J. MILNE ON THE SINAITIC PENINSULA AND N.W. ARABIA. 21 



From the observations made on these dykes at the various lo- 

 calities visited, which in part are confirmed by the specimens collected, 

 it would seem that they may be divided into two classes — those of 

 a red colour, and those of a dark green or black. 



As a general rule the former are the harder of the two, and 

 stand up as ridges which can be seen running up the sides of the 

 mountains and over their crests, or else appearing only as peaks, 

 but in all cases producing serrations ; whilst, on the other hand, the 

 latter are generally soft and form trenches and hollows where the 

 red ones would have formed ridges and peaks. Exceptional cases 

 are to be seen where the black dykes are hard and have resisted 

 degradation ; but in the case of the red ones no exceptions were 

 seen. 



Both classes of these dykes, like the granites they traverse, are 

 highly felspathic, the red ones being generally compact felsites or 

 fine-grained porphyrites, whilst those of a darker colour are gene- 

 rally porphyries in which small crystals of felspar are imbedded in 

 a dark- coloured base. 



Traversing several mountains near to Jebel el Nur, and noticeably 

 one called Jebel Atagtagheer, there are large dykes 12, 14, and 

 even 20 feet in width, almost wholly composed of a soft material ; 

 yet, through having hard exteriors, they stand up so as to form 

 a well-defined wall-like ridge. Through being thus composed of a 

 soft central part or core cased in between two slabs of a harder 

 material, disintegration has acted more rapidly on the interior por- 

 tion than on the exterior, and has cut them out into a trench. 



Up one of these trenches I ascended Mount Atagtagheer (see fig. 1). 

 The dyke was throughout of a dark-green material, but slightly lighter 

 in colour on its sides than in the middle. Its width was about 

 12 feet; 6 feet of the central part was soft and crumbled like 

 dry clay when struck with the sharp edge of a hammer, whilst the 

 3 feet of casing on either side into which it graduated was hard 

 and tough, in fact much more so than the granite through which 

 it pierced. 



The result of examinations of different portions of such dykes as 

 these is given in the following list of rocks from Jebel Atagtagheer, 

 from which it would appear that the interior portions of these dykes 

 are apparently more siliceous, contain more olivine, more magnetite, 

 and are decidedly more calcareous than the exterior portions ; but 

 as these aud other similar specimens are intended to form the sub- 

 ject of a future investigation, the present statement must be re- 

 ceived provisionally. 



Rocks from Jebel Atagtagheer (the first four of these were ex- 

 amined microscopically) : — 



1. Quartziferous dolerite, from the exterior of a dyke, of which No. 2 



is the interior. This is a dense, olive-green-coloured rock, readily 

 scratched by a knife to a light-green streak. 



2. Quartziferous dolerite from the interior of a dyke, of which No. 1 



is the exterior. This is of a reddish colour and more granular than 

 No. 1, from which it also differs in being decidedly calcareous and 

 magnetic, and apparently containing more olivine and quartz. 



