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



in broad slieets over the wliole surface. As a result, delta-like 

 deposits are produced at the mouths of the valleys, the Cooper Creek 

 delta in Australia, originally described by Eyre, being given as an 

 example of such a feature, which far exceeds in extent the great river 

 deltas of the Nile and the Danube. 



Section 17 is devoted to Deflation, or the activity of wind as 

 a transporting agent of the light materials formed by erosion or 

 composing the softer rocks. This agency Professor "Walther considers 

 to be the most important of those acting in desert regions. 



An illustration is given of the beginning of a dust-storm on an 

 Australian farm, but equally good examples are available from the 

 Sudan, very fine photographs having been taken of such phenomena 

 at Khartoum. Dust is carried for liundreds of metres into the air, 

 a sand-storm when viewed frqm the summit of a hill above its 

 influence being an impressive and awe-inspiring spectacle. Personally 

 I entirely agree with Professor Walther as to the importance of 

 deflation in the Libyan Desert ; in the Eastern Desert of Egypt water 

 action is probably as effective a factor. 



One of the noticeable effects of sand-laden winds is the formation 

 of hollows (often arranged in network patterns) in the faces of 

 sandstone cliffs or on the sides of granitic outliers. Walther, 

 following Eutterer, maintains that it is a mistake to ascribe their 

 genesis to the action of sand blown by wind only ; in the 

 majority of instances the cavities have been formed by the effects of 

 insolation on the solutions in the rock, the weathered material due 

 to the action being then carried away by the wind. In this way 

 granitic blocks may be completely eaten through, giving rise to arched 

 cavities. If these processes be continued the rocks are eventually 

 worn down to isolated blocks of remarkable shapes, in which the 

 ' mushroom ' form predominates. 



Sections 18-20 deal with the character of the desert surface 

 resulting from the sweeping away of all the lighter materials by 

 the wind. A series of illustrations exhibit the character of the great 

 plains in the Western Desert and Ethiopian region due to the removal 

 of the finer particles where limestone and conglomerates are pre- 

 dominant; in the sandstone regions a vast expanse is produced, out 

 of which rise isolated tabular outliers, sometimes protected by a hard 

 ferruginous cap. 



In Section 20 Professor Walther establishes five diverse regions 

 as regards the forms produced by denudation and their geological 

 history. 



1. The Serir, in which strata of sand and flint pebbles or boulders 

 have formerly spread over the surface. From these beds the sand 

 has been removed, leaving a waste of large fragments embedded in 

 the soft material which still remains. This flint desert gives rise to 

 gentle undulations in which no conspicuous features are developed, 

 and is characteristically displayed to the west of the Pyramids. 



2. The Hamada is a region where hard limestone bands alternate 

 with softer bands of marl and clay. The limestone beds form hard 

 surfaces, which, remaining after the removal of the softer strata, give 

 rise to broad and desolate plains extending unbroken in every 



