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



YI. — Professor Walther's Erosion in the Desert considered. 



By W. F. HuiME, D.Sc, F.E.S.E., F.G.S., 



Director of the Geological Survey of Egypt. 



IN the Cairo Scientific JournaP the first chapter of Professor 

 Walther's new edition of Das Gesetz der Wustenhildung was 

 considered in some detail. His second chapter, "Die Abtragung in 

 der Wiiste" (pp. 111-218), is no less stimulating to thought and 

 replete in observation. He opens this section of liis work with 

 a statement of the facts which point to great climatic stability in 

 Egypt during historical times. Two of these are specially mentioned. 

 First, the presence of a small sack of salt in a grave of the Eighteenth 

 Dynasty, the analysis of which agreed absolutely with that of the 

 salt at present obtained from the lakes of the Wadi Natrun. This 

 occurrence, according to hira, proves that the same salt mixtures 

 were being formed at periods 3,500 years apart. A statement as to 

 the nature of the resemblances which lead to so wide a conclusion 

 would have been of interest. Secondly, attention is also called to the 

 total absence of water in the Tombs of the Kings since they were first 

 excavated, pointing to the underground water having stood for many 

 thousands of years at more than 50 meti'es below present valley level. 



No doubt these conditions indicate climatic similarity, but other 

 arguments, such as the opening of routes across the desert in early 

 times, have been adduced in favour of a less marked aridity than at 

 present exists. The question is one calling for much wider treatment 

 than has yet been given it, but it is as well to have every thoughtful 

 observation recorded as material for further discussion. The uniform 

 character of the Nile alluvium, and the great development of the 

 longitudinal dunes in the Libyan desert, certainly point to very little 

 climatic modification during the historical period. 



A long section is devoted to the erosion of Egyptian monuments by 

 sand-blast, and as it is illustrated by photographs from the neighbour- 

 hood of the Pyramids it is of considerable interest to those dwelling 

 in this country. So also are the pages in which Professor Walther 

 regards the destruction of many of the monuments in Karnak and 

 elsewhere as due to weathering through temperature variations, and 

 not, as is commonly assumed, to earthquakes. The Memnon statues, 

 for instance, built as they are of Nubian Sandstone cemented by 

 siliceous material, and based on the Nile alluvium, are not readily 

 liable to fracture by earth-movements, while they have been subject 

 for many centuries to the great variations of temperature (insolation) 

 by day and by night. The musical notes of the Memnon, which 

 lasted for some two hundred years, are regarded by Professor AValther 

 as due to the wind traversing one of these fractured portions, the 

 sounds ceasing as soon as the statues were walled in by Septimus 

 Severus. 



Section 14 deals with " Dry "Weathering " (Trockene Verwitterung). 

 Attention is called to the very important activity of evaporation in 

 the desert areas, by which many solutions present in the upper 

 portions of the crust are being continually diawn to the surface, and 



^ Cairo Scientific Journal, No. 77, vol. vii, pp. 32-41, February, 1913. 



