Nov. 13, 1884] 



NATURE 



33 



ally finds himself again on the old platform of Archaean 

 gneiss exposed by denudation (see Section). Both the 

 zones of quartzite are then once more met with in their 

 normal order, the highest beds exposed on the western 

 shore of Loch Eriboll belonging to the horizon of the 

 "pipe-rock." On the eastern shore, at Ant Sron, on the 

 crest of a low anticlinal arch of the " pipe-rock," there is an 

 excellent section of the middle series between the quartz- 

 ites and the limestones. The two subdivisions of the 

 " Fucoid-beds" (No. 5) and the " Serpulite-grit " (No. 6 in 

 Section), which are typically developed at that locality, 

 pass underneath the Serpulite-limestone at the base of 

 Group I., exactly as they do at Durness. The dark leaden- 

 gray limestones of the lowest group (I.) are then rapidly 

 succeeded by flaggy limestones (Ant Sron, Chorrie Island, 

 Heilim) and dolomitic limestones which, probably the 

 equivalents of the Eilean Dubh Group in Durness, are 

 the highest members of the series here represented 

 (No. 7 in Section). A careful search among the Eriboll 

 limestones has failed to bring to light any organic remains 

 save Serpulites and certain minute spherical bodies which 

 may prove to be Foraminifera. A similar dearth of fossils 

 characterises the two lowest zones in Durness, so that this 

 feature is common to both areas. The non-occurrence of 

 the higher fossiliferous limestones in Eriboll may be 

 accounted for by the remarkable geological structure of 

 that region which is now to be described. 



As the observer passes eastwards along the magnificent 

 quartzite sections on Crag Dionard and Conamheall, south 

 of Loch Eriboll, he cannot fail to note the numerous 

 flexures of the strata, and especially the peculiar type of 

 their sharp anticlinal folds. As a rule, the eastern limb 

 of each of these folds dips at a gentle angle to the south- 

 east, while the west limb is highly inclined, vertical, even 

 inverted, or sometimes broken by a reversed fault the effect 

 of which is to bring lower over higher beds. These reversed 

 faults (/// in Section) become more numerous eastwards. 

 They are admirably displayed both in ground-plan and dip- 

 section on the shore north of Heilim, where they repeat the 

 various zones ranging from the "pipe-rock" to the Eilan 

 Dubh limestone (Group II.). The strike of the reversed 

 faults ranges on the whole with that of the strata tra- 

 versed, and their hade is inclined at a higher angle than 

 the dip of the latter, the difference generally amounting 

 to about 10°. Inland from the coast-section, north of 

 Heilim, to the base of Ben Arnaboll, the zones just men- 

 tioned are constantly repeated by a complicated system of 

 reversed faults and folds, the general inclination of the 

 strata being towards the south-east. As that hill is ap- 

 proached, the displacement produced by these faults in- 

 creases in amount ; hence the observer meets with beds 

 occupying a lower geological horizon the further east he 

 travels. At length this intricate system of faults and folds 

 culminates in a great dislocation which, for convenience 



" 



mm 



- ■ 



-> Schis 

 , (Frilled Schist 

 L \Green Schist 





: Durness Area; p. = Eriboll Area; ff 



of description, and to distinguish it from the ordinary 

 reversed faults, may be termed a Thrust-plane. By means 

 of it a great mass of coarsely crystalline gneiss with peg- 

 matite-veins, in places upwards of 400 feet thick, is placed 

 above the Silurian rocks (see Section). A careful examina- 

 tion of the mass which caps Ben Arnaboll clearly proves 

 that it rests transgressively on all the zones of lit, Silurian 

 rocks, from the lower zone of the quartzites {false-bedded 

 grits) up to the limestone which overlies the Serpulite-grit. 

 Owing partly to its low hade and partly to subsequent 

 folding, the outcrop of this thrust-plane resembles that of 

 an ordinary overlying formation cut into a sinuous line by 

 denudation. It is admirably seen in dip-section on the 

 east and north slopes of Ben Arnaboll, whence it can be 

 followed round the west face of the hill, descending into 

 the valley on the west, then bending back on itself, wind- 

 ing round the north slope of Druim Tun^i, and entering 

 Loch Eriboll in Heilim Bay. It reappears at the base of 

 Crag-na-Faolinn, and has been traced still farther to the 

 south, while northwards it can be followed to the Whitten 

 Head, at the mouth of Loch Eriboll. 



That the gneiss thus brought up on Ben Arnaboll and 

 elsewhere is in reality the Archaean gneiss is evident, for 

 two reasons. First, its lithological characters agree with 

 those of the typical Archaean area to the west, save in 

 certain cases where the original features have been effaced 

 by the crushing to be afterwards described. Near the 

 thrust-plane, this effacement is complete, but in the 

 heart of the mass the normal characters of the Archaean 



rocks, including in some instances their characteristic 

 north-west strike, are retained. The rocks consist of 

 coarsely crystalline hornblendic gneiss and pink granitoid 

 gneiss, with lenticular veins of hornblende-rock and kernels 

 of cleavable hornblende, while massive veins of pink 

 pegmatite are well developed. The soft greenish mineral 

 (agalmatolite?) already mentioned as characteristic of the 

 gneiss, where now or lately covered with quartzite, occurs 

 here in the pegmatites, and veins of epidosite are abundant. 

 Second, at various localities the brecciated conglomerate 

 and false -bedded quartzite at the base of the Silurian 

 strata are found resting on these crystalline rocks. Further, 

 the unconformable junction can on one line be traced 

 continuously for more than a mile. There can be no 

 doubt, therefore, that this mass is really a fragment of the 

 old platform of Archaean rocks on which the Silurian 

 strata were deposited. 



The occurrence of this Archaean gneiss in its present 

 position above much younger rocks is doubtless to be 

 ascribed to the same cause which elsewhere has resulted 

 in foldings of the strata. In the present instance we 

 see an attempt, as it were, to establish an anticlinal 

 fold of the type already described as occurring to 

 the west, with a steep westward and gentle eastward 

 slope. The west limb however has here given way along 

 a great dislocation or reversed fault, while the eastern 

 limb has been driven forwards until the Archaean rocks 

 have been carried over the truncated edges of the Silurian 

 strata. The vertical beds of the basal quartzites are still 



