DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



133 



run into the heart of the hills between the bedded group to the north and the 

 structureless group to the south. If the weather is favourable, some eight or more 

 prominent parallel bars of rock may be counted on the two higher cones to the right. 

 These bars are not quite level, but appear to have a gentle inclination from right 

 to left. They remind one of the terraced basalts of the plateaux, but present a 

 massiveness and a breadth of intervening bare talus-slope such as are not usual among 

 those rocks. 



Nor is this impression of regularity and bedded arrangement lessened when we 

 actually climb the slopes of the hills. I had for years been familiar with the outlines of 

 Eum as seen from a distance, and had sketched them from every side, but I shall never 

 forget the surprise and pleasure when my first ascent of the cones revealed to me the 

 meaning of these parallel tiers of rock. I found it to be the structure of the Cuillin 

 Hills repeated, but with some minor differences which are of interest, inasmuch as they 

 enlarge our conceptions of the process by which the gabbro-bosses were formed. 



The northern half of the island of Rum consists almost entirely of red sandstone, 

 which is obviously a continuation of the same massive formation so well developed around 



Fig. 39. — Outline of the Hills of the Island of Rum, sketched from near the Isle of Eigg. 



Loch Torridon and traceable between the Archaean gneiss and the Lower Silurian strata 

 up as far as Cape Wrath. The sandstones, though full of false bedding, show quite 

 distinctly their true stratification, which is inclined with singular persistence towards 

 W.N.W., at angles averaging from 15° to 20°. If they are not repeated by folds or faults, 

 they must reach in this island a thickness of some 10,000 feet. Their red or rather 

 pinkish tint seems mainly to arise from the pink felspar so abundant in them, for in 

 many places they really consist of a kind of arkose. Pebbly bands with rounded pieces 

 of quartz are of common occurrence throughout the whole formation. Dykes and veins 

 of basalt are profusely abundant. Sometimes these run with the bedding, and might at 

 a distance be taken for dark beds among the pink sandstones. They often also strike 

 obliquely up the face of the cliffs like ribbons. 



But, notwithstanding their apparent continuity, there can be no doubt that these 

 sandstones have suffered from those powerful terrestrial disturbances which have affected 

 all the older rocks of the North- West Highlands. On the west side, where they plunge 

 steeply into the sea, they have undergone a change into fine laminated rocks, which might 

 at first be mistaken for shales, but which owe their fissility to shearing movements. 



VOL. XXXV. PART 2. S 



