20 GEOLOGY OF ARRAN. 



The summit is 500 to 600 feet above the valley, and is more 

 than a quarter of a mile long, and from 250 to 300 yards 

 broad. It descends steeply towards Shiskin on the south- 

 west, and slopes gradually north-east towards Glen Loig. 

 The summit and sides of this plateau are formed of fine- 

 grained granite, very similar to that of Ploverfield. The 

 base of the cliff towards Mauchrie Water is covered by a long 

 talus of granite blocks and smaller fragments, reaching to 

 within 50 or 100 yards of the road, and appearing even at 

 that distance of so different an aspect from fallen masses of 

 sandstone that it is surprising it was not noticed till our first 

 visit to that side of the island in 1855. 



The granite here seems to rise either through the old red 

 sandstone or at the junction of this rock with the carbonife- 

 rous strata. The granite is nowhere seen in situ at a low 

 level ; the talus before mentioned obscures the rocks along 

 the base of the hill; and the ground by the roadside arid 

 along the valley is deeply covered with alluvium. At some 

 spots, however, rocks are seen apparently in situ. They are 

 not of a very marked character, but seem to be chiefly sand- 

 stone of the old red formation, greenstones and trap porphyries, 

 the sandstone having assumed a subcrystalline metamorphic 

 aspect from the intermixture. But at a high level on the 

 west, south, and east sides of the plateau, the granite is seen 

 to rise through a coarse conglomerate ; and numerous con- 

 tacts are observable. These are highly interesting, and 

 clearly indicate the intrusion of the granite subsequently to 

 the formation of the conglomerate. The base of this con- 

 glomerate is a coarse sand, and the imbedded fragments 

 sandstone, quartz, and granite. The base is highly indurated, 

 and assumes a porphyritic structure ; the sandstone is 

 rendered crystalline, and the quartz has been fused, and 

 reconsolidated into a substance resembling porcellanite. The 

 fragments of granite are of an elliptic form, less rounded 

 than the quartz, and are exactly like the adjoining mass of 

 granite in structure and component parts. Whence have 



