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.