Hornfels.
Associated with the schists of Gebel Abu Hamamid and the neighbouring mountains there are great masses of very hard horny-looking rock [10,401] of green to grey colour, breaking with a sub-conchoidal fracture, and of such close texture as to appear homogeneous even with a strong lens. The pyramidal peak called Gebel Um Semiuki, which rises to 1,282 metres above sea, three kilometres to the north-east of Gebel Abu Hamamid, is almost entirely composed of rocks of this type; in the mountain faces the rock looks red, but this is only due to a film covering weathered surfaces, the interior being of a green to grey colour. The rock, which has a sp. gr. of 2·71, is frequently beautifully banded, light and dark layers alternating with each other, and often contains tiny cubes of pyrites [10,399]. The microscopic slide from Gebel Um Semiuki shows a very fine-grained clouded compact rock, apparently consisting of glassy matter with minute granules of quartz and altered felspar, together with a little sericite, the latter especially along certain bands. The slide from Gebel Abu Hamamid is similar, but here the granules of quartz and felspar are a little larger, though they are still too small to be seen with a lens in the hand specimen; the appearance is that of a quartz felsite on a small scale. It has already been mentioned ([p. 281]) that the quartz felsites of Gebels Igli and Hadarba pass gradually into hornfels, and when we remember that the schists of the Abu Hamamid district are mostly crushed volcanic rocks, it becomes almost certain that the hornfels associated with them is a crushed and devitrified glassy lava of acid composition.
A yellowish horny rock with grey streaks [10,379], which occurs near Gebel Sabahia, is conspicuous in the field owing to its weathered surfaces being covered with a bright red ferruginous skin, resembling cinnabar in colour. The sp. gr. is 2·52. Examination with a lens shows the grey streaks to be filled with myriads of brilliant yellow specks of pyrites. The microscopic slide shows these to be aggregates of little cubes, while the bulk of the rock is a schistose felsitic mass of quartz and felspar, with scattered larger felspar crystals, much broken up and bent. In this rock too we have therefore a rolled up and altered felsite.