190 MB. A. HAEKBR ON THE OVERTHRUST [May I903, 



The Isle of Eum divides, on the broadest view, into a northerly 

 moorland-tract, the highest points of which fall a little short of 

 1000 feet, and a southerly mountain- tract, of much bolder relief, 

 and reaching a maximum altitude of 2659 feet. The mountains are 

 formed of massive plutonic rocks of Tertiary age, and, as Macculloch 

 remarked, these, at least in the eastern part of the island, overlie 

 the stratified rocks. Torridonian strata occupy about one-half of 

 the total area, including the northern moorland-tract and a strip 

 along the eastern coast, as shown in the accompanying sketch-map 

 (PI. XIV). The highly-disturbed strata to be particularly described 

 occur in two districts, namely, a small area in the north-western part 

 of the island, and a more extensive belt along the north-eastern and 

 eastern border of the mountains. 



I shall give, first, a short general account of the relatively un- 

 disturbed strata which occupy the greater part of the Torridonian 

 area. They consist principally of a monotonous succession of 

 sandstones, dipping north-westward or west-north-westward, at 

 angles usually between 20° and 33°; but below these emerges, 

 on the eastern coast, a group of shales with similar dip. 1 The total 

 thickness, as calculated from the extent of the rocks and the observed 

 dips, is more than 10,000 feet, without any natural base or summit, 

 and this thickness is distributed approximately as below : — 



Upper group : sandstones, 9000 feet seen. 

 Lower group : shales, 1400 feet seen. 



The upper group consists almost exclusively of felspathic sandstones, 

 which have a more or less pronounced reddish colour, except where 

 they have been bleached by metamorphism or some other agency. 

 The texture varies from fine to coarse, many beds containing abun- 

 dant small pebbles up to an inch or more in diameter. The lower 

 group consists essentially of dark shales, of very uniform aspect 

 where they have not been metamorphosed in the vicinity of igneous 

 intrusions. There are, however, occasional beds of fine grey sand- 

 stone at various horizons ; and some alternations of similar sand- 

 stones with shales at the summit of the group may be regarded as 

 passage-beds to the sandstones above. 



In this — which we have styled, with implied reservation, the 

 relatively-undisturbed tract — there is, apart from some faults of 

 moderate throw, the appearance of an unbroken succession with a 

 gentle and steady inclination. This apparent regularity is, however, 

 as Sir Archibald Geikie has remarked, in great measure illusory, and 

 the estimates of thickness given above must be qualified accordingly. 

 The stratification is in reality highly disturbed on a small scale. 

 The sandstones, which present so monotonous a succession of 

 steadily-inclined beds, show on closer examination in innumerable 

 places indications of violent contortion, these indications being 

 brought out by weathering in almost all parts of the coast-section, 



1 Macculloch ('Western Is.' vol. i, 1819, p. 481) observed these shales, but 

 identified them erroneously with the [Liassic] shales of Loch Eishort in Skye. 



