100 



66. The cliff on the north side of the corrie shows some interest- 

 ing dikes. One of these is of green pitchstone, and cuts the granite 

 sheer through in a N. and S. direction from bottom to top of the cliff. 

 It is four feet wide, prismatic across, and owing to the more rapid 

 disintegration, depressed below the level of the granite. The con- 

 tact does not present any peculiar change in either rock, such as 

 invariably marks the planes of contact of pitchstone and the sedi- 

 mentary strata. The pitchstone is decomposed into a thin white 

 film in many places along the outer edge of the dike, next the 

 granite, in consequence, probably, of the oxidation and removal of 

 the iron which enters into its composition. The dike is in some 

 parts of its course obscured by debris, but upon the whole is, per- 

 haps, the best defined dike of this rock occurring anywhere in the 

 granite of Arran. But phenomena of much greater interest will be 

 again noticed in connection with other pitchstones. 



Two basaltic dikes occur close together, about 100 yards east of 

 the dike we have been describing ; they traverse the granite preci- 

 pice in the same manner, but in a different direction, their course 

 being about 28 W. of N., subject, however, to undulations. These 

 dikes are from 18 inches to 2 feet broad, and are separated by a 

 granite band 8 or 10 feet in breadth ; elliptic masses of granite, of 

 which the largest we observed was about 18 inches by 9, are enclosed 

 in the trap, but very little altered. The alteration, indeed, is no- 

 where remarkable, the granite being in some places coarse, in others 

 fine-grained, along the planes of contact. Specimens may be ob- 

 tained of both rocks firmly adhering. 



Some pretty plants occur here in shady spots on the granite 

 ledges, where a little soil has accumulated ; they will reward the 

 young botanist for his long walk to their secluded habitat. The 

 Sedum rkodiola, Oxyria reniformis, Saxifraga stellaris, Alchemilla 

 alpina, and several others, rejoice in the temperature and humidity 

 which these heights supply. 



To reach the head of the valley, and the first ascent of the Pass 

 into Glen lorsa, we now direct our steps along the base of the pre- 

 cipice, where the grassy tufts and granite debris afford a safe footing 

 on the steep slope, keeping as high a level as possible, in order to 

 shorten the ascent by which the summit is reached. As we pass 

 along we notice several dikes of pitchstone and basalt, ranging 

 north-westwards up the precipices on our left towards the summit of 

 the ridge, and doubtless crossing down on the other side into Glen 

 lorsa ; but we may not now delay to trace them. The Ben-Talshan 

 ridge on our right, and that along which we have passed, coalesce at 



