OLENELLUS ZOTS^E IN THE NORTH-WEST HIGHLANDS. 



229 



mountain-path a broader channel, which is partly filled with gravel, 

 particularly after a heavy rainfall. Por this reason the higher 

 members of the zone are not so well exposed. 



At this part of the section the attention of the observer is at once 

 arrested by two prominent bands of dark blue shale, intercalated 

 in the normal dolomitic beds of the zone. The upper band is about 

 3 feet and the lower one about 9 feet from the top of the ' Eucoid 

 Beds.' Some of these dark blue shales are slightly calcareous and 

 are traversed by small worm-casts. The fragments of Olenellas 

 were found in the lower band, the best specimens being confined 

 to a seam less than an inch thick. 



Fig. 1.- 



w. 



■Section in AJlt liigli lan^ from the base of the ' Pij)( 

 to the Moine thrast-plane. 



Hill -road 

 to Dundonnell. 



-Rock ■ 



E. 



[Length of section=400 yards.] 



2. Thrust Torridon Sandstone. 



4. 'Pipe-Rock.' 



5. ' Fucoid Beds.' 



5'. Olenelhcs Zone in ' Fucoid Beds.' 



T. Glen Logan Thrust. 



6. ' Serpuhte Grit.' 



7. Basal Bands of Durness Limestone. 

 10. Eastern schists. 



T^ Moine Thrust. 



The upper limit of the ' Fucoid Beds ' is well defined, the base of 

 the ' Serpulite Grit ' forming a small cascade over which the stream 

 leaps on to the softer beds below. Along certain lines, the higher 

 zone, which here consists of a massive grit, is crowded with >S'gr^^^/?Y65 

 MaccuUochii (SaltereUa) in splendid preservation. 



From the base of the ' Pipe-llock ' to the highest visible portion 

 of the ' Serpulite Grit,' the strata have a persistent dip to the 

 south of east at an average angle of 16"^ ; the total thickness of beds 

 amounting to 400 feet. 



The normal sequence of the Durness Series is here interrupted by 

 a powerful thrust, already referred to as one of the striking geological 

 features of the district, which brings forward a thin wedge of 

 Torridon Sandstone, through which the stream has cut a small gorge. 

 Though changed in tint and considerably crushed, this mass can be 

 readily identified after a careful study of the geological structure of 

 the region. When traced southwards in the direction of Loch an 

 !Nid, this slice of Torridon Sandstone increases in thickness and is 

 seen to rest unconformably on undoubted Archaean rocks. Within 

 a short distance to the east, in Allt High Ian, the crushed Torridon 



