232 



MESSRS. PEACH AND HORNE ON THE 



by materials carried down during heavy floods and by morainic 

 matter. The following vertical section, measured with an Abney 

 level, is exposed in the small escarpment cut by this streamlet. 



feet, inches. 

 "Grey quartzose grit 18 — • 



Flaggj^ grits, with intercalated shales 7 — 



Zoneof'Ser- .^^^{j^^^l^^, ; ..,••,•........-■••. 1 - 



vj. p -4- ' < Dark blue shales containing C^ewem^s 8 



pu 1 e ri . J Qj-ey grit 2 — 



1 Dark grey shales 1 6 



l^Grey quartzite 7 3 



Top of ' Fii- f Brown dolomitic] shales, with bands 

 coid Beds.' \ of dark shales 



In the dark-blue fossiliferous shales we obtained a head-shield and 

 other fragments of Olenellus in fine preservation. These shales are 

 underlain by grey grits and quartzites forming the lower portion of 

 the escarpment of the ' Serpulite Grit.' When traced southwards 

 along the hill-slope, the upper and lower ledges of quartzose grit 

 coalesce and form a prominent escarpment overlying the ' Fucoid 

 Beds.' 



About 100 yards to the south of the foregoing section, a fragment 

 of Ole7iellus was found by Mr. Macconochie in a dark shale in the 

 ' Fucoid Beds,' exposed in a streamlet not far below the cliff of dis- 

 rupted gneiss. 



For nearly | mile to the north of Loch an ISTid, the ' Serpulite Grit ' 

 is overlain by a few feet of the basal bands of the Durness Limestone, 

 forming the lower plane or ' sole ' of the Glen Logan thrust. The 

 limestone is not visible in all the streamlets within the foregoing 

 limits, but where it is exposed and not much altered by the displace- 

 ment of the overlying materials, serpulites (Salterella) occur in fine 

 preservation. 



The geological structure of the disrupted masses overlying the Glen 

 Logan thrust-plane along this portion of the Loch an Nid Craig is 

 of absorbing interest. But the description of these complications 

 is foreign to the main object of this paper. It is sufiicient for our 

 present purpose to state that these displaced masses are abruptly 

 truncated at a higher level by the Moine thrust, which forms the 

 second great structural feature of this magnificent crag. 



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

 the dundonnell forest, with their prolongations to the 

 north and south oe that region. 



The discovery of Olenellus in the ' Fucoid Beds ' and ' Serpulite 

 Grit ' suggests certain interesting questions regarding the probable 

 variation of the subdivisions of the Durness Series along the belt 

 between Loch Eriboll and Strome Perry. One striking result of 

 the tracing of the various zones from Sutherland southwards into 

 Hoss-shire is the remarkable persistence of the sub-zones identified 

 in Assynt and at Loch Eriboll. 



