196 MR. A. HARKER ON THE OVERTHRUST [May I903, 



which has produced the existing arrangement. The infraposition 

 of the crushed limestone, and the fact that the evidences of profound 

 mechanical disturbance become more pronounced as we pass upward 

 in the section, suggest that the whole overthrust mass is in an 

 inverted position beneath a more important plane of overthrusting, 

 the position of which is not far above the present surface of the 

 ground. Such a hypothetical major overthrust might perhaps be 

 identified with that which traverses the centre of the island, to be 

 described below. The belt of country neighbouring the overthrust 

 area of Monadh Dubh shows, however, some phenomena which are 

 not without a bearing on the subject, although the evidence obtained 

 is only of a fragmentary nature. The red sandstones immediately 

 beneath the crushed and displaced rocks give, as has been stated, 

 no clear indication of any special disturbance. A little farther away, 

 however, towards Loch Sgaorishal, occurs a narrow band, along 

 which the rocks are highly inclined and greatly crushed. It follows 

 a rather irregular and curved course in a general south-westerly 

 to north-easterly direction, at a distance of 100 to 300 yards from 

 the outcrop of the Monadh-Dubh overthrust, and can be traced for 

 about 900 yards, dying out, so far as any palpable evidence is con- 

 cerned, in both directions. It is much obscured by a boss of picrite 

 and other smaller intrusions. Along this band the sandstone is not 

 only brecciated, but in certain places mylonitized. Further, a 

 certain proportion of crushed limestone is in some parts mingled 

 with the sandstone, and there are lenticles of less crushed lime- 

 stone, with cherts, like those noticed above. The appearances seem 

 to show that the north-western boundary of this crush-band is a 

 surface of discontinuity comparable with the Monadh-Dubh ' thrust- 

 plane,' but inclined at a high angle. Although much more narrowly 

 localized than in the other case, the differential movement has been 

 of an extreme kind ; and the highly-sheared sandstone, with its 

 abundant development of white mica in parallel flakes, is a typical 

 mylonite [10498]. 



III. The Overthrust Belt of the Mountain-Border. 



We now proceed to consider the more extensive area of overthrust 

 and highly-disturbed Torridonian strata in the east-central, eastern, 

 and south-eastern part of the island, where, as shown in the 

 sketch-map (PL XIV), it forms a belt along the north-eastern and 

 eastern border of the mountain-tract. The Tertiary plutonic rocks, 

 of which the mountains are built, consist in this district of 

 a succession of roughly-parallel and partly-interlacing sheets or 

 laccolitic bodies with a general inward dip. Viewed broadly, they 

 have been intruded not far from, and usually above, the main 

 surface of overthrusting. In places they transgress this surface, 

 cutting into the relatively -unmoved strata below. In the western 

 part of Rum the quasi-stratiform disposition of the plutonic masses 



