18 GEOLOGY OF ARRAN. 



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

 Blackhill a high, bold point separating Glen Dhu from 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 

 teems to be the northern front of a large dike ranging be- 

 tween west and north, and cutting off the granite. To the 

 south of it the hills are composed of sandstone, with occa- 

 sional 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 syenite 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 diminish- 

 ing 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 pitchstone, and of the coarse-grained 

 granite of the northern mountains. We must thus be con- 

 tent with a hypothetical boundary for the Ploverfield granite 

 on the west. Its southern boundary does not reach so far 

 as a line joining the heads of Clachan glen and Glen Cloy, 

 sandstone being seen continuously between them. On the 

 north side its boundary runs from the west base of the Wind- 

 mill Hill to a point considerably to the 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 is composed of a dark-red and 

 hard sandstone; the rocks are seen in close proximity, the 



