Alteration Products of Serpentine. Magnesite and Asbestos.
In many of the serpentine-masses the effects of solution and pressure can be traced in the formation of veins and pockets of magnesite, asbestos, and occasionally talc in the rock. Magnesite veinlets are frequently seen, for instance, round about Bir Meneiga [12,142] and Bir Muqur, while asbestos has been found in pieces of several kilogrammes weight in the Wadi Sherefa el Sherqi [12,122] and near Bir Muqur [12,153]. At the last-named locality it occurs along the crush-planes of the serpentine, and appears to be preceded by the formation of long fibres of a green substance.[133] Where thickest, the glaze on the slickensided surfaces of the serpentine fragments seems to consist of this same green substance, which splits into hard fibres often reaching ten centimetres or more in length and only a few millimetres in diameter [12,153 A] and these hard fibres, which have a sp. gr. of 2·64, break up gradually, apparently under weathering influences, into the much finer and more flexible fibres of white asbestos. The quality of the asbestos is, however, inferior, for a specimen of the softest kind sent to London for an expert opinion, was reported to be of no commercial value; even had it been otherwise, the quantity seen was too small to justify working, though of course a thorough search among the mountains might reveal the presence of bigger deposits than were encountered during the survey.
[125]From Marwa, the Arabic term for quartz.
[126]The numbers in square brackets in this and the following chapter are the registration numbers of the specimens in the Geological Museum, Cairo.
[127]I may here recall that the quartz mass which constitutes the so-called “alabaster” quarry north of Aswân, and for which I have inferred an igneous origin, is likewise closely associated with aplitic or pegmatitic intrusions. See A Description of the First or Aswân Cataract. Cairo, 1907, p. 84.
[128]This specimen was brought back by a guide sent for the purpose to Gebel Adar Qaqa. It is stated to be the main rock of the mountain, but as I did not visit the mountain I am unable to vouch personally for this being the case.
[129]From a specimen brought back by the guide who was sent to erect the triangulation beacon on Gebel Mishbih.
[130]I did not visit Gebel Shendib, but as seen from Gebel Elba the mountain appeared to be granite; the felsite probably forms a dyke passing through the summit.
[131]There is another Gebel el Anbat near the Wadi Hodein, but this latter consists of schists.
[132]Preliminary Report on the Geology of the Eastern Desert of Egypt, between latitude 22° N. and 25° N., by Dr. W. F. Hume. Cairo, 1907. p. 57.