=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.