THE OLENELLtrS ZONE IN THE NORTH-WEST HIGHLANDS. 227 



17. The Olenelltjs Zone in the North-west Highlands of Scotland. 

 By B. jS". Peach, Esq., F.E.S.E., F.G.S., and J. Horne, Esq., 

 F.R.S.E., F.G.S. (Eead Eebmary 10th, ]892.) 



[Communicated by permission of the Director- General of the 

 Geological Survey.] 



[Plate V.] 



Contents. 



Page 



§1. Physical Features of the Dundonnell Forest 227 



I 2. General Geological Features 228 



I 3. AUt Righ Ian Section 228 



§ 4. Sections north of Allt Righ Ian 230 



§ 5. Sections near Loch an Nid 231 



§ 6. Comparison of the Zones of the Durness Series, exposed in the 

 Dunclonnell Forest, with their Prolongations to the north 



and south of that region 232 



§ 7. Conclusions drawn from the Discovery of Olenellus 23.5 



I 8. Description of Fossil Eemains 235 



While tracing the various subdivisions of the Durness Series of 

 quartzites and limestones from Sutherland southwards into Ross- 

 shire, careful attention has been paid to any indications of f ossiliferous 

 zones which might throw additional light on the age of the strata. 

 During last season's campaign, certain sections in the Dundonnell 

 Forest happily furnished the evidence which has been eagerly sought 

 after. 



§ 1. Physical Features oe the Dundonnell Forest. 



Between Little Loch Broom and Loch Maree the members of this 

 series traverse one of the wildest tracts in the west of Eoss-shire. 

 The Dundonnell Forest lies mainly to the south of the head of Little 

 Loch Broom, stretching southwards by An Teallach (3483 ft.) to 

 Loch an Nid and Creag Bainich (2646 ft.). The southern portion 

 of the Forest is drained by the Loch an Md river, which, iinding 

 its source in the loch of that name, flows northwards for about 

 three miles towards the shooting-lodge of Achneigie. The river is 

 bounded on the east for part of this distance by a precipitous crag 

 (1000 ft. high), and on the west by the long dip-slopes of quartzite 

 and Torridon Sandstone of Sgurr Ban (3194 ft.) and Ben a 

 Chlaidheimh. Near Achneigie the river bends towards the north- 

 west, soon pouring its waters into Loch na Sheallag. From this 

 loch issues the Gruinard river, discharging into Gruinard Bay. 



The sections from which the fragments of Olenellus were obtained 

 occur along the eastern slope of the valley drained by the Loch an 

 Nid river, between Loch an Nid and Achneigie. Though there is 

 here conclusive evidence of the continuation of those terrestrial 

 displacements described in a former communication to this Society,^ 

 the area is comparatively free from those extreme complications 



^ ' Report on the Eecent Work of the Geological Survey,' &c., Quart. Journ. 

 Geol. Soc. vol. xliv. (1888) p, 378. 



