CHAP. XLVI 
THE ACID BOSSES OF SKYE 
38i 
mass of Beimi 11 a Caillieh and the two Beinn Deargs is the product of a 
distinct orifice, if not of more than one. Beinn na Cro', lying between its 
two deep bounding glens, is another protrusion. The western cones stand 
so closely together that their screes meet at the bottoms of the intervening 
valleys. Yet each group is not improbably the result of emission from an 
independent funnel, like the separate domite pays of Auvergne. 
But, though I believe this large area of granitoid rock to have proceeded 
not from one but from many orifices, I have only here and there obtained, 
from the individual hills themselves, indications of an internal structure 
suggestive of distinct and successive protrusions of material from the same 
vent of discharge. On the outer declivities of some of the cones we may 
detect a rudely bedded structure, which will he subsequently referred to as 
well displayed in Bum (p. 403). This structure is specially observable along 
the east side of Glen Sligachan. Down the northern slopes of Marsco the 
granophyre (here in part a hornblende-biotite-granite) is disposed in massive 
sheets or beds that plunge outwards from the centre of the hill at angles of 
30° to 40°. On the southern front of the same graceful cone, as well as 
on the flanks of its neighbour, Kuadli 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, 1 2 and in this internal arrangement of their 
materials, indefinite though it may be, there is a further resemblance to the 
onion-like coatings which Yon Buch and Scrope remarked in the structure 
of the interior of the Grand Sareoui.- 
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 11 a Caillieh. 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 conspicuous illustration may 
be noticed on the southern front of Beinn Dearg Meadhonach, north from 
Marsco. When I first saw that hillside 1 could not realize that the parallel 
1 Edin. New Phil. Jour. xl. p. 78. 
2 Von Buch, Geognostisehe Eeobachtuugen auf Beisen durch Deutschland und Italim, vol. ii. 
(1809) p. 245 ; Scrope, Geology and Extinct Volcanoes of Central France, 2nd edit. p. 68. Von 
Buch regarded the external form of this Puy as haying been determined by its internal structure. 
