98 



illustrative cut at e. The slate is here traversed by a basaltic dike, 

 still further modifying its altered structure. The dike intersects the 

 bed of the stream at a small angle, but is seen only on the east bank ; 

 it ranges about magnetic north and south, and has a breadth of ten 

 or fifteen feet. We shall meet with it farther up the glen on 

 another Excursion. Rising from the bed of the stream in great 

 rhomboidal masses, divided by partings here inclined towards the 

 slate, the granite extends in a slanting direction up the hill towards 

 Goatfell, and on the opposite side by the Garbh-Alt burn, so that 

 the line of junction almost coincides with the southern margin of the 

 stream. The remaining part of our walk is entirely on the granite. 



65. Our path now lies up the steep slope forming the western side 

 of the glen, a little to the north of the Grarbh-Alt, which we keep 

 on our left hand. Having reached the summit level we are on the 

 southern slope of the high rugged ridge called Ben-Talshan, which 

 forms the western boundary of Glen Rosa. In the hollow between 

 it and the lower swells of the Ben-Ghnuis range, the Garbh-Alt 

 pursues its rapid course from north to south, along a granite bed, 

 down a pretty fall, and then between perpendicular walls of granite 

 about twelve feet in height, till, escaping from this rocky barrier, it 

 sweeps round the south, end of the ridge, and plunges headlong into 

 the depths of Glen Rosa. The gorge has been excavated along the 

 line of a basaltic dike, which occupies the bottom of the stream 

 throughout, and retires from it at the base of the fall. These dikes, 

 as already remarked (Art. 40), are prismatic across; and this struc- 

 ture renders their disintegration much more easy than that of the 

 granite. The amount of wearing in this case is measured by the 

 depth of the chasm, and the distance to which the fall has receded. 

 The stream ran at first on the level of the top of the granite walls, 

 and the fall must have been at the southern extremity of the chasm ; 

 the recession would cease, or become extremely slow, 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 



