|Publication Date||June 21, 2012|
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The writer was led in the first instance to take up the study of this subject by having his attention drawn to some specimens of crystalU ne rock in the Cairo Geological Museum, which showed a peculiar black polished film on the outer surface. Of the specimens one was a coarse-grained red granite Q) from the First, or A swan, Cataract of theN ile, and the others were a fine-grained red granite (),.a basalt Q) and a dolerite () respectively, all from theS econd Cataract, above Wadi-H aifa. Two visits were subsequently made to A swan at a time of the year () when the river down-stream of the new dam was at its lowest level and when the rocks in the river channel were most exposed. Under these conditions the intense jet-black colour of the rocks forms one of the most noticeable features of the cataract area. The smaller rocks are entirely black, shine brilliantly, and have the general appearance of masses of coal or blocks of pitch, while the larger rocks show a broad band of polished black immediately above the water level. This remarkable phenomenon has been commented on by many different writers and the following references may be quoted :T he granite rocks at the Cataract of A swan have a very thin, dark-black, shining surface-coating, which gives them the appearance of having been pitched. () Les rochers qui sont baignes une partie de I annee par les eaux duN il sont quelquefois revtus kleur exterieur dune espece denduit noir lisse et brillant. () Lorsqu on parcourt en barque les mille meandres de ces canaux profonds on est tres frappe. de la couleur dun noire intense, ressemblant kun vernis que presentent ces rocfccrs toujours immerges pendant (1) Collected byS ir William Willcocks. OC ollected by Dr. W. P. Hume, Egyptian Geological Survey. (0 A pril 1904 and March 1905. () BussBGGBE quoted by Walther in Die Denudation in der Wuste. Leipzig, 1891, p. 153.
(Typographical errors above are due to OCR software and don't occur in the book.)