[Illustration: 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. [Illustration: 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.