162 DR GETKIE ON THE HISTORY OF VOLCANIC ACTION 



Stac, still plainer indications of a definite arrangement of the mass of the rock in irregular 

 lenticular beds may be noticed. These beds, folding over the axis of the hill, dip steeply 

 down as concentric coats of rock. The external resemblance of the red conical mountains 

 of Skye to the trachyte puys of Auvergne was long ago remarked by J. D. Forbes,* and 

 in this internal arrangement of their materials, indefinite though it may be, there is a 

 further resemblance to the onion-like coatings whieh Scrope thought he could detect in 

 the interior of the Grand Sarcoui.t 



Where the contour of the cones is regular, and the declivities are not marked by 

 prominent scars and ribs of rock, this monotony of feature betokens a corresponding 

 uniformity of petrographical character. But where, on the other hand, the slopes are 

 diversified by projecting crags and other varieties of outline, a greater range of texture 

 and composition in the material of the hills is indicated. This relation is well brought 

 out on the western front of Marsco, where numerous alternations of granitoid and felsitic 

 textures occur. On many declivities also, which at a distance look quite smooth, but 

 which are really rough with angular blocks detached from the parent mass underneath, an 

 occasional basalt-dyke will be observed to rise as a prominent dark rib. A good example 

 of this structure is to be seen on the south front of Beinn na Caillich. Where a group of 

 dark parallel dykes runs along the sides of one of these pale cones it sometimes produces 

 a curiously deceptive appearance of bedding. A good illustration may be noticed on the 

 southern front of Beinn Dearg Meadhonach, north from Marsco. When I first saw that 

 hillside I could not realise that the parallel bars were actually dykes until I had crossed 

 the valley and climbed the slopes of the hill.| 



Occasionally round the margin of the granophyre a singular brecciated structure is to 

 be seen. It is most marked on weathered faces, and may be observed on the flanks of 

 Glamaig and of Marsco. But when the rock is broken open, it is less easy to detect the 

 angular and subangular fragments from the surrounding matrix, which is finely crystalline 

 or felsitic. 



The actual junction of the eruptive mass with the surrounding rocks through which it 

 has ascended is generally a nearly vertical boundary, or the granophyre plunges at a 

 steep angle under the rocks that lie against or upon it. On the north of Glamaig, for 

 instance, the porphyritic and felsitic margin of the great body of eruptive rock descends 

 as a steeply inclined wall, against which the red sandstones and marls at the base of the 

 Secondary formations are sharply tilted. On the south side of the area, a similar steep 

 face of fine-grained rock forms the edge of the granophyre of the great southern cones, 

 and plunges down behind Lias limestone and shale, Lower Silurian limestone and 

 quartzite, or portions of the Tertiary volcanic series. Yet there can be no doubt that, 

 along many parts of the boundary-line, the eruptive mass extends underneath the surface 



* Edin. New Phil. Jour., xl. p. 78. t Geology and Extinct Volcanoes of Central France, 2d edit., p. 68. 



X The difference of contour and colour between the ordinary reddish smooth-sloped " syenite " and the black craggy 

 " hypersthene rock" and "greenstone" at the Glamaig group of hills caught the eyes of Von Oyenhauskn and Von 

 Dechen (Karsten's Archiv, i. p. 83). 



