OLENELLFS ZONE IN THE NORTH-WEST HIGHLANDS. 231 



of Serpulites MaccuUochii (Salterella) which are readily seen on the 

 weathered surfaces. These beds forming the base of Group I. of 

 the Durness Limestone Series ^ are truncated by the thrust traversing 

 Allt E-igh Ian, bringing forward a wedge of Torridon Sandstone 

 which is here only 15 feet thick. 



§ 5. Sections neae Loch an Nid. 



The locality where the trilobites were found in the zone of the 

 * Serpulite Grit ' is near Loch an Nid, about eight miles to the K.N.E. 

 of Loch Maree. 



Por nearly two miles to the north of Loch an Nid, the quartzites 

 form prominent escarpments on the eastern side of the valley and 

 rise to a height of several hundred feet above the level of the stream. 

 Ascending the rocky slope to the horizon of the highest zones of the 

 ' Pipe-Eock ' the observer finds a narrow belt of bright-green grassy 

 ground, formed by the surface-soil of the decomposing ' Fucoid Beds ' 

 and partly covered with morainic matter. The overlying ' Serpulite 

 Grit' gives rise to a more or less continuous escarpment, disap- 

 pearing at intervals underneath a thin coating of drift or surface- 

 soil. At this level the attention is at once arrested by a conspicuous 

 geological feature, forming one of the great structural lines on the 

 eastern side of the valley. By means of a powerful thrust a slice of 

 Archaean rocks, covered unconformably by the Torridon Sandstone 

 and basal quartzites, is made to overlie the members of the Durness 

 Series. This line of disruption, which is the northern continuation 

 of the well-known thrust-plane in Glen Logan near Kinlochewe, can 

 be traced with remarkable precision along the mountain -slope east 

 of the Loch an Nid river. The materials immediately overlying 

 the thrust-plane form a prominent cliflP, over which, after a heavy 

 rainfall, numerous rivulets leap on to the members of the Durness 

 Series. Many of these streamlets are not indicated on the six-inch 

 Ordnance map, but they are of especial importance, on account of 

 the excellent sections of the strata close to the line of displacement. 



Immediately to the north of Loch an Md there are three small 

 streams traversing this escarpment of disrupted gneiss. The first 

 of these is about 250 yards distant from the northern shore of Loch 

 an Nid ; after crossing the Glen Logan thrust-plane it descends a 

 grassy slope to the underlying quartzites, joining the river below a 

 fine waterfall near the mouth of the loch. As this stream is 

 indicated on the six-inch Ordnance map, it forms a base-line for 

 measuring the distance to the section which now falls to be 

 described. 



About 200 yards north of the above-mentioned rivulet there is a 

 small section of especial interest at the base of the escarpment of 

 displaced Archaean gneiss. It traverses the ' Serpulite Grit ' and 

 the top of the ' Fucoid Beds,' the underlying strata being concealed 



^ See our Eeport on the Geology of the North-west of Sutherland, ' Nature,' 

 vol. xxxi. p. 31. 



Q. J. G. S. No. 190. R 



