Dr. W. F. Hume — On Walther's Desert Erosion. 19 



there evaporated with deposition of saline and other dissolved 

 constituents. 



These are practically ' desert evaporation effects ', and in spite of 

 his statement that by the use of the term ' dry ' he does not mean 

 absolute absence of water, the title of this part of the section is 

 distinctly misleading, though applicable to features dealt with later 

 in the chapter. Futterer has given the name ' exsudation ' to these 

 phenomena, which appears preferable. The effects of these activities are 

 manifold and everywhere obvious, and those who desire a careful study 

 of this question in its economic aspects will find valuable details in 

 Mr. Lucas's Survey memoir on the Bisintegration of Building Stones. 



In considering the effects of these evaporating solutions, attention 

 is called to what the author terms ' mass-movements ' in the rocks. 

 According to him (p. 124) there have been upward movements of 

 materials in certain cases, and as an example he mentions one of the 

 stones in a wall on the west side of the Great Pyramid which has 

 ' squeezed out ' some 8 centimetres bej-ond the regular line of blocks 

 of which it forms a part. He also cites cases of smaller fragments 

 found on the surface, obviously splintered from the parent foundations, 

 and now separated from them by several metres of made ground. It 

 would be interesting to have the views of archaeologists as to these 

 anti-gravitational movements suggested for pottery and other objects 

 of human construction embedded in soft muds. 



A small section is devoted to the very beautiful ' rippled limestones ' 

 often found on the surface in the Egyptian and Atacaraa deserts. 

 These were regarded by Abel as being fragments lifted into the air 

 during heavy wind-storms, rotated rapidly, and etched on all sides 

 by streams of blown sand. The size of the blocks seems, however, to 

 exclude any such explanation. One would be extremely sorry to face 

 a sand-storm such as would move the fine specimens now in the 

 Geological Museum of Cairo. Beadnell, who collected these and 

 described them as ' vermicular ', justly called attention to the fact 

 that these structures only occur (at any rate as a rule) in a hard grey 

 crystalline limestone, and he connected their origin with drifting 

 sand. According to him they would appear to depend on the internal 

 structure of the rock rather than on any external conditions, but may 

 perhaps be partly due to the irregular paths taken by sand grains in 

 motion. Professor Walther points out that they occur both on 

 angular and rolled limestone fragments, and these of very varied 

 size, excluding, as above stated, the idea of movement. Where the 

 * liillensteine ' or ripple-stones lie on a clayey basis the ripples are 

 sharp and resemble the solution figures which are produced when 

 limestones are etched with weak acid. Calcite veins in the pebbles 

 are more completely dissolved, and leave sliarply marked furrows 

 with a breadth at the surface of 1 to 4 millimetres. Where, however, 

 sand is spread over the desert floor, or the surfaces indicate previous 

 sand action, the ripples are smoothed down and disappear on the lee 

 side. In digging down they are found with well-emphasized markings 

 to a depth of 10 to 20 centimetres ; in large fragments Professor 

 Walther noted that the continuous ripples beneath the surface were 

 clearly marked, but above it were smoothed, rounded, and polished. 



