Vol. 59.] TOERIDONIAN, ETC. OF THE ISLE OF RTJM. 203 



with its greatest extension conforming with the local strike of the 

 disturbed strata in its vicinity. A number of detached and partly- 

 detached areas of gneiss occur in the central part of Rum, near the 

 western termination of the overthrust belt, the bare white outcrops 

 about Priomh-loch being very conspicuous at a distance. This is a 

 good district for studying the petrography of the rocks, but not 

 their relation to the Torridonian, the map being complicated by 

 irregular intrusions of peridotite, gabbro, and granite. Following 

 the disturbed belt from Loch Bealach Mhic Neill eastward and 

 southward, no gneiss is seen for a long distance, except a small 

 strip, in contact with Torridon Sandstone, involved in the por- 

 phyritic felsite on the hill east of Loch Gainmhich. Next, a 

 lenticular mass of gneiss occurs among the altered shales just above 

 the overthrust surface on the lower slope of Beinn nan Stac (fig. 4, 

 p. 202). A strip runs for nearly 400 yards along the Dibidil River, 

 from the ford to the coast, and there are several other occurrences to 

 the west and south-west. It is only in this district, where the strata 

 overridden by the great displacement are unusually disturbed, that 

 gneissic rocks are found below the main overthrust. In addition 

 to these occurrences, all more or less closely bound up with the 

 displaced Torridonian strata, gneisses are found about Loch Sgathaig 

 bounded only by granite, gabbro, and picrite ; while, farther west, 

 an isolated patch a quarter of a mile long occurs in the interior of 

 the main granite-area, forming the summit of Beinn a' Bharr-shaibh, 

 to the east of Orval. 



These rocks, with well-marked parallel banding and foliation, 

 and frequent alternations of different lithological types, are perfectly 

 characteristic gneisses in the ordinary descriptive sense of the word. 

 Indeed, their appearance led Sir Archibald Geikie to assign them 

 to an Archaean age. 1 Closer examination, however, compels me to 

 dismiss decisively the hypothesis that these rocks are portions of 

 a pre-Torridonian formation brought up by overthrusting. The 

 gneisses are clearly intrusive in the Torridonian strata. Not only 

 do they penetrate these in an intimate manner, but they sometimes 

 enclose fragments of them in a highly metamorphosed state. Thermal 

 metamorphism, as I have remarked, has affected in varying degree 

 a large portion of the disturbed strata along the mountain-border, 

 and I have ascribed it in general to the intrusion of the gabbro 

 and other plutonic rocks of Tertiary age ; but the highest grade of 

 metamorphism 4s found in the rocks bordering the relatively-small 

 intrusions of gneiss. On the lower slope of Beinn nan Stac, for 

 example, for a considerable distance from the lenticle of gneiss, the 

 contorted shales are converted into a hard black, almost flinty rock, 

 resembling a compact basalt. Elsewhere sandstone has been 

 transformed into quartzite. 



We have, then, full assurance that the gneisses are younger than 



1 'That some of these rocks are portions of the Lewisian Series can hardly 

 be doubted, and their structure and relations are probably repetitions of those 

 between the Lewisian gneiss and Torridon Sandstone of Sleat in Skye,' Ancient 

 Volcanoes of Great Britain, vol. ii (1897) p. 351. 



p2 



