Chlorite-schists.
Though many of the decomposed hornblende-schists contain more or less chlorite, I have only in two localities come across rocks in situ which contain so large a proportion of this mineral as to deserve the name of chlorite-schists. The first is in the hills of Um el Huetat (latitude 25°), where typical chlorite-schists are mixed with mica, talc, and hornblende schists. The second locality is between Gebels Ras Shait and Nugrus, where the rock [10,388] is remarkable not only in its peculiar appearance but also by its strongly magnetic character. It is a thoroughly schistose rock of a rather pale greyish-green colour and rather silky appearance, with rusty looking spots. The sp. gr. is 2·77. The microscopic slide shows the stone to consist essentially of an aggregate of elongated plates and fibres of low double refraction, which from the hand specimen seem to be chlorite, but in the slide look more like antigorite. Magnetite grains are liberally scattered through the chloritic mass. The rusty spots visible in the hand specimen are translucent foxy red in the slide, in irregular broken forms with well-marked cleavage and nearly straight extinction. They are somewhat doubtfully regarded as deeply iron-stained hornblende. Mixed with the foxy red material are aggregates of granules of a highly refracting but isotropic mineral of deep bottle-green colour (? spinel), and chloritic wisps. There are also some clear colourless grains, resembling apatite in appearance except that they sometimes show well-marked vertical cleavage and high extinction angles; these are possibly a colourless augite, but the double-refraction colours are far lower than is usual with this mineral.
Typical chlorite-schists occur in the Wadi Salib Abiad, and near Gebels Ribdab and Muqsim, in the extreme south-west portion of the region. Hearing, from some wandering Arabs while at Gebel Abu Dahr in February 1907, that prospectors were at work in the Wadi Salib Abiad, I sent a guide to find out who they were and what they were doing; the guide reported that on his arrival they had gone away, but there were some old workings in a green rock of which he brought a sample. I did not get an opportunity of visiting the locality personally, but the specimen brought back by the guide [11,523] is a beautiful apple-green chlorite-schist, with some brownish calcareous-looking bands. The microscopic slide shows some little quartz and talc besides the chlorite, and there are scattered minute highly-refracting rounded grains of a feebly translucent mineral of a reddish-brown colour, probably rutile. A similar rock from the eastern side of Gebel Muqsim has been reported by Mr. Charteris Stewart, who also records a normal chlorite-schist as occurring on the north side of Gebel Ribdab.