E0CKS OF THE 15TOETHEEN HIGHLANDS. 397 



characters. Besides this, though a few typical specimens may 

 resemble some varieties of the old gneiss, there is no great difficulty 

 in differentiating the two gneisses when we compare them in the 

 mass. In the newer group mica is incomparably more abundant ; 

 there is a larger proportion of a pinkish-grey, rather fine-grained, 

 granitoid gneiss, quite different from any I have seen in the Hebri- 

 dean ; and there is that undcfinable assemblage of minute characters 

 which we may call the " habit''' of a group. 



Further light is thrown upon this question by the gneiss at Sango 

 Bay, Durness, described in my former paper. The resemblance of 

 this rock to the Arnaboll group struck me in 1880. The Durness 

 gneiss has undergone considerable alteration, chloritic minerals 

 being developed ; but the general characters of the two rocks are 

 similar. In Durness the Hebridean steadily preserves its typical 

 north-westerly strike, the beds being nearly or quite vertical ; while 

 the Sango Bay gneiss dips at low angles conformably, though not 

 in actual contact, with the flaggy gneiss of Far-out Head. If then, 

 as is probable, the Sango Bay gneiss is the equivalent of the Arnaboll 

 rock, we are furnished with an additional proof of discordance 

 between the Hebridean and the Arnaboll group. 



But the undoubted Arnaboll gneiss, in its north-easterly extension, 

 is only separated from the Hebridean of Ben Keannabin by the 

 breadth of Loch Emboli, within 2 miles ; yet the strike of the newer 

 series is as steadily to the N.N.E. as that of the old gneiss is to the 

 KW. 



The Arnaboll gneiss is literally " riddled " by a binary granite of 

 quartz and felspar, which is sometimes more prominent than the 

 gneiss ; and this has apparently led Nicol to the conclusion that it 

 was the prevailing rock, which had " caught up fragments of the 

 mica slate." Further particulars of this series will appear in the 

 sequel. 



B. The Assynt Series folded bach upon itself. 



Structure of Druim-an-tenigh. — This quartzite ridge overhangs the 

 road from above Heilem Inn to the west end of Arnaboll valley, a 

 distance of about two thirds of a mile. It strikes due N. and S. 

 It is the first ground which should be examined by any one wishing 

 to read the riddle of the district. 



I ascended the ridge from the east side, just at the north end. 

 Here I found quartzite of the seamy type, with a grit band 10 feet 

 above the base, just as in A ssynt. The beds were horizontal or with a 

 slight dip to the E. Following the strata across the ridge, they are 

 seen to bend abruptly over, and plunge down at a high angle to the 

 "W. We now turn south, and, keeping along the strike of these 

 strata, we find the dips grow steeper and steeper till the vertical is 

 reached. Then the beds begin to overhang, the overthrow increasing 

 till, at the southern end, just at the opening of the Arnaboll valley, 

 they dip easterly, apparently passing below the Arnaboll gneiss at 

 moderate angles, from 50° to 60°. The quartzite has still the seamy 



