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Glen Dhu, and composed of quartziferous porphyry. On its south- 

 west base, the granite first appears, separated, in some places at least, 

 from the porphyry by a narrow band of altered sandstone. Thence, 

 keeping at a high level, it extends along the hill slopes at the head 

 of Glen Dhu, and terminates southwards against a ledge or low 

 cliff of syenitic greenstone, a little west of the top of the Blackhill 

 a high, bold point running forward between Glen Dhu and the upper 

 part of Glen Cloy. The top and front of this hill toward both glens 

 are composed of altered sandstone, resembling quartz rock, the 

 change of structure being, no doubt, due to the proximity of this 

 syenitic greenstone. The ledge or cliff seems to be the northern 

 front of a large dike ranging between west and north, and cutting 

 off the granite. To the south of it, the hills are composed of sand- 

 stone, with occasional knolls of overlying common trap. Westwards, 

 the granite extends across the moorland tracts, which lie between 

 Glen Cloy and a high ridge forming the watershed or axis of the 

 island at this part. It is seen in a few rocky points, but chiefly 

 rising through the sandy substratum of the peat, where this has been 

 worn away, or in the beds of the small streams. As we advance 

 westwards, it becomes gradually intermingled with S3 r enite and 

 porphyry ; but the relation of the three rocks cannot be made out, 

 owing to the nature of the ground. They appear to alternate, the 

 granite diminishing in quantity toward the west. On the summit 

 of the highest ridge or watershed, loose pieces of granite strew the 

 surface, mixed with syenite and porphyry. Bosses of the latter rise 

 through the broken-up masses of the sandstone, forming the main 

 body of this ridge. The surface is also strewn with masses of pitch- 

 stone, and of the coarse grained granite of the northern mountains. 

 Such must be taken as the hypothetical boundary of the Ploverfield 

 granite on the west. Its south boundary does not reach so far as a 

 line joining the heads of the Clachan glen and Glen Cloy, sandstone 

 being seen continuously between them. On the north side its 

 boundary runs from the west base of the Windmill hill to a point a 

 little west of the summit level of the String road. Here a high 

 ridge on the south side of the glen, and west of the watershed, 

 presents granite in its upper part ; the lower portion, on the level of 

 the watershed, is composed of sandstone ; but the junction is not 

 seen. These limits mark out an area much larger than that 

 assigned to the granite in Mr. Ramsay's map and descriptions. He 

 states, indeed, his inability clearly to define it. " From the depth 

 of the moss on the flat surface of the hills, the boundaries of this 

 granite have not yet been determined ; but by a little labour, property 



