34 



NA TURE 



[Nov. 13, 1884 



to be seen on the west limb of the anticline on Ben 

 Arnaboll, Crag-na- Faolinn, and on Whitten Head (see Sec- 

 tion). The quartzites on Druim Tungi, and indeed all the 

 Silurian strata on the east side of Loch Eriboll, between 

 Heilim and Crag-na-Faolinn, form part of a syncline 

 which has been pushed westwards in front of the anti- 

 cline along this thrust-plane. This structure explains the 

 origin of the inversion of the Silurian rocks along the 

 junction line east of Camas-an-Duin, and the occurrence 

 of the lower limestone groups in a shallow trough at 

 Eriboll. Of special interest is the occurrence of a small 

 outlier of Archaean gneiss on the crest of a hill (Sithean- 

 na-Cuag) north-west of Ben Arnaboll. This mass rests 

 on the Fucoid-shales, Serpulite-grit, and limestone. 

 Though now isolated by denudation, it was obviously 

 originally continuous with the mass on Ben Arnaboll, and 

 it thus furnishes striking proof of the westward extension 

 of displaced gneiss, and of the thrust-plane on which it 

 travelled. 



The effects of this great movement on the Silurian 

 strata which have been over-ridden by the gneiss are 

 somewhat remarkable. The pipes or vertical worm-tubes 

 in the quartzites have been flattened, drawn out, and bent 

 over in a direction perpendicular to the strike of the 

 thrust-plane. The false-bedded grits and quartzites pre- 

 sent a streaky appearance resembling fluxion-structure, 

 due to the elongation of the fragments of orthoclase- 

 feldspar and the quartz grains. The fine-grained rocks, 

 especially the compact quartzites and the Fucoid-beds, are 

 highly cleaved, the strike of the cleavage-planes being 

 parallel with that of the thrust-plane, and this parallelism 

 being maintained quite irrespective of any variation in 

 the direction of dip of the strata next the gneiss. On the 

 surface of the cleavage-planes also there is a series of 

 parallel lines like slickensides which will be described 

 presently ; and lastly, there is a slight development of 

 sericitic mica along many of the cleaved surfaces. No 

 less important is the alteration produced on the overlying 

 Archaean gneiss. In the heart of the mass, as already 

 stated, there is little apparent change, but near the thrust- 

 plane the beds are drawn in towards it till their strike 

 roughly coincides with that of the thrust-plane. The 

 inclosed hornblende merges into a green chloritoid 

 product, the hornblendic gneiss has been converted into 

 ,1 fine green schistose rock, while the quartz and feldspar of 

 the pegmatites have been drawn out into streaky or wavy 

 lines, so as to assume somewhat the appearance of rhyo- 

 lites. Finally a new set of divisional planes has been super- 

 induced on the mass, the strike of which is parallel with 

 that of the plane of thrust. 



Again, there is clear evidence to show that the thrust- 

 plane just described was followed by minor movements of 

 .1 similar nature in the gneiss itself, whereby different 

 portions of the mass were made to slip over each other, 

 i .'ccasionally a thin lenticular mass of the bottom-quartzite 

 has been caught in these planes of disruption. 



But all these evidences of displacement are merely the 

 irecursors of a still more powerful thrust-plane, which 

 lias been traced continuously from the shore east of 

 Whitten Head to the crest of Crag-na-Faolinn, and at 

 ntervals for many miles t.i the southward into the Assynt 

 country. The strike of the strata overlying this plane is, 

 v>n the whole, north-north-east and south-south-west, with 

 ,1 general east-south-easterly dip, usually at comparatively 

 I >w tingles. Though roughly parallel with it, this greater 

 thrust-plane here and there overrides the lower or more 

 westerly one, for the rocks on its upper side may be seen to 

 pass across all the zones of the Silurian series up to the 

 limestones. A recognisable and tolerably persistent order 

 of succession has been observed in the rocks on the upper 

 ■-ide of this thrust-plane. At the base, and resting on 

 different platforms, there usually lies a belt of striped 



-ile schist, followed by green schist with alternations of 

 gneiss, which, though it has lost nearly all trace of its 



original foliation, is probably a portion of the Archaean 

 gneiss. A number of lenticular masses of Silurian 

 quartzite occur on this horizon between the Whitten 

 Head coast and Crag-na-Faolinn. In some cases, the 

 basal breccia and portions of the overlying false-bedded 

 grits are clearly seen resting on the Archaean rock, 

 the planes of foliation of the gneiss being roughly 

 parallel with the bedding of the quartzites. On closer 

 examination, however, it is observable that successive 

 folia of the gneiss impinge against the basal breccia. In 

 other cases, wedges of the false-bedded grits, without the 

 basal breccia, are caught between two thrust-planes. Per- 

 haps the most remarkable example of these isolated 

 masses of Silurian rocks, is the limestone intercalated 

 among the green schists, on the hill-slope above Eriboll 

 House. This mass appears to belong to one of the higher 

 limestones of the Durness Basin which have not elsewhere 

 been noticed in the Eriboll area. It lies not far above 

 the great thrust-plane, and though its relations to the 

 schists are not as well shown as could be desired, its 

 presence here is evidently due to the same series of move- 

 ments that brought in the intercalations of quartzite. 

 Passing eastwards we find, next in order, a belt of frilled 

 green schists (No. 8 in Section) with a well-marked cal- 

 careous zone near the top, which has been traced from 

 the shore east of Whitten Head for a distance of ten 

 miles in a south-west direction, and which extends still 

 further to the south. To these succeed a thin band of 

 compact siliceous schists (No. 9 in Section) overlain by 

 hornblendic and micaceous gneiss, which is succeeded 

 by a great development of gneissoid flagstones (No. 10) 

 with occasional bands of hornblendic and micaceous 

 garnetiferous schists. 



This order of succession in the rocks above the upper 

 thrust-plane is also recognisable far to the west in Sango- 

 more Bay and on Farrid Head in the Durness area. 

 It is evident that there has been an extraordinary 

 amount of movement of these rocks along the upper 

 thrust-plane, since they override all the other rocks pushed 

 forward by the lower thrust-planes in the Eriboll area, 

 and rest directly on the limestones of the Durness Basin. 

 The thin band of shattery quartzite between the scriped 

 fissile schist and the limestone in Sangomore Bay is a frag- 

 ment of the false-bedded quartzite zone which has been 

 pushed forward along the surface of the thrust-plane, — a 

 characteristic feature of the thrust-planes in Eriboll. 



The microscopic characters of the rocks from the dif- 

 ferent zones above the upper thrust-plane have yet to be 

 studied. Much fresh light may thence be expected on 

 the Hindus operandi of the processes involved in the 

 extraordinary lithological changes which the rocks have 

 undergone. Meanwhile a careful examination of the 

 various masses in the field points very clearly to the 

 nature of these processes. The striped green fissile 

 schist which occurs along the thrust-plane presents an 

 exceedingly compact texture with a remarkable streaked 

 structure which at once recalls the fluxion-lines of an 

 eruptive rock. Still more conspicuously is this structure 

 displayed by the masses of pegmatite in the gneiss ; they 

 lose their ordinary character and assume more that of 

 rhyolite. The intercalations of quartzite are marked 

 likewise by the same streaked appearance, their compo- 

 nent particles of quartz and feldspar being all elongated 

 in one common direction. The gneiss associated with the 

 schists above the thrust-plane, though its original foliation 

 can still in places be detected, has had a new set of 

 schistose planes superinduced in it which are on the 

 whole parallel with the thrust-plane. Bands of horn- 

 blendic gneiss merge into hornblende-schist and chlorite- 

 schist, and these again into finely-frilled schists. All 

 these new structures, which are quite independent of the 

 original bedding or foliation of the rocks, were obviously 

 connected with the production of the great thrust-plane, 

 with which they lie more or less parallel. They point to 



