Dr. W. F. Hume — On Walthers Desert Erosion. 77 



direction. Change to a higher limestone stratum is marked by a low 

 step only a few metres in height, leading to another extensive plateau 

 in whicli the hamada character is faithfully reproduced. Professor 

 Walther could have carried his description further, for these surfaces 

 vary according to the nature of the limestone stratum. Sometimes 

 the desert floor is entirely composed of fossil remains, ovsters or 

 nummulites, at others of sun-warped sub-crystalline limestone scoured 

 deep and even cut into long hummocks by the wind-borne sand. 



3. The third place is given to the Wadis or Dry Desert Yalleys, 

 special attention being called by picture and description to their 

 precipitous terminations, often in great amphitheatres. In the 

 formation of these deep ravines, erosion by water and deflation are 

 regarded as both liaving taken part, the mighty effects of one day of 

 rain-storm being followed by the activities of the wind during long 

 periods of dryness. Here main channel and tributary have little 

 meaning, and valley slope is often difficult to determine, except in 

 the major watercourses of the country. Narrow ledges often separate 

 two branches until some great rainfall sweeps them away. If these 

 sudden torrents are so effective in the dry sun-cracked wilderness, 

 the greater streams, such as the Colorado and the Nile, must be more 

 intensely destructive, and erosion works on a scale to which no 

 parallel exists in more temperate climes. 



4. From the valleys, eating backward into the desert plateaux, 

 there is but a short step to the Outliers, which, first extending as 

 bold and precipitous projections from the main cliff face (Zungenberge), 

 (see fig. 120, p. 205), finally become separated, forming isolated hills. 

 In this section it is necessary to point out that a factor works of 

 which, if I mistake not, Professor Walther has taken no notice. 



In the Lower Eocene series huge masses of limestone rest on clays. 

 The rain falling on the plateau surface percolates through the lime- 

 stone and forms a water-cushion over the impermeable beds beneath, 

 upon which the heavy superincumbent strata tend to slide. This 

 geological factor is of the highest importance in a wide region of 

 Upper Egypt. 



5. Naturally the great depressions of the Oases attract special 

 notice, and Professor Walther holds that after weighing all the facts 

 there is only one conclusion possible, viz. that the whole of the mass 

 of material removed from these areas has been carried away by the 

 wind. This seems to me to assign far too important a role to 

 deflation, and to ignore the vast amount of marine erosion which 

 must have taken place during the elevation above sea-level of the 

 strata deposited during Eocene and Cretaceous times. 



6. The Nubian Sandstone scenery is mentioned in which pj-ramidal 

 and tabular outliers rise out of vast plains with compact floors 

 composed of the coarse grains not carried away by the wind. In the 

 Geological Survey memoirs it has repeatedly been pointed out that 

 many of the longitudinal valleys in the Eastern Desert of Egypt owe 

 their origin to the presence of the Nubian Sandstone, and result from 

 its easy denudation, while further south this formation is traversed 

 by fine ravines where the transverse valleys draining from the Red 

 Sea Hills cross it on their way to the Nile. 



