128 DRGEIKIE ON THE HISTORY OF VOLCANIC ACTION 



Through all these rocks numerous parallel basalt-dykes, running in a general persistent 

 N.N.W. direction, rise from below the sea-level up even to the very crests of the Cuillins. 



The sections on the westerD side of the area thus prove that the gabbro inosculates 

 with the bedded basalts by sending into them, between their bedding planes, sheets 

 which vary in texture from fine dolerites at the outside into coarse gabbros further 

 towards the central mass, and that this intrusion has been accompanied by a certain 

 amount of induration of the older rocks. 



On the eastern side, the same structure can be even more distinctly seen, for it is 

 not only exposed in gullies and steep declivities, but can be traced outward into the basalt- 

 plateau. In the promontory of Strathaird, Jurassic sandstones and shales, which form 

 almost the whole of the coast-line and lower grounds, are surmounted by the bedded 

 basalts. Denudation has cut the plateau into two parts. The smaller of these makes the 

 outlier that rises into Ben Meabost (1128 feet). The larger stretches continuously from 

 Glen Scaladal and Strathaird House northward into Blath Bheinn. Hence from the 

 ordinary terraced basalts, with their amygdaloids. thin tuffs, red partings, and seams of 

 lignite, every step can be followed into the huge gabbro mountain. Starting from the 

 black shales on which the lowest basalt lies, we walk over the successive terraces up into 

 the projecting ridge of An da Bheinn. But as we ascend, sheets of dolerite and gabbro 

 make their appearance between the basalts, which gradually assume the altered aspect 

 already noticed. The dip of the whole series is at a low angle northwards, and the beds 

 can be followed round the head of the Glen nac Leac into the southern slopes of Blath 

 Bheinn. Seen from the eastern side of this valley, the bedded character of that mountain 

 is remarkably distinct, but it becomes less marked towards the upper part of the ridge 

 where the gabbros preponderate. One of the most striking features of the locality is the 

 number and persistence of the N.W. dykes, which strike across from the ordinary 

 unaltered basalts of the plateau up into the highest gabbros of the range. Less durable 

 than the intractable gabbro, they have weathered out where they run up its precipices, 

 thereby causing the vertical rifts and gashes and the deep notches on the crest that 

 form so marked a feature in the scenery. On the other hand, they are often less 

 destructible than the plateau-basalts, and hence in the Glen nac Leac they may be seen 

 projecting as low dams across the stream which throws itself over them in picturesque 

 waterfalls. 



The deep dark hollow of the Coire Uaigneich has been cut out of the very core of 

 Blath Bheinn, and lays bare the structure of the east part of the mountain in the most 

 impressive as well as instructive way (Fig. 37). By ascending into this recess from Loch 

 Slapin, we pass over the whole series of rocks, and can examine them in an almost con- 

 tinuous section in the bed of the stream and on the bare rocky slopes on either side. 

 Sandstones and shales of the Jurassic series extend up the Allt na Dunaiche for nearly a 

 mile, much veined with basalt and quartz-porphyry, by which the sandstones are locally 

 indurated into quartzite. At last these strata are overlapped by the basalts of the Strath- 

 aird plateau, which with a marked inclination to N.N.W., here dip towards the mountains. 



