J. MILNE ON THE SINAITIC PENINSULA AND N.W. AKABIA. 15 



sand and small pebbles, and, as compared with the higher walls 

 made up of sand, stones like cocoa-nuts, and large boulders, are 

 of a noticeably fine material — the former looking like a face of 

 Roman cement, and the others like a conglomerate. 



The most striking point, however, about these walls, especially 

 in those about 6 or 10 feet in height, is the almost perfect and 

 unbroken square edge they form with the plain from which they 

 descend, these clear edges being in lengths varying from a few yards 

 up to 100 yards. Comparing the various walls together, it is seen 

 that these several characters depend upon the fineness or coarseness 

 of the materials of which they are composed ; and it may be generally 

 stated that their length, their fine finish, and the squareness of 

 edge they form with the upper plain, vaiw inversely with their 

 coarseness, whilst their height varies directly; the coarser the 

 material, the higher the wall. In taking a section transversely 

 to the length of one of these wadies, we may obtain a step-like 

 outline descending from the mountains on either side ; but more 

 generally the form obtained is that of two rapid slopes from the 

 hills, each terminating in a wall, leaving between them the level 

 central part of the wady, described as being in some respects analo- 

 gous to a river-channel. This central channel, in which the bould- 

 ers, which are often of great size, are found lying in heaps and 

 lines parallel to the bounding walls, may vary from 50 to 200 yards 

 in width. From the same characters being often seen in opposite 

 walls, it is probable that before an initial slope was formed, down 

 which water and materials in general would tend to travel, they were 

 joined from side to side. 



Their growth into the truly perpendicular forms which they now 

 present, evidently arises from the materials of which they are 

 built up being so regularly disposed that there is nothing left 

 to produce unequal disintegration ; that is to say, a disintegration 

 commenced at any one point is at once or very rapidly carried in 

 a perpendicular direction equally over the plain in which the 

 commencement of the disintegration took place, the materials 

 being so loosely placed together that for support they are mutually 

 dependent ; take one particle away and its neighbour falls. This 

 cliff-formation is strikingly seen in the lower and more common of 

 these walls, which are made up of pebbles, grit, and sand. On 

 attempting to walk within a foot of the upper edge of one of these, 

 a vertical layer separates from the top of the wall and falls to form 

 a conical heap below, which is afterwards removed by wind and 

 water. In nature, however, instead of an external pressure 

 acting on the upper surface, a similar result is produced by the 

 action of the little water which occasionally trickles down these 

 wadies, and still more by the almost continuous working of a 

 sand-drift along the lower portion of the face of these walls, 

 by which they are slightly undermined. When sufficiently under- 

 mined in this way (seldom more than 6 inches), the unsupported 

 material above, having little or no lateral attachment to the 

 contiguous mass, of necessity falls. After a little rain this action is 



