring241003590Spring of good water in diorite hillssouth-east of Gebel Hamata.
Well in Wadi Abu Hamamid247034260Wells sunk in wadi floor. Dry in1906.
Bir Helie245034320Well in Wadi Helie. Excellent water inspring of 1906, but rather salty later on in the year. Qualityevidently varies considerably with interval since rainfall.
Megal Um Gunud235850351050A smaller rock basin in Gebel Um Gunud,off Wadi Lahami, near to Galt Um Gunud.
Galt Um Gunud235810351120Rock basin, in Gebel Um Gunud. Yieldedsupplies in 1906. Accessible from Wadi Lahami.
Megal el Selaia235655345250Small spring in sand at foot of rocksnear Gebel Selaia. Yield small and not to be depended on for longafter rain.
Galt Batoga235120352130Rock basin at foot of Gebel Batoga offWadi Kalalat. Water good, plentiful, and fairly easy of access in1907.
Galt Um Maiyat23502351815—
Galt in Gebel Dahanib23444535950Rock basin in ravine, containing waterafter rain; about a cubic metre in 1907.
Bir Shenshef2344035230Several small wells in Wadi Shenshef nearruins. Water good.
Bir Abu Hashim23415634426Several wells in alluvial floor of wadi.Water good and plentiful, about 8 metres down in 1907.
Bir Betan, west well2338035315Well in floor of Wadi Betan, about 600metres lower down than foregoing, close to where Wadi Um Eleigaenters Wadi Betan. Filled by downwash in 1907.
Bir Betan, east well23375535335Well in floor of Wadi Betan, near smallseyal tree. Filled by alluvium in 1900.
Galt near Gebel Abu Dahr2337035520Rock basin in north-west flank of GebelAbu Dahr. Was yielding a good supply in 1907.
Bir Rahaba23333035954Well in alluvium of Wadi Rahaba. Filledby downwash in 1907.
Bir Abu Reye233303590—
Galt el Aguz23323034350Large rock basin in sandstone, near easypass over watershed at head of Wadi Arned. Yields excellent waterfor short time only after rain.
Bir el Gahlia233103580—
Bir Abu Beid2331034580Well in Wadi Abu Beid. Was yielding afair quantity of good water in 1906.
Bir el Sunta232550344740Spring forming pool at foot of sandstonescarp of Gebel Abraq. Good water.
Bir Abu Dibesat23256344710Spring forming small pool at foot ofsandstone scarp of Gebel Abraq. Water good.
Bir Abraq232456344720Spring forming pool in gully of GebelAbraq. Supply constant, good, and plentiful.
Bir Gumbit2324034490At edge of sandstone hills; probably aspring similar to those of Abraq.
Bir Orga232103530Well stated to exist in Wadi Orga elRayani.
Megal near Abu Saafa231930344840Spring? in small wadi off Wadi Hodein.Unimportant, being close to Abu Saafa Springs.
Abu Saafa Springs23189344745Four springs yielding constant supply ofvery pure water, at foot of sandstone scarp in Wadi Hodein. Poolseasy of access to camels.
Galt in Wadi Gihab23175344810Large rock basin in Wadi Gihab Yakub AbuDerb. Pool of good rain water 6 metres diameter in 1907. Easilyaccessible to camels.
Bir Um Reit23133634353Well sunk in floor of Wadi Um Reit. Waterof mediocre quality.
Bir Dif231242344955Spring forming two pools in the stony bedof Wadi Dif. Water good.
Bir Shalatein2385353628Very salt well near sea at mouth of WadiHodein. Drinkable only by camels. Point on administrative boundarybetween Egypt and the Sudan.
Bir Beida, north well22586351810In Wadi Beida, about 500 metres furtherdown than south well, at a bend in the wadi. Good water, yieldsthree years after rainfall, but supply infiltrates slowly.
Bir Beida, south well225753351810In Wadi Beida, close to where track toBir Meneiga leaves it. Dry in 1907.
Bir Madi22474635148Well sunk in alluvium of Wadi Madi, at abend in a narrow gorge at foot of a high granite hill. Plenty ofgood water in 1907-1908.
Bir Meneiga, north spring22478351220Similar to Bir Meneiga, south spring, towhich it is very close. Point on administrative boundary betweenEgypt and the Sudan.
Bir Meneiga, south spring224655351215Tiny pool in rocky floor of Wadi Meneiga,which refills as fast as emptied. Easily accessible to camels.Water excellent.
Bir Korbiai224655351035Two wells of excellent water in the rockygorge of Wadi Korbiai. Said to yield for three or four years afterrain.
Bir Muqur22422635185Trickling spring with pools inserpentine, in Wadi Muqur. Was yielding about five litres perminute in 1908.
Bir Sararat Seyet22411035159Deep well in Wadi Seyet; filled bydownwash in 1907-1908.
Bir Diqdib2241035130Spring? high up in Wadi Diqdib, southpart of Gebel Gerf.
Bir Adal Deib22403036430Salt well, close to sea.
Ti Dabei Hamra Dom223955353840Shallow excavations in alluvium and sandin the hills of Hamra Dom, only yielding supplies for a short timeafter rainfall.
Bir Baaneit223930351840Spring? situated a short distance up WadiBaaneit. Said to yield constant but limited supplies.
Bir Bint el Dreb223403590—
Bir Um Rasein222910352020Near Gebel Um Rasein.
Bir Nabit222650362150Close to sea. Probably salty.
Bir Um Bishtit222633353345In narrow winding gorge off Wadi UmBishtit. Said to yield good water supplies for a year afterrainfall and small supplies longer. Full of downwash in 1908.
Galt Osnei222535355040Rock basin in small granite hill; fullonly after rain.
Meis-heit-ar, north galt22224353512Galt in rocky gorge off Wadi Meisah.Yielding good supply in 1908; preferred to Bir Meisah forquality.
Bir Abu Hodeid2222035170Large and important spring in Wadi AbuHodeid.
Bir Odis Maaleq2222035160Large and important spring in Wadi Odis,reached by a mountain track from Wadi Delawet.
Bir Meisah22218353550Well-known well in Wadi Meisah. Filled bydownwash in 1908. Water said to be of only moderate quality.
Bir Qidmib2221035290—
Bir Abu Ramad222040362650Close to sea. Probably salty.
Bir Shinai2220035150—
Meis-heit-ar, south galt222115353520Galt in rocky gorge off Wadi Meisah.Yielding good water in 1908.
Megwel Adar Aqdeib221750353840—
Megwel Um Edwa221650353245Chain of rock basins and small spring instony gorge on east side of Hadal Aweib Meisah. Rather difficultfor camels. Water good.
Megwel Didaut221625353710—
Megwel Akau221450362110Small spring? on north side of GebelElba.
Bir Kansisrob22143536220Large well; water good and plentiful.Bisharin camping ground.
Megwel Hamida22142535470A tiny trickling spring of saline waterin a narrow gorge. Rock basin above it, dry in 1908.
Halaib wells221325363840Several wells in village. Waterplentiful, strongly purgative to Europeans, though it has no effecton the natives of the place.
Bir Akwamtra22131361758An excellent well at the foot of GebelElba; important Bisharin camping ground.
Bir Hilwit Hasium2213035140—
Bir Nubitra221250354620Said to yield only small supply.
Megwel Um Ein221230353940—
Bir Meheriqa221210355635Spring on east side of Wadi Di-ib. Waterrather salty, but drinkable and clear.
Bir Kagog2211035160Small well in Wadi Hasium, near itshead.
Galt in Wadi Kirir2210035190—
Bir Sararat Serimtai22850362415—
Bir Mashushenai22830354930—
Megwel Aqwem228035410—
Bir Guqub228034270Galt and spring? Water good but variablein quantity. Was yielding in 1906.
Bir Salalat O Sir2272336190—
Ti Kureitra22550364640Four wells sunk in gypseous strata nearsea; water strongly purgative.
Bir Egat225034540—
Bir Frukit222036360Well in Wadi Aqilhoq; water good andplentiful.
Bir Himeitra220035150—
Bir Qabatit215755365117Well in Wadi Qabatit, 1½ kilometres fromsea; water salty, but said not to be purgative.

[113]Part of this chapter is taken from my paper on “Desert Water Supplies” in the Cairo Scientific Journal for 1908 (Vol. II), pp. 234-242.

[114]I except the tiny spring called Megwel Hamida, which is a mere trickle of very salty water.

[115]Samples collected on two different dates; the figures show the variability of the water at the same well.

[116]Not determined.

[117]Berichte der Commission für Oceanographische Forschungen, Sechste Reihe, Wien 1898, p. 510.

[118]Cf. the Marah of the Israelites, Exodus 15, 23. “They could not drink of the waters of Marah, for they were bitter; therefore, the name of it was called Marah.”

[119]This is in latitude 27° 35′ N., and therefore outside the limits of the region specially described in this memoir, but is quoted as a useful example.

[120]Pronounced to rhyme with the English word shalt.

[121]The map of the roads and water sources on [Plate III,] p. 26, should be referred to in connexion with this list.

GEOLOGICAL MAP OF SOUTH-EASTERN EGYPT.

Ball. Geography & Geology of South-Eastern Egypt.PLATE XX.

Photo-Metal-Process. Survey Dept. Cairo 1910. (60-190)

(Largest-size: [upper], [lower], [scale])

CHAPTER VIII.

GEOLOGY.—SEDIMENTARY ROCKS.

Only a small proportion of South-Eastern Egypt is covered by sedimentary rocks. As will be seen from a glance at the geological map on [Plate XX,] sedimentary strata are to be found only in the north-western and central portions of the area, where the Nubian sandstone which covers so much of the country close east of the Nile terminates in a long south-easterly projection, and along certain portions of the coast, where small outliers of Nubian sandstone and patches and strips of gypseous limestones and coral reef occur. The oldest rocks of undoubted sedimentary origin, the Nubian sandstones, are of Cretaceous age. Tertiary beds are absent, except for the gypseous strata of Ras Benas, which may possibly be Miocene. Quaternary deposits are represented by Pleistocene and Recent coral reefs and beach deposits, and by accumulations of alluvial downwash, blown sand, and calcareous tufa.

The sedimentary deposits may thus be classified into:—

(1) Recent Deposits ([coral reefs and raised beaches], [blown sand], [alluvial deposits], and [calcareous tufa]).

(2) [Gypsum and Gypseous Limestones] (of uncertain age, possibly Miocene).

(3) [Nubian Sandstone] (Cretaceous).

Coral Reefs and Raised Beaches.

Coral reefs fringe almost the entire coast-line of South-Eastern Egypt, rendering the coast one of the foulest in the world for shipping. Coral forms the islands of Wadi Gemal, Gulhan, Mukawar, Mirear, Seyal, and Halaib, these rising to a few metres above the sea; but in general it is not met with on the main land except close to the shore. The reefs may extend, slightly raised, some distance inland, but cannot be seen owing to the sand and downwash from the mountains which covers the sloping coast-plain. At low water great expanses of nearly level coral reef, stretching out for a kilometre or more from the shore, are exposed in places, especially near Ras Benas. Frequently a line of reefs can be seen running for many miles parallel to the coast at a distance of a kilometre or more from it; these outer reefs can be traced in windy weather by the breaking of waves on them, and in calm weather by the lighter colour of the water over them. There is frequently deep water between the outer reefs and the shore. Openings in the outer reefs, through which boats can pass into the sheltered water within, are called mersas by the Arab boatmen; they frequently occur opposite the mouths of the larger wadis. Coral reef forms a tough, hard, and very porous limestone, with an extremely rough surface on which the structure of the astraean and other corals composing it can be clearly seen. It has been used for building the jetty at Halaib, and rough blocks of it were anciently employed for building at Berenice, as well as in the tombs at Suakin el Qadim; but its porosity and intractability would never recommend it as a building stone if any other could be found locally.

In close association with coral reefs are beach deposits of Pleistocene age. These occur round the hills near the tip and on the north side of the peninsula of Ras Benas, where they cover the slopes of the gypseous limestone hills. Low hills and banks of very white aspect occur near the sea further north; these are probably also raised beaches, though they have not been visited and may possibly be gypsum. Near the tip of Ras Benas these beach deposits are gritty limestones with abundant casts of lamellibranchs and gasteropods, mostly in bad preservation, as well as various echinoids, among which M. Fourtau has identified the species Brissus carinatus as one of the most abundant.

Blown Sand.

The dune-forming sand in South-Eastern Egypt is exactly similar to that which covers such large areas in the Libyan Desert. It consists of well-rounded quartz grains, averaging about a millimetre in diameter; the grains are usually of a golden-yellow colour, owing to their being coated with a thin superficial film of iron oxide.

The only area where sand dunes are of any considerable magnitude or extent in South-Eastern Egypt is the tract called I Hubâl (lit. “the sand dunes”) which extends from Wadi Meisah to Wadi Aideib, fronting the hills of Ti Keferiai, Geror, Balatitda, Sul Hamid, and El Sela. The north faces of all these hills and the smaller hills north of them are swathed in thickly accumulated sand to such an extent as to make travelling over them difficult. The reason for the heavy sand accumulation at this particular place is that the north-west wind, which is the prevalent wind over Egypt proper, here meets with south-easterly winds coming up the Red Sea, causing a local calm and a consequent dropping of the sand borne from the Nubian sandstone areas by the north-west wind. Thus the same climatic conditions which give rise to the rich vegetation on the Elba mountains cause at the same time extreme desert conditions in the tract immediately north-west of them.

Sand accumulations of less magnitude occur in and about the lower reaches of the Wadi Hodein, in the great valley to the north of Berenice, on the plain west of Gebel Um Harba, and at some few other places, but they are seldom so extensive as to cause a serious hindrance to travel.

Besides transporting the ordinary dune-forming sand, the action of the wind is responsible for a considerable amount of redistribution of the coarser granitic and other sand produced by the disintegration of igneous rocks. In high winds this coarse sand, in angular grains as large as peas, is blown about on the coast-plain to considerable distances from the places where it was originally deposited by water transport.

Alluvial Deposits.

The degradation of the mountain and hill masses which is constantly going on under sub-aerial agencies (disintegration by variations of temperature, and transport by water during occasional rain storms) has in the course of centuries resulted in immense accumulations of alluvial material (sand and gravel) along all the drainage lines and on the plains. Disintegration is in many places greatly facilitated by the crushed nature of the rocks, due to the tectonic movements which they have suffered during the process of folding and mountain formation. This fact is specially evident in the case of the more basic rocks. Serpentines, for instance, are frequently so crushed and cracked that one may search in vain over a whole mountain for a place where the rock is sufficiently free from cracks to allow of a hand-specimen of the ordinary size being extracted without breaking up into fragments under the hammer. In the more acid rocks, like the granites, crushing has been a less powerful aid to disintegration, and here the diurnal temperature-range has been the principal factor; the rock, being an aggregate of crystals of different materials with different coefficients of expansion, has been easily broken up at its surface by the constant differential motion of its component grains, resulting in the formation of coarse granitic sand. Chemical action in denudation is of quite subordinate importance. Though some chemical action has taken place, as for instance in the kaolinisation of the felspars, we find as a rule that the alluvial accumulations consist principally of unaltered rock fragments.

In material, the alluvial accumulations are of the same varied nature as the mountains from whose degradation they have originated, ranging from very basic mineral substances, such as serpentine, to the most acid, such as quartz. The relative proportions of the different materials in the alluvial deposits do not, however, correspond as a rule with the proportions in which they enter into the mountain masses. Not only are heterogeneous rocks, such as granites and diorites, far more rapidly disintegrated by temperature-changes than are the more homogeneous rocks such as felsites and serpentines, but the fragments produced by the disintegration of granitic rocks are rounder and of much smaller size than the fragments resulting from the breaking up of felsites, schists, and serpentines. The result is that granitic detritus is transported to greater distances and distributed over larger areas than the detritus of the closer-grained and more homogeneous rocks. In approaching a mountain composed of about equal parts of, say, granite and serpentine, we accordingly find the lower portions of the wadi consist chiefly of granitic sand, while the coarser and more angular detritus of serpentine is only seen in abundance in the higher parts of the drainage channel. The nature of the alluvium of the wadi floor occasionally gives its name to the wadi; thus, of the two wadis which by their junction form Wadi Khoda, one is called Wadi Salib el Azrak, because its alluvium consists of dark rock fragments, while the other is called Wadi Salib el Abiad, because its floor is formed of white granitic sand. (Azrak = dark; Abiad = white).

In size, the fragments composing alluvial detritus vary from huge blocks weighing tons down to the finest sand whose grains are only a fraction of a millimetre in diameter. As a rule, of course, the larger fragments are found near the heads of wadis, and the finer sands in their lower reaches and on the plains. But often we find great boulders mixed with the sand at immense distances from the parent mountain mass; the heavier rain storms which occasionally break over the mountains produce for short periods such immense rushes of water down the wadis, that great boulders are swept along and incorporated in the finer material which is deposited in times of more normal rainfall. Thus it is no uncommon thing in sinking a well in a wadi fifty kilometres or more from the mountains to encounter great boulders in the sandy alluvium.

In shape, the alluvial fragments vary very much, according to their mineral nature and the amount of rolling they have received in transport. Foliated rocks, like schists and slates, produce typically flaky fragments which seldom attain any high degree of rounding; the same is true of serpentines which have been shattered by crushing. Granitic rocks produce generally more symmetrical fragments, though granitic sand is typically angular unless it has been transported for considerable distances. The larger blocks are generally rounded in the case of granitic rocks, even when they have not travelled for any great distance; for if a cubical block of granite be exposed to weathering, disintegration takes place most rapidly at the corners, and the block eventually becomes more or less spheroidal without having moved at all. This can be well seen in such masses as Gebel Selaia, where the granite boss is thoroughly well rounded in situ. The same phenomenon can be noticed in certain dolerites, where portions of the rock detached by joint planes have weathered in situ into forms like cannon balls. The more homogeneous rocks like felsite and serpentine, as well as the foliated rocks like gneisses, schists, and slates, do not show much of this rounding in situ, as they yield to the stresses of expansion and contraction by breaking along planes of crushing or foliation, producing separate masses which preserve most of their angular shape. In the lower reaches of the wadis and on the plains, of course, all the materials are more or less rounded, owing to the attrition to which they have been subjected during transport.

Of the thickness of the alluvial sands and gravels very little is known. But that it exceeds eight metres in many of the wadis is proved by wells sunk to that depth. On the coast-plain it may be vastly thicker, for Grabham[122] mentions that borings made in the maritime plain near Port Sudan have been carried down to 1,000 feet (305 metres) in the deposits. Only a very small proportion of the alluvial detritus brought down the wadis enters either the sea or the Nile; for the storm-formed streams, though violent in the mountainous tracts, commonly cease by being absorbed in the thirsty wadi floors and plains before reaching either the sea or the river. Bearing this in mind, and the ages through which the actions of denudation and transport have gone on, we can well imagine how great must be the total quantity of detritus accumulated.

In some wadis there are high banks of old detritus through which the streams have cut their way. In the Wadi Meneiga, for instance, there are terraces of alluvium, some eight or ten metres high, on either side of the present water channel; these terraces, which are about two kilometres lower down the wadi than the wells, are covered with rude rubble ruins, the remains of dwellings erected out of reach of the occasional streams which rush down the wadi after rain.

At the east foot of Gebel Hamra Dom are some well stratified friable sand rocks and sandy clays, about ten metres thick, ending abruptly against the granites and schists of the hill-mass. These beds appear to be ancient alluvial deposits. They contain ferruginous scales and show sun-cracks in places, while the materials are finer and more distinctly stratified than one would expect to be brought down from the mountain under present conditions.

It is to the alluvial material in the wadis that the Eastern Desert owes most of its perennial water supplies and the vegetation it possesses. The alluvial deposits are the great conservers of water. Pools form, it is true, in the bare rocky beds of the higher drainage lines, and may last for many months where they are screened by the walls of a gorge from wind and sun, and are in consequence not subject to rapid evaporation. But such pools are difficult of access, and afford but a precarious source of water supply because they are liable to dry up if a long period passes without rain. The alluvium of the wadis absorbs the rainfall and protects it from evaporation, so that even in very dry years water may be found by excavating in it at suitable places to depths of a few metres. Almost all the wells which are so important to travellers crossing the desert from the Nile are of this character. The abundance of trees which flourish in so many of the wadis likewise derive their nourishment from the water conserved in the alluvium of the wadi floor.

Calcareous Tufa.

Small deposits of calcareous tufa of recent origin have been noted in the Wadi Um Tundeba and in a gorge on the east side of Gebel Ghuel, as well as round the little trickling spring called Megwel Hamida in the south part of the region.

At Um Tundeba the deposit occurs in a little gully close to a well known galt or pool of rain water. The deposit is not extensive, and has doubtless been formed by the evaporation of trickling drainage-water which had absorbed lime from the rocks. The tufa (10,374) is a pale brown rock of rather porous nature; it envelops fragments of schist-debris from the surrounding rocks.

The calcareous tufa of Gebel Ghuel is a more impressive deposit, though its total mass is not very great. It occurs at a point of sudden fall in a narrow rocky gorge leading to the Wadi Ghadir in about latitude 22° 53′. Proceeding up the gully one is confronted with a great curtain-like mass of tufa covering the face of a high ledge like a solidified cascade. Here also the origin is clearly due to trickling of lime-laden drainage-waters over the ledge of rock, which forms a step in the wadi floor.

Fragments of tufa were also seen round about Gebel Allawi, though the rock was not traced to its source. As only a relatively small number of rocky gullies have been explored, it is likely that similar deposits to those above described occur in many other places.

Gypsum and Gypseous Limestones.

The occurrences of gypsum and gypseous limestone beds in South-Eastern Egypt are restricted to the coastal regions. The most prominent of the deposits are those on Ras Benas (see map on [Plate XXI]), where the beds form white hills rising to 188 metres above sea-level. Further north, gypseous deposits have been found by Dr. Hume to exist near the sea at Bir el Ranga, and by Mr. Ferrar the same beds have been recorded as occurring near the coast in the neighbourhood of Wadi Igli. It appears probable that the gypseous strata form a continuous or nearly continuous strip extending along the coast down to latitude 24° 22′, as shown on the geological map on [Plate XX,] but more complete observations may show that the distribution of the beds is somewhat different from that indicated. On the coast-plain south of Ras Benas, gypseous beds are not exposed except in the extreme south-east corner of Egypt, where they form small patches at Halaib and round the wells of Ti Kureitra.

At Ras Benas, where the beds have been studied in most detail, they consist of gypsum and anhydrite [11,513][123] alternating with sandy marls and marly sands, forming hills much cut-up by steep sided narrow ravines. The weathered faces of the rocks are very soft, and the disintegrated material forms a stretch of soft gypseous sand, into which one’s feet sink four or five centimetres at every step, between the hills and the shore. At Halaib, gypseous limestones crop out from under the gravel of the coast-plain and form low banks; the gypsum is here associated with calcareous grits [12,152, 12,114] and conglomerates, the latter having boulders of igneous rock set in a calcareous matrix. Near Ti Kureitra wells, the gypsum exposures contain much crystalline selenite, blocks of this material being used in the masonry lining of the wells. Both at Bir el Ranga and on Ras Benas the gypsum is found to contain small pockets of native sulphur.

No fossils have been found in the gypseous strata, and their geological age is uncertain. The beds are younger than the Nubian sandstone, since they overlie that formation at Bir el Ranga. Their occurrence only near the coast leads one to regard them as having been formed after the Red Sea occupied its present position; but whether they originated as direct gypseous deposits, or were produced by the alteration of pre-existent Cretaceous or Tertiary limestones is not yet quite certain. It is noteworthy that at Ras Benas, where the gypseous strata rest on diorite and hornblende granite, the igneous rocks are considerably altered as if by weathering, with a strong brick-red colour due to the oxidation of ferruginous matter.

GEOLOGICAL MAP OF RAS BENAS

Ball. Geography & Geology of South-Eastern Egypt.PLATE XXI.

Photo-Metal-Process. Survey Dept. Cairo 1910. (60-190)

The Nubian Sandstone.

The Nubian sandstone covers a comparatively small portion of South-Eastern Egypt, occupying only about one-tenth of the total area under consideration. The greater portion of it forms the eastern termination of the vast sandstone plateaux which stretch eastward from the Nile towards the igneous and metamorphic back-bone of the Red Sea mountains, but there are in addition some small patches on the eastern side of the watershed which are interesting as showing that the deposition of the sandstone was not confined to the western side of the mountain ranges.

The most northerly exposure of Nubian sandstone in the district here described extends with some breaks southward from near Gebel Sufra to near Gebel Homr Akarim, forming low hills. Further south, it comes in again on both sides of the Wadi Garara, forming the hills of Felieiti and Abu Hashim. In the sandy plain around the Wadi Timsah are outlying sandstone hills of considerable height and extent, the principal being Gebels Nuggur, Mulgata, and Ziraga. Further south-west, the Nubian sandstone forms an extensive high broken plateau stretching southward from near Gebel Zergat Naam to near Gebel Um Reit, sending out a long tongue south-eastwards across Wadi Dif to near Gebel Nigrub el Tahtani. This tract of broken sandstone plateaux bears various names in different parts, Gebels Um Harba, Um Khafur, Dagalai, Shebakhit, Awamtib, Um Sididad, Abraq, Hodein, Dif, Anfeib, and Kala, all forming parts of the same great sandstone mass, though separated by wadis from each other. Small outliers of Nubian sandstone cap the hills of Gebels Reietit, just north of latitude 23°, near the meridian of 34°, and Gebel Seiga, in latitude 22° 44′, longitude 34° 16′.

On the eastern side of the watershed, Nubian sandstone extends for some distance near the coast opposite the Gulhan islands, and also forms small thin outliers south of the Wadi Hodein. The most southerly point where the sandstone has been met with in the area is a small outlier in latitude 22° 36′, a few kilometres east of Gebel Hamra Dom.

In petrographical characters the Nubian sandstone is remarkably uniform, consisting of medium-sized silica grains set in a more or less ferruginous cement; the colour varies from nearly white, through various shades of brown, to nearly black, according to the amount of iron present. It is generally well bedded, and frequently much jointed. There is usually a marked absence of the clay beds which are associated with the Nubian sandstone in other parts of Egypt. The basal beds are generally pebbly, forming conglomerates in some places. Concretions are found in the rock at some points, notably to the north of Gebel Um Harba. Some of the concretions are hollow, the shell being of hard dark ferruginous sandstone while the interior is filled with a powder of snow-white chalky matter containing sand grains. The only fossils observed in the sandstone within the area described were collected by Mr. Charteris Stewart in the plain of Um Harba; they consist of crocodile scales and various shells. From these and the fossil shells found further west in sinking a well in Wadi Abu Rahal,[124] it may be concluded that the Nubian sandstone here, as in the Nile Valley, is of Cretaceous age.

Intrusions of igneous rock into the sandstone have only been observed at two points in the area, viz., near the western foot of Gebel Awamtib, where a basic dyke runs through the sandstone, and on the Red Sea coast, near Bir el Ranga, where Dr. Hume has found andesite interbedded in the sandstone. A bed of diabase, now much altered, underlies the sandstone near Wadi Muelih, and is possibly of contemporaneous origin.

In thickness, the Nubian sandstone attains a maximum of about 350 metres in several places round about Bir Abraq and the Wadi Hodein. Its thickness must at one time have exceeded this, for the upper surfaces have suffered much denudation, and are not capped by younger rocks.

Some interest attaches to the maximum altitude reached by the Nubian sandstone beds. The highest point at which it has been observed is at Gebel Seiga, whose sandstone cap is 905 metres above sea-level. The highest point reached by the rock on the great plateau round about Wadi Hodein is Gebel Kala, 846 metres above sea.

In contradistinction to the same beds which form the plateau between longitude 34° and the Nile, the Nubian sandstones within the area here described show considerable disturbances from their original horizontal bedding, the dips being as a rule greater and more variable the nearer one approaches to the watershed mountain-ranges. These tectonic disturbances, which are important in connexion with the geological history of the region, will be considered in [Chapter XI.]

[122]Geol. Mag., Decade V, Vol. VI (1909), p. 271.

[123]The numbers in square brackets are the specimen-numbers in the Cairo Geological Museum.

[124]Wadi Abu Rahal is a small feeder of Wadi Abad, joining the latter from the south in latitude 25° 0′, longitude 33° 30′. At the point of junction of the two wadis, which lies on the usual camel road from Edfu to the Baramia mine, a well was sunk by the Mines Department in 1906, in the hope of obtaining a water supply. I visited the well in May 1906, when it had attained a depth of fifty-four metres. The strata passed through were ten metres alluvium, then thirty-seven metres of sandstones and clays, followed by a thin band of bituminous shale, and seven metres of dark grey clays. From near the bottom of the well I collected specimens of Lingula and a mytiloid shell which Mr. Bullen Newton compared to Septifer linearis; the latter shell differs but little from specimens obtained from the English Gault, and thus tends to show that the Abu Rahal beds are of Cretaceous age. (See Hume, Preliminary Report on the Geology of the Eastern Desert of Egypt. Cairo, 1907. p. 29). Since my visit, the well has been deepened to seventy-three metres in sandstones with a bituminous seam, but water was not reached, and the well has been abandoned.

CHAPTER IX.

IGNEOUS ROCKS.

Igneous rocks cover about one-third of the entire area of South-Eastern Egypt, forming irregularly distributed tracts, alternating with others occupied by metamorphic rocks, entering largely into the composition of the principal mountain masses and also underlying a large portion of the areas covered by sand on the coast-plain and elsewhere.

Regarded broadly, the igneous rocks of this portion of the Eastern Desert form two main divisions, namely, an acid division typified by granite rich in felspar, and a basic division typified by gabbro. Intermediate types occur, as well as ultra-acid and ultra-basic rocks, but these are less abundant, and are generally found in close association with one or other of the two main types. Rocks of the acid type predominate to the north of latitude 24°, while further south basic rocks form large portions of the main mountain tracts, with acid rocks at intervals on either side down to the Sudan frontier. Most of the igneous rocks are plutonic. There is a striking paucity of volcanic rocks, a circumstance doubtless to be ascribed largely to the enormous denudation which has taken place in the district. The plutonic masses, with the dykes seaming them, have been elevated and laid bare, while most of the volcanic outpourings have been denuded away and others have been altered or devitrified till their original volcanic nature has become less apparent. Metamorphism has also affected many of the plutonic masses to such a degree that it is often difficult to decide whether the rocks should now be placed in the igneous group, or classed as metamorphic rocks.

For purposes of classification, the igneous rocks of South-Eastern Egypt may be divided into five main groups, based on silica-percentage:—

1. [Ultra-acid rocks,] containing over 80 per cent of silica.

2. [Acid rocks,] containing from 65 to 80 per cent of silica.

3. Intermediate rocks:—

(a) [Sub-acid rocks,] with from 60 to 65 per cent of silica.

(b) [Sub-basic rocks,] with from 55 to 60 per cent of silica.

4. [Basic rocks,] containing from 45 to 55 per cent of silica.

5. [Ultra-basic rocks,] with less than 45 per cent of silica.

The classification is based on silica percentage, for although up to the present it has not been found possible to undertake the chemical analysis of the rocks, the microscopic examination of thin sections itself gives a very fair guide to the chemical composition, and the specific gravities of the different rocks, which increase progressively from the acid to basic groups, afford a further guide in this direction. The table on the following page shows the different rocks comprised within the five main classes.

The five classes are not all of equal importance. As already mentioned, the acid and basic types are predominant. The ultra-acid and sub-acid rocks are present in relatively small quantity, and are in close relationship with those of the acid series which occur in great abundance; the rocks of the sub-basic and ultra-basic groups, on the other hand, though by no means insignificant in their distribution (diorites and serpentines cover very large areas), appear to be closely linked in the field with those of the basic group. The district is so large, and the investigation of it has been so limited, that it would be unwise to draw from the above remarks the deduction that the rocks have originated from two main magmas; the statements are merely those of the facts of observation, so far as observation has proceeded.

The scheme tabulated above differs from established classifications only in the inclusion of an ultra-acid class of rocks. This inclusion is necessitated by the presence in the district of huge masses of quartz-rocks which are almost certainly igneous in origin. These rocks are almost pure silica, and though they are properly regarded as an extreme form of pegmatite, their silica-percentage (over 95) is too high for them to be placed with their near relations the granites; and on grounds of symmetry of classification it appears justifiable to have an “ultra-acid” class corresponding with that of ultra-basic rocks.

Classification of Igneous Rocks.

Ultra-acid.Acid.Intermediate.Basic.Ultra-basic.
Sub-acid.Sub-basic.
Quartz-rock.Granite.Syenite.Diorite.Gabbro.Pyroxenite.
Normal pink granite.Augite-diorite.Troctolite.Amphibolite.
Red pegmatitic granite.Mica-diorite.Pyroxene-granulite.Peridotite.
Aplite.(Serpentine).
Biotite-granite.
Muscovite-granite.
Hornblende-granite.
Granite-porphyry.Syenite-porphyry.Diorite-porphyrite.Diabase.
Augite-porphyrite.Mica-diabase.
Kersantite.
Quartz-felsite.Trachyte.Andesite.Basalt.

ULTRA-ACID ROCKS.

Quartz-rock of igneous origin occurs in the form of intrusive bosses and veins penetrating older igneous and metamorphic rocks at Marwot Elemikan and elsewhere. The rock is practically pure vitreous quartz. Its dazzling white aspect in the sun renders it very conspicuous in the field. The shape of many of the masses (bosses), their close association with pegmatite and aplite in at least one locality, the absence from the veins of other minerals (calcite and various ores) such as usually accompany vein-quartz deposited from solution, and the occasional presence in the rock of mica flakes, all point to an igneous origin, and justify us in regarding this quartz-rock as the final product of differentiation of a granitic magma, representing its ultra-acid portion.

The most conspicuous occurrence of quartz-rock is in the three nearly conical hills called Marwot[125] Elemikan, which rise from the granitic plain at the head of Wadi Elemikan in latitude 23° 53′, and which from their snow-white colour form landmarks visible from great distances. The largest and most northerly hill of the three is a boss about 300 metres in diameter, rising sixty-eight metres above the plain, which itself has here an altitude of 580 metres above sea. The other two hills are somewhat smaller; they lie respectively two kilometres south-east, and two and half kilometres south, of the largest one; a black hill of about equal size rises from the plain mid-way between them. The highest of the three white hills, which was occupied as a triangulation station, is composed almost entirely of glassy quartz, the only other mineral seen in it being a white mica, of which a few flakes were visible in the outer portions. The mass is cracked in all directions, and the faces of the separated blocks frequently show a slight yellowish staining.

Another remarkable boss of quartz, similar to that just described and likewise rising through granite, occurs on the south side of the Wadi Khoda (latitude 23° 42′) about seven kilometres above the point where the wadi emerges from the hills on to the coast-plain. A third occurrence, of much less size, in the floor of Wadi Abu Marwa, on the track from Wadi Lahami to Berenice via the Wadi Naait; the first-mentioned wadi derives its name from the quartz occurrence in it.

Of igneous quartz veins, the best example is the huge dyke which forms the back-bone of the ridge called Erf el Fahid, in latitude 25° 0′. Here the vein, which strikes east and west, is in schistose country. It is at least ten metres wide, rising to a height of ninety-five metres above the wadi, and can be traced for a length of nearly two kilometres. Like the rock of Marwot Elemikan, the quartz of Erf el Fahid shows faint iron-staining in places, but appears to contain no other minerals. It is weathered to a sintery appearance in places [10,362],[126] and contains some cavities, from one of which I extracted a handful of loose quartz crystals [10,363] with pyramids developed at both ends; these last are evidently deposited from solution, but it is difficult to assign other than an igneous source for the vein, and the cavities and loose crystals are probably the product of solution subsequent to the igneous intrusion.

Going south-west from Erf el Fahid, down the Wadi Muelih, is another line of ridges, of which the back-bone is a great quartz vein traceable for some two and a half kilometres along its strike. Further down the wadi are networks of quartz veins seaming diorite in all directions on either side of a horse-shoe-shaped hill called Marwot Rod el Ligaia; this hill is of aplite, and probably represents a less acid part of the same magma which formed the quartz veins.[127]

It is significant that there are no traces of mine workings at any of the places where quartz veins and bosses of the igneous type occur, notwithstanding the fact that the masses are so conspicuous as to have surely attracted the eye of every gold-seeking prospector. Though the loose crystals found at Erf el Fahid show that there at least aqueous solutions have acted on the rock to a slight extent, these great igneous quartz masses appear never to have been impregnated with gold or other ores.

ACID IGNEOUS ROCKS.

Granites.

Granites are the most abundant and most widely distributed of the igneous rocks of South-Eastern Egypt. They form a large proportion of the most prominent mountain masses, such as Gebels Hamrat Wogud, Nugrus, Hamrat Mukbud, Hamata, Faraid, Um Reit, Mishbih, Adar Qaqa, Adar Aweib, and Elba. They also occur in some great plain tracts, such as those round Gebel Selaia and to the west of Gebel Um Reit, where low hills of the rock rise through the coarse granitic sand which covers most of the plain.

In point of geological age, the granites appear to be the youngest of the plutonic rocks of the country, forming great intrusions in the more ancient schists and diorites.

Though sometimes occurring as rounded bosses, as at Gebels Muelih, Selaia, and Faraid, granite typically forms rather jagged mountains, more especially in the south parts of the area; the mountains of Mishbih and Elba, for instance, abound in spiky peaks, while Qash Amir, the “Scragged Hill” of the Admiralty Chart, is an extreme example of this mode of weathering. The spikes are often made up of more or less rounded blocks piled one on another, the separation and rounding of the blocks being brought about by jointing and the more rapid weathering of the corners of the separated masses. Granite mountains, though sometimes white, are usually of a pink or red colour and are often named accordingly by the Arabs. In Ababda country, all mountains named Hamrat, as for instance Hamrat Wogud and Hamrat Mukbud, are composed of red granite or granitoid gneiss; while in Bisharin country the corresponding name Adar, as used for example in Adar Qaqa and Adar Aweib, has the same significance, both the names meaning red. The beds of wadis draining from granitic mountains, and plains adjacent to them, are invariably covered with a thick accumulation of coarse felspathic and quartzose sand derived from the weathering of the rock. This sand is quite firm to walk on, and in this respect forms a pleasing contrast to the finer wind-borne sand derived from the disintegration of sandstone. The granitic sand being generally white in colour (the redness of the felspars having largely disappeared in the process of weathering), the wadis in whose floors it is largely displayed are frequently named “Wadi el Abiad” (abiad = white); there are numerous wadis called by this name, all possessing the common characteristic of draining from granitic mountains and consequently having a floor of granitic sand.

Besides the great mountain-forming masses, granites also occur frequently in the form of dykes or veins, penetrating the gneisses, schists, and other rocks.

Viewed as a whole, the granites of South-Eastern Egypt are characterised by their strongly acid composition. Though perfectly normal granites are found in many places, and a gradual passage into quartz-syenite may be occasionally traced, yet on the whole the rocks approximate to the aplitic and pegmatitic types in which quartz and felspar are associated with very small quantities of ferro-magnesian minerals.

We may consider the granites as falling mainly into the following classes:—

1. Normal pink granite.

2. Red pegmatitic granite.

3. Aplite.

4. Biotite-granite.

5. Muscovite-granite.

6. Hornblende-granite.

Ball.—Geography & Geology of South-Eastern Egypt.PLATE XXII.

ACID IGNEOUS ROCKS.

PORPHYRITIC GRANITE.
Gebel Fereyid.

PEGMATITIC RED GRANITE.
Wadi Gemal.

BIOTITE-GRANITE.
Gebel Abu Hegilig.

HORNBLENDE-GRANITE.
Gebel Elba.

QUARTZ-FELSITE.
Near Gebel Kolaiqo.

QUARTZ-FELSITE.
Near Gebel Kolaiqo.

NATURAL SIZE.

Normal pink granite is the form of granite most commonly met with in South-Eastern Egypt. It forms the principal rock of many of the mountains, such as Gebels Nugrus, Selaia, Faraid, Um Reit, Shigigat, Niqrub, Abu Brush, and Hamra Dom, and also covers large expanses of low hill country, and occurs as knolls scattered over sandy plains such as that of Selaia and the tract west of Um Reit. The rock [11,505], a full size representation of which is shown on [Plate XXII,] is composed of pink orthoclase, white oligoclase, quartz, and biotite, with hornblende, sphene, apatite, and magnetite as accessory minerals. The pink orthoclase is the dominant constituent, and gives the colour to the mass; it is sometimes in grains of about the same size as those of the quartz, viz., two to four millimetres in diameter, but frequently tends to assume a porphyritic habit, the crystals then measuring a centimetre or more across, and inclining to idiomorphic forms. Microcline is occasionally sparingly present. The oligoclase is white, and is far less abundant and less conspicuous than the orthoclase; in the porphyritic varieties of the rock, the oligoclase is mixed with the quartz and mica of the ground mass. In some specimens the felspars are fresh, in others [12,133] they show clouding due to decomposition, with formation of kaolin and occasionally of sericite. Quartz is present to about half the amount of the felspars, in grains ranging up to three millimetres in diameter. The biotite is frequently the only ferro-magnesian mineral present, occurring in wisps and little nests among the other minerals; under the microscope it is strongly pleochroic, the colour varying from usually a pale olive brown to nearly black when a crystal is rotated over a nicol prism, though in a specimen [11,531] from the little hill called Sikeit, about five kilometres west of Berenice, colour range is from pale yellow to deep green. Hornblende is typically either absent or present only in a very subordinate amount to the biotite. It is sometimes altered to chlorite, with separation of granules of iron oxide and formation of epidote. Sphene, apatite, and magnetite are usually only very sparingly present, and are only visible on microscopic examination. In a slide cut from the rock of Gebel Fereyid, however, sphene forms a crystal measuring 1·6 millimetres across, of the characteristic wedgelike form (see [Fig. 4]).

Fig. 4.—Granite, Gebel Fereyid [11,505], × 10. f, felspar (mostly orthoclase), somewhat clouded by decomposition; q, quartz; b, biotite; s, sphene.

The principal variation in mineral composition of the rock is the greater or less abundance of the biotite, and the occasional presence of appreciable amounts of hornblende. In a specimen from Wadi Kreiga [12,133], the biotite is almost absent, its place being taken by green hornblende, now largely altered to chlorite and epidote. The ferro-magnesian minerals are as a rule present in smaller amounts than in average granites, and in some places, especially near the periphery of the great intrusions, they vanish almost entirely, and the rock passes gradually into a pegmatite.

Variations in appearance of the rock are also conditioned by the size and habit assumed by the felspar crystals, by the variations in general coarseness or fineness of grain, and by the pressures and weathering influences to which the masses have been subjected. Where the rock has been crushed, or subjected to unequal pressures in different directions during consolidation, the porphyritic crystals tend to lie along definite planes, and the rock may approximate in appearance to a gneiss. The microscopic sections from these places show further evidence of crushing in the cracking of crystals and undulose extinction between crossed nicols. Where the rock is much weathered it usually takes on a whiter aspect due to the bleaching of the orthoclase, and sometimes exhibits green spots due to the formation of chlorite from the decomposition of biotite and hornblende.

It is the pink granite which by its disintegration gives rise to much of the granitic sand which is met with in wadis and on plains. The rock disintegrates very readily under changes of temperature, the cleavage planes of the orthoclase aiding in the process. Change of the felspar to kaolin goes on under weathering influences, and the felspars become bleached, but the bulk of the disintegration is a mechanical process, a large part of the sand consisting of unaltered felspar.

Red pegmatitic granite is closely associated with the normal pink granite, occurring in great masses peripherally to the latter, as for instance at Gebel Hamrat Mukbud and in the low hills about the lower parts of Wadi Gemal and Wadi Kreiga. The red pegmatitic granite is in fact simply an extreme variation of the normal pink granite in which the ferro-magnesian minerals (biotite and hornblende) are either absent or present in very small amounts. That the rock often forms dykes is probably a consequence of its peripheral position rather than of any special manner of formation. The great masses such as Gebel Hamrat Mukbud appear, like the normal granites, to have solidified under plutonic conditions, and the dykes are off-shoots from the main mass. There is, therefore, no reason in this locality for classifying the pegmatites separately from the granites into a special division of dyke-rocks, as is done by Prof. Rosenbusch, and we shall consider the red pegmatitic granite as simply a coarse grained mica-free granite rich in orthoclase.

The pegmatitic granite is usually of a far more pronounced red colour than the normal granite. This is doubtless in part due to the greater abundance of orthoclase, but it also arises in some cases from the orthoclase having itself a deeper colour. The rocks about the lower parts of Wadi Gemal and Wadi Kreiga have a strong brick-red aspect. Their proximity to the coast suggests that there may possibly be some connexion between the intensity of their coloration and their position near the sea; this idea is supported in some measure by the strong red colouration of the dioritic rocks of Ras Benas, which occupy a similar position, but the actual manner of production of the oxide of iron which gives the colour is not clear.

Fig. 5.—Red pegmatitic granite, from low hills near the mouth of Wadi Gemal [12,106], × 10. q, quartz; f, felspar (orthoclase) turbid and full of finely disseminated red oxide of iron.

In the hand specimen (see [Plate XXII]), the red pegmatitic granite [10,389] is a coarse-grained aggregate of red orthoclase and quartz. The quartz is generally of grey aspect in the mass, running in strings and networks through the felspar. The rock usually breaks easily into irregular pieces under the hammer, owing to the facility with which the large felspar crystals can be cleaved, but the very red variety from near the mouth of Wadi Gemal [12,106] is extremely hard and tough, and strikes fire very easily under the hammer. Under the microscope (see [Fig. 5]) the red colour of the orthoclase often persists even in the thinnest sections, being evidently due to finely disseminated iron oxide which clouds the felspars. The quartz shows sharply defined angular forms, and appears to have got somewhat the start of the felspar in crystallisation. Owing to its cloudy nature, the felspar looks like a ground mass, and between crossed nicols shows a peculiar patchy appearance, so that a first glance at the slide suggests a quartz porphyry; but the whole of the patches forming a crystal extinguish at once, and moreover the characteristic twinning of the orthoclase can be seen in some of the crystals. A little oligoclase is also present in addition to the orthoclase. Crushing of the rock is frequently shown by cracked and brecciated crystals, more especially in the hard form of the rock from Wadi Gemal.

Dykes of pegmatitic granite are very frequent in the neighbourhood of Gebel Hamrat Mukbud; they are typically rather paler in colour than the larger masses, and occasionally show graphic structure.

In ascending Gebel Migif, the gneiss slopes of the mountain were found to be strewn here and there with crystals of green microcline [10,366], similar to the well-known Pike’s Peak mineral. The crystals, which are of imperfect idiomorphic form, measuring about one to two centimetres in length, sometimes show cross-striations on certain faces. Though the rock was not traced in situ, the crystals are doubtless derived from pegmatite dykes traversing the gneiss.

Aplite, or fine-grained binary granite consisting essentially of quartz and felspar, with very little or no mica or hornblende, differs from pegmatite not only in its greater fineness of grain, but in its structure, which is essentially granitic rather than pegmatitic, the two main minerals being both in allotriomorphic grains. In colour, different aplites vary with that of their felspars from red to white, and most of them have tiny dark spots in them, sometimes only visible with a lens, due to the presence of small quantities of hornblende or biotite. As already mentioned on [p. 268,] most of the granites of South-Eastern Egypt incline to the acid or aplitic type, and one is frequently in doubt whether to classify an acid granite, such as that of Gebel Elba for instance, as an aplite with a little hornblende, or as a hornblende granite. Under aplites will be here considered only those granites in which the proportion of ferro-magnesian minerals is extremely small, not forming more than about one or two per cent of the rock. When the definition of an aplite is thus limited, aplites are comparatively scarce in the area.

Gebel Abu Hireiq consists of a red granitic rock; I did not visit the range, but the guide sent to erect the triangulation beacon brought back as the typical rock a pink aplite [12,134] of rather fine grain, consisting essentially of quartz and pink felspar, with sparsely scattered specks of hornblende.

The dykes [11,536] which seam the granite and diorite near where the Wadi el Kreim joins Wadi Garara may be classed as siliceous altered aplites. They are greyish-white to purplish-pink rocks of rather fine grain, with a few blackish specks, the latter visible only with a lens. The microscopic slide shows granitic quartz and red-stained decomposed matter, with a rather plentiful sprinkling of grains and powder of iron oxide; the decomposed matter is almost certainly the result of alteration of felspar and hornblende, the latter in very small proportion.

The hill called Marwot Rod el Ligaia, near the head of Wadi Muelih, is a boss of pink aplite, and is of interest as lying in close connexion with certain quartz veins which are believed to be of igneous origin (see [p. 266]).

Fig. 6.—Biotite-granite near Gebel Abu Hegilig [10,390], × 17. q, quartz; f, felspar (orthoclase and oligoclase), somewhat turbid by decomposition; b, biotite; s, sphene.

Biotite-granite of a well-defined type [10,390] occurs somewhat largely in the mountains round the heads of Wadi Abu Ghusun and Wadi el Abiad. It is a whitish rock with dark-brown to black spots (see [Plate XXII]). The white portion of the rock consists of quartz and felspar; the dark spots are biotite. Generally the rock is of medium grain, the biotite being in nests two to four millimetres diameter scattered through the mass, giving it a speckled appearance. Sometimes the biotite is more uniformly scattered, giving the rock a grey aspect. At some places the rock shows signs of crushing, this being indicated in hand specimens by a tendency to laminar arrangement of the biotite patches. Under the microscope (see [Fig. 6]), the felspars, which are somewhat more abundantly present than the quartz, are seen to consist of orthoclase and oligoclase, with here and there crystals showing the characteristic cross hatching of microcline. The biotite is mostly in nests and clusters of ragged-looking crystals, but it also occurs in tiny flakes included in the felspars, sometimes showing a distinct arrangement along the cleavage planes of the latter. The biotite is usually fairly fresh; it is highly pleochroic, with colour ranging from pale yellow to deep olive-green in different positions over the nicol. Associated with the biotite are a few wisps of colourless mica, probably muscovite, and irregular granules of opaque iron oxides. Granules of epidote are to be seen in the altering felspars, and there are a few small irregular granules of a brownish highly refracting mineral, probably sphene.

A much more acid type of biotite granite [11,507] occurs in close proximity to serpentine in low hills near the wells of Abraq. In this the proportion of biotite is so small that the rock might almost be classed as an aplite. It is a fine-grained white rock, very fresh-looking, with dark spots; in hand specimens it almost exactly resembles the hornblende granite of Gebel Elba (see [Plate XXII]), but examination with a lens shows the dark spots to consist of biotite instead of hornblende. The microscopic slide shows quartz, orthoclase, and oligoclase with granitic structure, with here and there small straggling crystals of brown biotite; the brown biotite is sometimes altering to chlorite with change of colour from brown to green.

Muscovite-granite is rarely met with in South-Eastern Egypt. A specimen from Gebel Adar Qaqa[128] [12,138] is a fine-grained hard pinkish rock of sp. gr. 2·62, composed of quartz, orthoclase, oligoclase, muscovite, and garnet.

A very coarse-grained variety of muscovite-granite, in which the individual crystals measure several centimetres across, forms white hills to the east of Wadi Nugrus.

Hornblende-granite occurs in several forms, giving rise to rocks of different aspects in different parts of the district.

The most important and widely distributed form of hornblende-granite is a fine-grained and very acid rock of sp. gr. 2·59, which but for the manner in which it occurs might almost be spoken of as an aplite. It is a white rock, sometimes with a faintly pink or greenish cast, of medium to fine grain, with black specks of hornblende about a millimetre in diameter scattered through it (see [Plate XXII]). This rock forms Gebels Muelih and Elba; it also occurs in the low country round Gebel Hamrat Mukbud, and a gneissose variation of it forms the great boss of Um Rasein. Dykes of the same rock traverse the more basic rocks of Um Bisilla.

Fig. 7.—Hornblende-granite, Gebel Elba [12,118], × 17. q, quartz; f, felspar (mostly orthoclase), clouded by kaolinisation; h, hornblende; b, biotite, altering with separation of flakes of limonite.

Under the microscope the rock from Gebel Elba [12,118] is seen to consist of quartz, orthoclase, a little oligoclase, hornblende, and small quantities of biotite (see [Fig. 7]). The orthoclase is somewhat clouded in strips parallel to the clino-axis of the crystals. The hornblende is in irregular grains of sharply marked curved outlines; it is of a very deep bottle-green colour. The accessory biotite is brown, occurring as small wisps associated with the hornblende.

The rock of Gebel Muelih [10,355] is similar to that of Gebel Elba, but the hornblende here is of an extremely pale greenish brown colour, only slightly pleochroic, with well-marked vertical cleavage and a very small extinction angle, so that it might be mistaken for biotite, which latter mineral appears to be absent.

Fig. 8.—Hornblende-granite, Gebel Hamata [10,405], × 10. q, quartz (the rock contains a larger percentage of quartz than appears in the drawn portion of the slide); f, felspar (orthoclase), clouded; h, hornblende, altered.

Another type of hornblende-granite, differing from that last described in being of coarser grain and containing much more abundant hornblende, and thus having much more the appearance of an ordinary grey granite, occurs in the lower part of Gebel Hamata and the surrounding mountains. This rock [10,405] is slightly heavier than the foregoing type, having a sp. gr. of 2·66. Under the microscope (see [Fig. 8]) the hornblende is seen to be of a dark green colour, very much altered and clouded by separated iron oxides and epidote.

A third type of hornblende-granite is formed by the variation of the normal pink granite already referred to, in which the place of biotite is largely taken by hornblende. The mass of Gebel Mishbih appears[129] to consist of a rock of this type, which passes by insensible gradations into a syenite.

Granite-porphyry.

Fig. 9.—Granite-porphyry, Kreishim Hill [12,150], viewed between crossed nicols, × 10. q, quartz (the dark triangular area to the left of figure is also a quartz crystal in the position of extinction); f, felspar (oligoclase); g, micro-granitic ground mass.

Under this heading are comprised rocks of granitic composition in which porphyritic crystals of quartz, felspar, and mica are surrounded by a fine-grained ground mass of entirely granitic (holocrystalline) character. They differ from porphyritic granites in the much finer grain of the general body of the rock, and from the quartz felsites in the entire absence of glassy matter from the ground mass. Rocks of this type are scarce in South-Eastern Egypt. An example [12,150] occurs at the hill called Kreishim, sixteen kilometres west of Halaib. Here the rock, which appears to form a small boss, consists of rounded white porphyritic crystals set in a finely crystalline grey ground mass. The sp. gr. is 2·69. Under the microscope ([Fig. 9]) the porphyritic crystals are seen to consist of quartz and oligoclase, the latter in approximately idiomorphic crystals, often beautifully zoned; the ground mass, of micro-granitic structure, is made up of smaller allotriomorphic crystals of quartz, felspar, and biotite.

Dykes of granite-porphyry also occur traversing the granite of Gebel Kahfa [11,537 B] and in the basic rocks of Gebel Um Bisilla [11,518]. At both these places the rocks are far less fresh than that above described. In the hand specimen they are of a greyish-white or greenish-white colour, strongly resembling fine-grained aplites in appearance. With the lens, crystals of quartz and felspar can be made out, and specks of hornblende and chlorite. The microscopic slides show the characteristic structure of granite porphyry, but the felspars are all clouded by decomposition products, and the hornblende, which is rather sparingly present in the ground mass, is mostly changed to chlorite and epidote. The specimen from Gebel Kahfa contains a small amount of muscovite.

Quartz-felsite.

Though covering only small areas, quartz-felsite is of very wide distribution in South-Eastern Egypt. It typically forms dykes and intrusive sheets traversing the plutonic and metamorphic rocks, but likewise occurs occasionally in larger masses. Felsite dykes are specially abundant in the neighbourhood of Gebels Muelih, Zergat Naam, and Um Reit. Larger masses occur in the Wadi Huluz, and form the summits of Gebels Nigrub el Foqani and Hamata.

Felsites are frequently met with in a highly altered condition. The commonest alteration, especially in dykes, is kaolinisation of the felspars and a clouding of the whole rock with finely disseminated iron oxides. In some dykes traversing the granite of Um Reit, a bleaching action has reduced the felsite to the appearance of a limestone. In other cases the rock has been devitrified and indurated to a high degree; this is well seen in Gebel Igli el Iswid (latitude 25°) and at Gebel Hadarba (latitude 22°), where extensive hill-tracts consist of felsitic rocks of almost flinty hardness.

The quartz-felsites are in general among the youngest of the igneous rocks of the district, since they commonly form dykes and intrusions, not only in the schists, but also in the granites and other eruptive rocks.

Fig. 10.—Quartz-felsite, Wadi Huluz [10,394], × 10. q, quartz; f, felspar (orthoclase and oligoclase); g, ground mass, showing flow structure round the porphyritic quartz and felspar.

The quartz-felsite of Wadi Huluz [10,394] occurs in considerable masses about a kilometre below the water holes of Um Gerifat. In the hand specimen, it is of granitoid appearance, with opalescent quartz crystals two millimetres or more in diameter plentifully scattered in a light greyish ground mass. The sp. gr. is 2·71. Under the microscope the crystals of quartz are seen to have rounded forms and to be accompanied by other porphyritic crystals of orthoclase and oligoclase, often approximating to idiomorphic shape (see [Fig. 10]). The porphyritic constituents are embedded in a cryptocrystalline ground mass, in which augite and minute grains of quartz, felspar, and biotite can be made out, and there are some strings and granules of epidote and fairly large specks of iron oxides. The ground mass shows a fluidal arrangement, the little biotite flakes in it being often arranged in lines which sweep round the porphyritic crystals. The rock has undergone considerable alteration, the felspar crystals being clouded and full of tiny micaceous flakes of high double refraction; the epidote is also doubtless due to the alteration of augite and other minerals in the ground mass.

The quartz-felsite which forms the upper part of Gebel Hamata [10,906] is a dark brown coarse-textured rock, with remarkably glassy porphyritic quartz in granules about two millimetres diameter scattered plentifully through it. It is a very hard rock which rings under the hammer and weathers into rusty brown blocks. The sp. gr. is 2·71. Under the microscope the quartz crystals are seen to be much cracked. There are also porphyritic crystals of orthoclase and oligoclase, mostly of irregular shape, and a few crystals, of still more irregular form, of dark green hornblende. The cryptocrystalline ground mass consists chiefly of quartz and felspar, through which are scattered tiny granules of dark green hornblende. The ground mass shows a matted texture between crossed nicols; there is no trace of fluidal movement. Like the foregoing, this rock is somewhat altered, the hornblende in particular being very much attacked; the felspars are fairly fresh, but the crystals, like those of the quartz, are often cracked, and thus show the rock to have undergone considerable crushing.

Fig. 11.—Quartz-felsite from dyke at Gebel Kolaiqo [12,145], × 10. q, quartz (a group of idiomorphic crystals); f, felspar (mostly orthoclase); g, microgranitic ground mass of quartz and felspar, with wisps of hornblende.

One of the quartz-felsite dykes traversing the schists at Gebel Kolaiqo [12,145] is a very similar rock to that of Gebel Hamata, except that here the ground mass, instead of being dark brown in the hand specimen, is of a reddish brown colour (see [Plate XXII]), and the porphyritic crystals are partly pink felspar and partly glassy quartz. The sp. gr. is 2·62. Under the microscope the quartz and felspar crystals are seen to be less crushed than in the Hamata rock, while the ground mass, instead of being of a matted structure, is microgranitic, with tiny elongated grains of green hornblende scattered through it (see [Fig. 11]).

Another quartz-felsite dyke at Gebel Kolaiqo [12,135] has a rather different composition, and in the hand specimen has more the appearance of a felspar porphyry, porphyritic white felspars being scattered with quartz grains through a dark grey to black ground mass (see [Plate XXII]). Its quartz crystals frequently approximate to idiomorphic forms; they are much corroded by the ground mass. The felspar is chiefly oligoclase, in more or less idiomorphic crystals, a little clouded by decomposition products. The ferro-magnesian mineral here is chiefly brown biotite, in crystals of smaller size than those of the quartz and felspar, scattered porphyritically through the ground mass. One of the quartz crystals includes a crystal of biotite. The ground mass is crystalline, and appears to consist of quartz and felspar with a little biotite and some glassy matter.

A peculiar type of quartz-felsite [10,377] occurs associated with serpentine and ophicalcites in the upper part of Gebel Ghadir. The rock, which has a sp. gr. of 2·66, is nearly white, with a faint greyish tinge and scattered reddish-brown specks. At first the rock was taken for a granulite, which it very much resembles in appearance, and the reddish-brown specks for garnet. But examination with a lens shows the specks to be ferric oxide, and here and there little crystals of glassy quartz can be seen. Under the microscope the rock is found to consist mainly of an extremely fine-grained semi-granulitic colourless ground mass in which are sparsely scattered clear crystals of quartz, sometimes showing corrosion, and straggling irregular patches of opaque iron oxide, often mixed with nearly colourless wisps of mica. A few of the clear crystals are in four-sided forms, and may possibly be felspar; but they are perfectly free from alteration or twinning. The iron oxide is doubtless the product of alteration of a ferro-magnesian mineral, probably biotite. The minerals of the ground mass are difficult of determination owing to the minuteness of the grains and the absence of any colour, but apparently consist of quartz, felspar, a colourless hornblende, and sericite.

The quartz-felsite dykes [10,356] which cut through the granite and schists of Gebel Muelih appear as grooves in the granite, and as back-bone ridges in the surrounding schists. They are pinkish-brown close-textured rocks, with a dark marbling in places. The microscopic slides show quartz and felspar in a confused ground mass, with little flakes of a white micaceous mineral (sericite?) aggregated round the porphyritic crystals and distributed through the felspars and the ground mass.

The highly altered quartz-felsite [12,158] which forms Gebel Butitelib, near Gebel Niqrub el Tahtani, appears to have formed a boss or neck in the surrounding schists. The rock is a very fine-grained grey to purplish one, in which no crystals can usually be made out with the unaided eye. The sp. gr. is very low, being only 2·32. Under the microscope one sees small highly corroded crystals of quartz scattered through a clouded and glassy ground mass. No traces of felspars can be seen, the crystals, if they existed, having been decomposed and become undistinguishable from the ground mass; nor can any ferro-magnesian minerals be made out, though there are tiny granules and wisps of iron oxides which in places show an arrangement suggestive of their having been derived from the decomposition of such minerals.

The specimen [12,121] brought back from the summit of Gebel Shendib, by the guide who was sent to erect the triangulation beacon on it, is a compact brown rock with reddish patches, which under the microscope appears to be an altered and brecciated felsite.[130] The slide shows quartz and altered felspars in a spherulitic ground mass, the whole of the mineral being very much clouded by finely disseminated ferric oxide.

The felsite dykes [11,538] which seam the granite of Gebel Um Reit are a still more highly altered rock. The dykes are about two metres wide, of a white colour, and resemble limestone in appearance; being soft, they have weathered more rapidly than the granite around them, leaving vertical-sided chasms. But for the manner of its occurrence, the rock would have been easily mistaken in the field for a sedimentary one. The microscope, however, confirms its eruptive origin. The slides cut from the rock show a confused and clouded semi-crystalline aggregate, containing clearer small areas of quartz. Even the quartz crystals are full of specks, and only the faintest traces can be seen of the felspars, which are so decomposed as to be hardly distinguishable from the ground mass.

Fig. 12.—Altered quartz-felsite, Gebel Igli el Iswid [10,372], as seen between crossed nicols, × 40. f, felspar (mostly orthoclase in the position of extinction); h, a crystal of hornblende; g, hemi-crystalline ground mass; m, micropegmatitic intergrowth of quartz and felspar, separating the porphyritic crystals.

In the rocks of Gebels Igli el Iswid, Mahali, and Hadarba, we have quartz-felsites which have been altered in quite a different way. The rocks are extremely hard, and almost flinty, breaking with a sub-conchoidal fracture. They are typically of a brown to nearly black colour, with white spots measuring a millimetre or two across. Under the microscope these white spots are seen in the rock from Gebel Igli el Iswid [10,372] to be chiefly porphyritic felspar and quartz crystals, sometimes corroded by the ground mass; while the black ground mass is largely composed of extremely fine micro-pegmatitic intergrowths of quartz and felspar, with some glassy matter, and here and there irregular small clouded and altered crystals of hornblende. The micro-pegmatitic material has possibly originated by devitrification of an originally glassy ground mass. Variations of the rock occur in which the porphyritic felspars are infrequent or even altogether absent, the rock passing gradually into a hornfels [10,371]. The slide from Gebel Mahali [10,402] exhibits clear porphyritic crystals of quartz and orthoclase, both minerals in more or less idiomorphic forms, embedded in, and occasionally corroded by, a semi-crystalline ground mass. The ground mass contains some calcite and iron oxides, probably arising from the decomposition of a hornblendic or micaceous mineral.

Fig. 13.—Microperthitic structure in felspar of quartz-felsite, Gebel Hadarba [12,147], as seen between crossed nicols, × 40.

The rocks of Gebel Hadarba [12,146-12,149] are essentially similar to those of Gebel Igli el Iswid, but in some cases they show bright red veining; where they have been exposed to the polishing agency of the sand blast, these veined varieties look as though they had been streaked with melted sealing-wax. In some of the slides the felspars show a well marked microperthitic structure, while in others they are so decomposed that they are barely distinguishable from the ground mass. The ground mass sometimes contains granules and strings of iron-oxide, possibly referable to alteration of a hornblendic constituent, but the rock is too much altered for one to be sure.

Fig. 14.—Crushed oligoclase crystal in quartz-felsite, Wadi Huluz [10,404], as seen between crossed nicols, × 40.

An altered and crushed quartz-felsite [10,404] which occurs in the Wadi Huluz, near where the Wadi el Abiad joins it about eight kilometres to the north-west of Gebel Hamata, somewhat resembles the rock of Gebel Hadarba in appearance. It is a jaspery looking rock of dark colour with red and greenish patches, in which with a lens one can see scattered grains of glassy quartz. The slide cut from the rock shows clear porphyritic crystals of quartz, orthoclase and oligoclase in a fine grained ground mass. The porphyritic crystals are mostly in rounded forms, but some of the felspars show a tendency to idiomorphism. Many of the crystals are smashed, and some show undulose extinction. The ground mass consists of quartz, felspar, and some glassy matter, with abundant epidote in nests and strings, and a small amount of green hornblende in straggling forms.

The east-and-west dykes which traverse the syenite of Gebel Zergat Naam, and form the actual summit of that mountain, consist of a very hard compact flesh-coloured rock which weathers brown on the surface. The microscopic section [11,525] shows the rock to be a highly indurated felsite. The original felspar crystals can be made out by their shapes, but between crossed nicols they are seen to consist of a very fine-grained mosaic polarising in low greys, probably the result of alteration by siliceous solutions. These altered felspars are scattered with some quartz in a cryptocrystalline ground mass.

No volcanic rocks of acid type have been with certainty identified in South-Eastern Egypt. It is, however, possible that some of the rocks above described are in reality of volcanic origin, though the manner of their occurrence is more suggestive of intruded masses and sheets. It is also quite likely that some of the hornfels found associated with the schists, as for instance at Gebel Um Semiuki, near Abu Hamamid, are altered forms of glassy acid lavas; but their vast age and the intense metamorphism to which they and the surrounding rocks have been subjected render it impossible to be certain of the manner of their origin.