74 Dr. W. F. Hume — On Walthers Desert Erosion. 



umber are dominant, and even where vegetation is present the tints 

 of green pass insensibly into delicate shades of grey. Only at sunset 

 is the stern scene for a short period replaced by the riotous colouring 

 of a brilliant fairy -land, then with the fall of night the pall of a great 

 desolation is spread over the wild and inhospitable waste. 



This year as we steamed down the Gulf of Suez, past the granitic 

 hills of Gebel Zeit, there stood by me three earnest geological students 

 on their way to study the intricacies of these desert regions. In front 

 of those rugged ranges were low foothills of dark and forbidden aspect. 

 "And what are those?" askeil my companions, as they gazed on 

 their sombre outlines, recalling the outpourings of some destructive 

 volcano. As I replied "Raised coral-reefs " a silence fell, and it was 

 evident that the questioners found it difficult to believe the statement. 

 Later on, at Abu Mingar, one of them was deeply impressed when he 

 found these strangely scoriaceous masses filled with fossils of the most 

 pronounced character, and realized that the blackened surface was 

 but a thin crust on a limestone which had been eroded by sand and 

 chemical agencies. 



Professor Walther distinguishes three types of desert films. The 

 first is the dust film, due to the fine dust driven with great force 

 against the rocks. This forms a thin covering, giving a yellow tint 

 to many of the snow-white limestones of Egypt, this being a case 

 therefore where the colouring of the stone covaQs, from without. 



Most striking is the second group of cataract films, beautifully 

 displayed in the cataract regions, where a brilliantly polished crust 

 of graphitic appearance spreads over igneous rocks of every shade and 

 type. Taking as his basis the well-known memoir by Lucas (misspelt 

 Lukas in the text) on the " Blackened llocks" of the Mle Cataracts, 

 he points out that the materials i-ecorded (much manganese dioxide, 

 ferric oxide, lime, silica, magnesia, with traces of potash, sulphuric 

 and phosphoric acids) are such as would occur in the rocks themselves, 

 which ai-e permeated with water at high Nile. The solutions so 

 formed are subsequently evaporated during the low-water period, 

 with deposition of the dissolved substances as a thin surface film. 



In Professor Walther's discussion of this subject he has assumed 

 that the crusts are only formed on igneoiis rocks. This is not, 

 however, the case, for at Ragama, near Kom Ombo, some distance 

 north of Aswan, a white chalky limestone is blackened to a remarkable 

 extent. In this case it seems difficult to imagine that the film is due 

 to evaporation of internal solutions, and the question cannot yet be 

 considered as finally settled. 



The third type, the brown protective film, is one of the most 

 characteristic features in desert regions. Its formation is frequently 

 capricious. Fossils in rocks often stand out, intensely blackened, 

 from a light-tinted limestone, and siliceous limestones display the 

 darkening better than finer varieties. The crust is purely a surface 

 one, and only a millimetre thick. It often gives a blood-red or 

 yellow streak, indicating iron oxide where the colouring is yellow or 

 grey ; hydrated oxides of manganese and iron are dominant. The 

 brown crust marks the beginning of the change, the red one is the 

 same after loss of water, the addition of moisture restoring the yellow 



