EXCURSION I. 57 



near the junction are of inconsiderable thickness, and that 

 the granite exists beneath at a small depth. 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. 



27. Our path now lies up the steep slope forming the 

 western side of the glen, a little to the north of the Garbh- 

 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-Cliabhain, 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 are 

 prismatic across; and this structure renders their disintegra- 

 tion 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, 



