J. MILNE ON THE SINAITIC PENINSULA AND N.W. AKABIA. 17 



scribed, on looking up, long lines of boulders were often seen waiting 

 to be undermined and to iall below. Many could be seen that had 

 fallen, whilst others were barely balanced and ready to topple over 

 on the least disturbance. 



Each time one of these falls it travels a certain distance forwards ; 

 and as cliffs are continually being formed in the centre of the wady 

 to work back towards the hills, steps are continually approaching 

 these boulders, down which they may roll and approach the central 

 line of cliff-formation, where those from one side of the valley meet, 

 stop, and accumulate with those coming from the opposite side. 



Such modes of transit as these may be suggestive in accounting 

 for the presence of erratic blocks so often seen not only in various 

 parts of Arabia, but also in other countries, as, for example, in 

 Persia, where they have been seen to have travelled distances of 

 five and six miles — in certain cases, perhaps, giving a clue to those 

 phenomena which otherwise might have found a satisfactory solu- 

 tion either in a coat of glaciers or a sea of icebergs. 



In the cases quoted large blocks have apparently travelled dis- 

 tances of a quarter of a mile by the breaking down of about a hun- 

 dred feet of modern alluvium. How far, it may be asked, would 

 blocks have travelled had the strata measured thousands instead of 

 hundreds of feet ? 



With regard, therefore, to the general appearance of the beds of 

 these mountain wadies, it may be briefly stated, in conclusion, that 

 their characters are, in the main, rather due to a stream of sand 

 than to water ; small furrows formed in the central parts of the 

 wady retreat towards the hills by being undermined and then 

 falling by their weight. By this falling, boulders, often 20 feet 

 in diameter, are rolled forward, and strewn across the plain from 

 the hills towards a central line in which they accumulate. Whilst 

 all this is going on, an almost continuous draft of air up or down 

 these funnel-like denies is in operation, carrying sand to polish the 

 scattered debris, thus helping in the production of appearances not 

 unlike those of some ancient river-bed, in which action it is aided 

 by a slight trickling of water after the winter showers. 



Sand-blast. — Having spoken of the movement of sand as an agent 

 in the undermining of cliffs and the polishing of rocks, although, 

 perhaps, often before observed by others, I may here mention what 

 was seen of its other effects in these districts. 



A great portion of the country lying between Nackhl and Suez is 

 covered with a thick superficial deposit of fine reddish sand, which, 

 like all other sand, is set in motion whenever there is the slightest 

 movement in the air. 



This, although an almost perpetual action, is only to be seen under 

 very favourable circumstances. By placing yourself so that the sand- 

 bank, or piece of ground you are observing, is between yourself and 

 the sun, a slight smoke-like vapour, which from other positions would 

 be invisible, is to be seen sweeping over the surface of the ground. 

 The presence of this drift may also be recognized by placing the face 

 within 10 or 12 inches of the ground, when fine particles of sand 



Q. J. G. S. No. 121. c 



