132 DRGE1KIE ON THE HISTORY OF VOLCANIC ACTION 



a conspicous crag on the east side of Strath More, immediately to the north of Beinn na Cro. 

 It consists of beds of coarse gabbro, with others of dolerite and basalt, and is traversed 

 by veins from the granophyre of the glen, as well as by the usual N.W. basalt dykes 

 (fig. 55). It appears to be a marginal portion of the main gabbro area separated by 

 the intrusion of the great granitoid boss of the Red Hills. On the north-eastern side of 

 Beinn na Caillich numerous intrusive sheets of gabbro and dolerite traverse the quartzite 

 and limestone, and extend down to the sea-margin in the Sound of Scalpa. 



(b) Rum. — The mountains of the island of Rum, rising as they do from a wide 

 expanse of open sea, present one of the most prominent and picturesque outlines in the 

 West Highlands. Less accessible than most of the other parts of the volcanic region, 

 they have been less visited by geologists. They were described by Macculloch as 

 composed of varieties of " augite rock." He noticed in this rock "a tendency to the 

 same obscurely bedded disposition as is observed in other rocks of the trap family," 

 and found at one place that it assumed "a regularly bedded form, being disposed in 

 thin horizontal strata, among which are interposed equally thin beds of a rock resembling 

 basalt in its general characters."""" Professor Judd repeats Macculloch's observation, that 

 " the great masses of gabbro in Rum often exhibit that pseudo-stratfication so often 

 observed in igneous rocks." He regards these masses, like those of Skye and Mull, as 

 representing the core of a volcano from which the superficial discharges have been 

 entirely removed, and he gives a section of the island in which the gabbro is repre- 

 sented as an amorphous boss sending veins into a surrounding mass of granite.t In a 

 more recent paper he has given an excellent detailed account of the mineralogical 

 composition of some of the remarkably varied and beautiful basic rocks constituting the 

 hills of Rum, but adds no further information regarding the geological structure of the 

 island.^ 



Even from a distance of eight or ten miles, the hills of Rum are seen to be obviously 

 built up of successive nearly horizontal tiers of rock. As the summer tourist is carried 

 past the island, in that wonderful moving panorama revealed to him by the " swift 

 steamer " of modern days, these great dark cones remind him of colossal pyramids, and 

 as the ever- varying lights and shadows reveal more prominently the alternate nearly 

 level bars of crag and stripes of slope, the resemblance to architectural forms stamps 

 these hills with an individuality which strikes his imagination and fixes itself in his 

 memory. If choice or chance should give him a nearer view of the place, he would 

 not fail to notice that it is among the northern hills of the island that the bedded 

 character is so conspicuous, and that it ceases to be prominent in the southern 

 heights. Crossing over from Eigg, he would recognise each of the features represented 

 in the sketch reproduced in fig. 39. Along the shore, red (Torridon) sandstones 

 rise in naked cliffs, from the top of which the ground seems to slope upward in 

 brown moors to the bare rocky declivities. A deep valley (Glen Dibidil) is seen to 



* Western Islands, i. p. 486. t Quart. Jour. Geol. Soc, xxx. p. 253. 



X Op. cit., xli. (1885) p. 354. See also his paper in vol. xlii. of the same Journal. 



