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



^ inch in thickness and having a glistening crystalline surface, 

 which fill up joints in the rock. All the rock containing this 

 carbonate of lime (not only the dykes but also the granite) crumbles 

 under the hammer like a dry clay, whilst at the distance of a yard 

 from the dykes, where this carbonate of lime does not exist, the 

 stone is hard and compact, and when struck gives a sharp clear ring. 



Roclcs from AJcaba. 

 (The first three of these were examined microscopically.) 



1. Dolerite, large-grained, containing some acicular crystals, which are probably 



apatite. This is an even-grained compact rock of a reddish-grey colour. 



2. Dolerite similar to No. 1 ; but the felspar is more degraded, and the rock 



itself of rather a darker colour. 



3. Syenite with altered hornblende, orthoclase, a little triclinic felspar, mica and 



quartz. In general appearance the rock is very like Nos. 1 and 2. 



4. Granite, of a pinkish- white colour, and with a scarcity of mica. From the 



vicinity of a doleritic dyke. 



5. Granite, consisting of white and pink felspar, mica, and quartz. 



6. Granite, with chlorite, and fissures filled with crystalline calcite. 



7. Granite with more chlorite than No. 6. 



From Akaba our journey eastwards was confined to Wady Ithm 

 and the various wadies and plains which branch out of it. 



General appearance of Mountain- Wadies. — These wadies winding 

 in and out between the granite hills, may be described as narrow 

 defiles of great length. They vary in width from 100 yards to half 

 a mile, and wind in and out between almost perpendicular walls of 

 granite, making the approach to every turn or bend in their course 

 appear as if it were a terminus. 



Under foot are large boulders, stones of various sizes, small 

 pebbles and sand, giving the place the appearance of a dried-up 

 channel, which formed the bed of some large and rapid river. On 

 inquiry it was found that no body of water ever flowed down these 

 defiles — a fact that might have been anticipated by observing that 

 the beds of grit and sand were cut through by small channels not 

 6 inches in depth, instead of being left, as would have been the case 

 in a river, in one flat stone-covered surface. Whilst amongst these 

 mountains, I experienced three days of continuous rain, after which 

 I did not see anywhere more than the faintest trickling of water — 

 from which fact, in conjunction with others, I think we may conclude 

 that in these wadies there are conditions very analogous to those of 

 river-beds, but that in their formation water has played but little 

 part. 



Another striking phenomenon of these wadies is the presence 

 of perfectly perpendicular walls of debris, which often form boun- 

 daries upon both right and left. 



These walls vary considerably in their height ; sometimes they 

 are only 1 or 2 feet in height, but generally from 6 to 10 feet, 

 whilst in many places, by actual measurement, they were from 

 30 to 60 feet, and occasionally even still higher. The lower ones 

 (which are more generally met with) are formed of greyish gritty 



