16 J. MILNE ON THE SINAITIC PENINSULA AND N.W. ARABIA. 



strikingly rapid, the slight bond between the particles being loos- 

 ened by the soaking-in of the water. 



As these walls are cut further back and approach the hills, the 

 mass of material in which they are formed being thicker, they are 

 naturally higher, in addition to which it may be noted that they are 

 also coarser and have lost much of their smooth finish, which latter 

 character is apparently due to the larger masses of which they are 

 built up having more hold upon each other, one of them not moving 

 without disturbing its neighbour. 



Had the materials of which these walls are built been inter- 

 laminated or cemented in any way, no portion of it could have given 

 way without disturbing that which was contiguous to it, by acting 

 on it as a cantilever. 



This may be looked at generally by considering cliffs or walls 

 the component parts of which are so arranged that their greatest 

 length lies in a horizontal direction. In such walls, where we get 

 this horizontal interlamination, whether of massive bands of rock, 

 fissile shales, or only layers of stone, on their being undermined, 

 generally speaking, no portion of them can give way without dis- 

 turbing those parts with which they are in contact, especially those 

 lying above, which, cantilever-like, they tend to prize upwards and 

 then cause to fall outwards, this outward tendency being aided by the 

 material from above slipping down over that which has fallen from 

 below. The result of this is the production of a slope, instead of a 

 clear perpendicular wall, such as is produced by the direct fall of an 

 uncemented fine material. 



The unbroken edges of these cliffs, although in part due to the 

 nature and arrangement of the material of which they are formed, 

 are also in part due to a cause similar to that assigned for the 

 unworn edges of some of the American canons, namely the 

 comparative absence of rain — the little that does fall being 

 hardly sufficient to affect those of coarse material, whilst those 

 made of fine material are immediately soaked, and the under- 

 mined portions at once fall instead of remaining to be channelled 

 down with gutters. 



It has been observed that the great heaps and long lines of 

 boulders so often seen in the centre and other parts of these wadies 

 can hardly be thought to have assumed their rounded forms and to 

 have come into their present positions by the agency of water 

 (which at first sight is so suggestive both as a motive power and 

 also as a polishing agent), the district being riverless and also, com- 

 patively speaking, rainless. 



The reason of their waterworn appearance is apparently in great 

 part due to the cutting effect of an almost perpetual sand-blast ; 

 but the cause of the central position they so commonly occupy is 

 not so obvious. It may have been acquired by their having simply 

 rolled down the sides of the mountains when they extended further 

 into the wadies than they do at present; but in many cases it is 

 probable that the descent was far more gradual. Whilst riding along 

 the base of some of the cliffs of sand and conglomerate just de- 



