=Gebel Um Harba= is a conspicuous sandstone peak marked by a triangulation beacon in latitude 23° 36′ 56″, longitude 34° 30′ 38″. Rising to a height of 688 metres above sea-level, it forms the highest and central point of a long range running north-north-west and south-south-east from the Wadi el Garara past the Galt el Aguz. The north end of this range is forked, the east and west arms being separated by a broad high valley, filled with blown sand, draining from near the peak of Gebel Um Harba northwards to Garara. A marked feature of the range is the almost constant north-easterly dip of the sandstone beds of which it is composed; the ridges follow the strike of the beds, and present gentle dip slopes to the east with steep basset edges to the west. Gebel Um Harba overlooks to the west a great sandy plain, so that it is a good landmark for travellers coming from that direction. In occupying it as a station, I approached it by the broad sandy valley above-mentioned, and fixed the camp at 487 metres above sea-level and 800 metres north of the summit; the hill can also be reached from the west, though the climb from the plain is about one hundred metres more. The ascent becomes rather steep near the top of the hill, but presents no great difficulty. For a short time after rain, water can be found in pools in the gullies close to the east of the hill, but owing to the porosity of the rock these never last long. The Galt el Aguz, nine and a half kilometres south-south-east of Gebel Um Harba, in a gully on the east side of the road to Bir Abu Saafa, lasts longer than other pools in the neighbourhood, but even it only yields supplies within a few months at most after rain, for the same reason. =Gebel Um Sididad= is a mass of high sandstone hills on the north side of Wadi Silsila, thirty kilometres east-south-east of Gebel Um Harba and overlooking the broad and sandy Wadi Naam to the east. The hill I ascended, one of the highest, was 623 metres above sea-level and nearly 300 metres above the plain at its foot. High sandstone hills extend north-westwards from Gebel Um Sididad to Gebel Zergat Naam, forming the western side of Wadi Naam.