62 GEOLOGY OF ARRAX. 



dikes. The horizontally prismatic structure of these sub- 

 jects them to a more rapid decay than even the friable 

 coarse granite ; and hence most of the deep notches of this 

 jagged ridge mark the situations of whin dikes. The fact is 

 curious and interesting, and has not been before noticed. 

 Ranging up the front of the precipice from Glen Rosa, these 

 dikes cut right through the crest of the ridge, and pass down 

 towards the fine-grained granite, which occupies most of the 

 lorsa valley, and rises up on the back of the Beu-Ghnuis 

 range and the Ceims as far as the level of many of the cols 

 that is, the lowest parts of the ridges between the glens. 

 There can be no doubt, from the direction in which these 

 and the other dikes already mentioned range, that they are 

 the same as those which are found crossing the fine-grained 

 granite tract on the west side of the lorsa valley, and will be 

 mentioned again. 



30. The jagged and notched character of the ridge makes 

 it impossible for us to pass along it on our way to Cior- 

 Mhor ; neither can we safely cross the huge granite slabs at 

 the back of the ridge, as they are smooth, slippery, and con- 

 siderably inclined. But below these a safe, though rough 

 and irregular, pathway will be found ; and from this we can 

 occasionally pass upwards towards the ridge, along the clefts 

 in which the dikes lie, to have a peep down into Glen Rosa, or 

 to scan the frowning cliffs on the north side of Ben-Tarsuin. 

 For the latter purpose a telescope will be useful. We must 

 be careful, however, to return by the cleft by which we came 

 up, till we reach the path. The granite sheets must not be 

 attempted, as a single false step upon them might hurl the 

 climber with fearful velocity into the valley at their base, 

 filled with blocks and debris. The range, width, and struc- 

 ture of the dikes are well seen as we pass along ; the width 

 is various, in some five or six feet only, in others ten to 

 twelve, and fifteen to eighteen feet. The broadest, being of 

 this latter width, is one on the south side of the col, between 

 Glen Rosa and Glen lorsa. It consists of a crumbling green- 



