(e) Hornblende-picrites, containing olivine and hornblende.
(f) Mica-peridotites, containing olivine and biotite.
Most if not all of these classes are represented in South-Eastern Egypt, but in the altered form of serpentine. The change to serpentine has been so complete that it is now hardly possible to extract even a small specimen of the primitive rock; indeed, careful search is often required to obtain even specimens containing any unaltered mineral whatever, and one has frequently to rely on the structure of the serpentine for the identification of the rocks from which it originated.
Serpentine.
Serpentines cover about 400 square kilometres of South-Eastern Egypt, forming the principal rock of several remarkable mountain groups and also occurring in lower hill country.
The largest occurrence is that of the great mountain mass of Gebel Gerf, where serpentine with alternations of gabbroid and dioritic rocks can be followed from Bir Meneiga southward for some thirty kilometres to beyond the Sherefa pass, while the breadth of the tract from east to west is some fifteen kilometres or more at its widest part; this tract includes a vast assemblage of high peaks and ridges, towering up in many cases to more than a kilometre above sea-level. To the north, south, and east, the serpentine is bounded by gabbros and diorites, while on the west a tract of schistose rocks separates it from another great serpentine mass which forms Gebel Korabkansi.
Other remarkable serpentine masses are Gebel Abu Dahr and the upper part of Gebel Sikait; serpentines also enter into the composition of Gebels Ghadir and Um Tenedba, while lower hills formed of similar rocks occur near Bir Murra in Wadi Shait, in Wadi Um Khariga, near Gebel Kalalat, and on the plain east of Abraq Springs.
The foxy red colour of exposed faces of serpentine hills, and the generally shattered nature of the rock, have already been referred to. When one approaches the masses closely, the red colour often becomes less marked, because the fresh debris flanking the hills is of a darker aspect. The slopes of serpentine hills are usually steep, and this combined with the rotten nature of the rock renders their climbing not always quite free from danger.
Any attempt to map out in the field the precise limits of the different peridotites which were the parents of the serpentine ends in failure, partly because the constituent minerals can generally only be identified on microscopic examination, and partly because the different peridotites pass gradually one into another, and are evidently only produced by slight variations in the composition or conditions of consolidation of a single magma. Even the limits of the serpentine itself are not always very clear; for where the associated rock is gabbro, as for instance to the north of Gebel Gerf, there is a gradual passage through more or less serpentinised olivine gabbro to the true serpentine. Golden yellow veinlets of fibrous chrysotile can be seen running through the rock at many places, and occasionally veins and pockets of magnesite and an inferior kind of asbestos occur (see [p. 330]).
The serpentines are nearly always more or less magnetic, and sometimes show strong polarity. The compass was disturbed by 40° at the triangulation station on Gebel Sikait; the amount of disturbance changes greatly when one moves even a short distance, and all estimations of direction from compass readings in serpentine country are therefore liable to enormous error.