Gebel Um Bisilla is a dark mountain of gabbro rising conspicuously among lower hills twenty-two kilometres west of Gebel Dahanib. The triangulation beacon on its summit is in latitude 23° 45′ 34″, longitude 34° 57′ 39″, and 824 metres above sea-level. Gebel Um Bisilla is drained by Wadi Um Bisilla, the head of which encircles its southern half, while the northern flanks are drained by feeders of the Wadi el Khiua, which courses south-west to join Wadi Um Bisilla about ten kilometres west of the mountain. There is an easy passage for camels round the east side of the mountain from the head of Wadi Um Bisilla into a feeder of Wadi Abu Nilih, a tributary of Wadi El Khiua. In ascending the mountain I pitched my camp in one of the gullies draining from it to Wadi Um Bisilla, at an altitude of 513 metres above sea and 1,300 metres south-east of the beacon. The climb of 311 metres from this camp to the summit presented no difficulties, and occupied only an hour and a half.
Erf el Gimal is a high dark ridge surrounded by lower hills ten kilometres west-south-west of Gebel Um Bisilla. Its highest point, the west end of the ridge, is 673 metres above sea. About seven kilometres further west is a conspicuous conical hill called Gebel Abu Shigelat.
Gebel Zergat Naam, sometimes called Hagar el Fil, is a mountain mass on the main watershed thirty kilometres west of Gebel Um Bisilla. It forms a curved range round the head of the Wadi el Kreim, a tributary of Garara. Its northern part is drained partly by the Wadi Abu Had, a feeder of the Rod el Kharuf, and partly by the heads of the Wadi el Fil, which is itself the head of Wadi Naam. The highest peak of the range is a conical one near its south end, 845 metres above sea-level, but the northern peaks are more conspicuous from some points of view, and it is on the highest of these, 823 metres above sea, in latitude 23° 45′ 28″, longitude 34° 40′ 34″, that the triangulation beacon has been erected. My camp at the base was in one of the heads of Wadi el Fil, 527 metres above the sea, and about 700 metres north-east of the beacon. The ascent from here on foot to the beacon was fairly easy. Zergat Naam is a mass of syenite rising through schists, but the northern summits are formed by east and west bands of a very hard quartz felsite. From the summit, looking northwards one sees long lines of north-and-south felsite dykes forming ridges on either side of the head of Rod el Kharuf, while close south of one is the enclosed sloping plain across which run the feeders of Wadi el Kreim. To the south-south-east, a range of lower hills runs north-westward from the end of the main range, and between the two there appears to be a remarkably sudden drop into the head of Wadi Abu Seyal. The Arabs state that there is a very large galt at this point, reached by ascending the Wadi Abu Seyal; the galt is said to hold water for five months after rain. But during my occupation of the station recent rains had filled other smaller rock basins in the gullies to the north, and it was unnecessary to send far from the camp to get supplies. It is interesting to observe from the summit of Zergat Naam the disposition of the rocks forming the lower hills to the south and west; these are of sandstones with marked dips, the area being one of much disturbance by faulting along the Wadi el Kreim.
On the great sandy plain to the north and north-east of Zergat Naam are two extensive groups of low hills, which stand out remarkably owing to their dark colour. One of these groups, twenty kilometres north of Zergat Naam, is called Gebel Geneina Gharbi; it is a mass of hills composed chiefly of dioritic and gabbroid rocks (through there are some schists and granites intermingled) and is drained northward by the Rod el Geneina, a feeder of Wadi Kharit. The other group, called Gebel Geneina Sharqi, is smaller and situated fifteen kilometres further east; I did not visit it, but from its dark colour it is probable that it consists of similar rocks to those above-mentioned. The hills of both groups rise to about 550 metres above the sea, or about eighty to one hundred metres above the plain.
In the low country to the west of Zergat Naam, between the Wadi Garara and the Rod el Kharuf, the most conspicuous hills are Erf el Dubeis, a ridge twenty-six kilometres due west of the beacon, Gebel el Nikeiba, a granite hill marked by a triangulation cairn, 570 metres above sea-level and thirty-four kilometres west-north-west of Zergat Naam; Gebel Felieiti, a sandstone hill, also marked by a triangulation cairn, 500 metres above sea and sixteen kilometres south-west of Gebel Nikeiba; Erf Abu Homur, a ridge at the head of Wadi Abu Homur (a tributary of Garara), about seven kilometres north of Gebel Felieiti; Erf el Mohaib, marked by a triangulation cairn 465 metres above sea-level, about nine kilometres north-west of the last-named hill; and Gebel el Ghar, a hill on the south side of Wadi el Kharit, close to the point where the Rod el Kharuf enters it.
The Abu Dahr — Orga — Um Tenedba Group.
The mountains of this group occupy the tract between the Wadi Naam and Gebel Faraid, and are limited on the south by the Wadi Hodein.
Gebel Abu Dahr, the highest mountain of the group, is a great mass of serpentine, situated in latitude 23° 35′ and about forty kilometres from the coast. Its summit, marked by a triangulation beacon in latitude 23° 36′ 8″, longitude 35° 5′ 46″, is 1,131 metres above sea-level. Its western face is drained by the heads of Wadi Betan, its north-east flanks by those of Wadi Khoda, and its south-east parts by feeders of Wadi Rahaba. Abu Dahr is surrounded by high hills except on the west, where there is low hilly country containing the wells of Betan and the old mining ruins of Um Eleiga (see [Plate IV,] p. 30). The mountain can be most easily reached either by following up the Wadi Betan to its head, or by ascending the Wadi Salib el Azrak (a tributary of Wadi Khoda) and its feeder the Wadi Um Karaba. There is an easy pass 1,800 metres north of the beacon, between the central mass of Abu Dahr and the high hills north of it, connecting the heads of Wadi Betan and Wadi Um Karaba. A short distance to the south-west of this pass there is a galt in a spur of the mountain which holds water for some time after rain, and which furnished supplies during my stay at the place in 1907. In occupying the triangulation station, I fixed my camp in a small stony wadi draining from the north of the mountain, on the east side of the pass just referred to; the camp was 600 metres above sea-level and 1,200 metres north-north-east of the beacon. The ascent, which occupied three hours, was rather difficult and dangerous, the mountain being not only steep[110] but composed of rotten highly crushed rock which came away in tons at a mere touch. There is a very extensive view from the summit, the striking peaks of Faraid being a marked feature to the east and south-east, while to the south are the great masses of Mishbih, Niqrub and Gerf, with the sandstone plateaux of Dif, Hodein and Abraq limiting the view to the south-west. The rock of Gebel Abu Dahr is highly magnetic, and several pieces which I examined showed strong polarity. The compass needle points in all sorts of directions as one moves about the mountain.
Gebel Abu Dahr and the high hills round it, especially to the south, form an important rain-collecting area, the drainage from which feeds, wholly or in part, the wells of Betan, Rahaba, Abu Reye, Abu Beid, and Gahlia, all of which are within a radius of about fifteen kilometres of the summit of the mountain. The relative abundance of water in this area is caused by two main factors. In the first place, the situation of the mountain on the eastward side of the watershed, only about forty kilometres from the coast, and its considerable height, condition the frequent accumulation of clouds about its summit, with relatively high rainfall as a consequence. And in the second place, the steepness of the slopes, and the fact that the serpentine rocks are not only of an impermeable nature, but are so crushed that the downwash into the gullies around is chiefly composed of smooth faced blocks rather than of absorbent sand, condition a higher ratio of run off to rainfall than exists in the case of most other localities in the Eastern Desert. A similar state of things is found in the case of Gebel Gerf, which, as will be mentioned further on, likewise consists of serpentine and is the centre of a district relatively rich in good wells; while in the case of the mountains of Faraid, which are much higher and closer to the sea, but consist of granite weathering into sand, though the rainfall is probably greater, the absorption by the sand is so rapid that there are no wells known to exist in their neighbourhood.
Between Gebel Abu Dahr and Gebel Um Bisilla there are three remarkable little groups of granite hills. Gebel Abu Arta is a small range of red peaks six kilometres south of Um Bisilla; Gebel Hendusi, five kilometres further south, is a very conspicuous sharp granite peak rising to 678 metres above the sea and forming a good landmark; while Gebel Abu Husenat, four kilometres east of Hendusi, has another well-marked peak rising to 725 metres above sea-level.