Vol. 59.] TORRIDONIAN, ETC. OF THE ISLE OF RUM. 201 



special signs of disturbance, excepting the contortions on a small 

 scale which seem to affect most of the Torridonian rocks of Rum. 

 The overthrust shales are soon cut out again by the gabbro, and 

 only reappear for a short distance about three-quarters of a mile 

 farther south. 



We pass on to Beinn nan Stac, a hill about 1850 feet high, the 

 south-eastern slope of which descends rather sharply to the sea 

 (see fig. 4, p. 202). The average inclination of the overthrust- 

 surface in this place does not differ greatly from that of the ground, 

 so that the displaced shales make a considerable spread on the 

 slope. They may be regarded as of the nature of an outlier, this 

 relation being obscured, however, by the subsequently-intruded 

 gabbro and felsite to the north-west. The shales are highly 

 inclined, often vertical, and their strike varies rapidly from point 

 to point. They are also violently contorted on a small scale, and 

 indurated in consequence of metamorphism. Fine grey sandstone, 

 for the most part thoroughly brecciated, forms the actual summit of 

 the hill, this mode of crushing having been especially operative, as 

 usual, at the passage from shale to sandstone. Nearly along this 

 zone of weakness has been intruded a sheet-like mass of porphyritic 

 felsite similar to that of Meall JBreac, and here, too, it has in many 

 places enclosed fragments of sandstone from the crush-breccia. The 

 shales themselves are brecciated in some places, but not on an 

 extensive scale. The relatively-unmoved sandstones below the over- 

 thrust are more disturbed on Beinn nan Stac than elsewhere along 

 the line that we have followed, and immediately below the main 

 surface of displacement they show high and reversed dips. 



The overthrust outlier of Beinn nan Stac, with much reduced 

 width, comes down to the sea a little east of the outlet of the 

 Dibidil River. Beyond this the strata beneath the overthrust show 

 much more evident disturbance than heretofore, the sandstones 

 being extensively brecciated. There is in places considerable meta- 

 morphism, which is here connected with the occurrence of several 

 patches and lenticles of gneiss. Highly -disturbed Torridonian 

 rocks extend up to a rather high altitude on the east side of Sgurr 

 nan Gillean, and run along the coast for some distance towards 

 Papadil; but our detailed survey has not yet covered the actual 

 termination of the belt of displacement in this direction. 



IV. The Geological Relations of the Gneisses. 



I have next to notice what is, in some respects, the most 

 interesting feature of the tract under consideration, namely, the 

 occurrence of gneissic rocks at numerous places along the border 

 of the mountains, usually in immediate association with the highly- 

 disturbed Torridonian strata. 



The individual occurrences are never of large dimensions, the 

 length being usually less than a quarter of a mile, and sometimes 

 as little as 100 yards. Where the natural boundary is clearly 

 shown, it approximates more or less closely to a lenticular form, 



Q. J. G. S. No. 234. p 



