58 GEOLOGY OF ARRAN. 



when the present situation of the fall was reached, as the 

 dike here retires from the stream. The dike is ten to fifteen 

 feet wide, and ranges nearly due north and south. The rocks 

 adhere firmly at the junction, but the alteration on the 

 granite is not remarkable. Similar cases of the excavation 

 of river channels along the line of dikes are frequent in 

 Arran. As a general rule, fractures or faults determine the 

 course of streams in the first instance; along such lines the 

 excavation is much more rapidly effected. 



The steep brow on the eastern side of the wide hollow 

 where we now are, exhibits many rounded masses of granite, 

 presenting the "moutonneV character of surface, as if 

 moulded by the action of ice ; but as no striae were observed, 

 we can hardly ascribe them to the action of glaciers, as the 

 forms may be due to the effects of disintegration on the con- 

 centric structure upon the large scale, so often seen in 

 granite. The bed of the stream, as we pass up, is strewed 

 with many loose rounded masses of pitchstone and trap, 

 indicating the existence of dikes or beds of these rocks 

 among the lofty precipices on the west. Mounting this 

 steep brow, to reach the corrie under the north front of Ben- 

 Ghnuis, we meet with a dike of spheroidal trap in the bed 

 of one of the streams ; it is about seven feet wide, and ranges 

 35 W. of N. The rounded masses of granite here may 

 have received their forms from the long-continued action 

 of water trickling over them, and torrents occasionally 

 sweeping along gravel and large stones. Arrived at this 

 corrie, we are in the midst of a scene wild, lonely, and 

 picturesque. The bare and rugged precipices of Beu-Ghnuis 

 rise high into middle air on the south, with their immense 

 sheets and rhombic masses of granite, from six to twelve feet 

 in the side, piled up, block on block, in massive courses, like 

 the huge rough masonry of giants. The topmost row, broken 

 by clefts and deep gashes, due to irregular disintegration, 

 shews grandly in its perfect definition against the clear sky. 

 Along the front, which sweeps round to form one side of the 



