228 MESSES. PEACH AND HOENE ON THE 



SO characteristic of the region between Assynt and Loch Eriboll. 

 Owing to the admirable exposures on the mountain-slopes and to the 

 well-nigh continuous sections in some of the streams the relations 

 of the strata are clearly displayed. 



§ 2. Geneeal Geological Featuees. 



In this portion of the Dundonnell Porest the basal quartzites rest 

 with a marked unconformability on the Torridon Sandstone, which is 

 admirably defined on the dip-slopes of Sgurr Ban and Ben a 

 Chlaidheimh. Bed after bed of the underlying Torridon Sandstone 

 is transgressed by the pebbly grit at the base of the quartzites. 

 Here, as elsewhere, the evidence plainly demonstrates that, prior to 

 the deposition of the quartzites, the older rocks were elevated, gently 

 inclined, and subsequently worn down to a great plane of marine 

 denudation (see fig. 2, p. 233). 



From the base of the quartzites there is an unbroken sequence, 

 in certain sections, either to the ' Serpulite Grit ' or to the basal bands 

 of the Durness Limestone. At these horizons the fossiliferous 

 strata are truncated by a powerful thrust, which, at Loch an Md, 

 brings forward a slice of Archaean rocks with the Torridon Sandstone 

 and basal quartzites, forming one of the striking geological features 

 of the district. The fragments of Olenellus were discovered in the 

 undisturbed area to the west of this important structural line. 



§ 3. Allt Eigh Ian Section. 



The locality where the fossiliferous dark blue shales in the 

 ' Fucoid Beds' were first observed while mapping this part of the 

 Forest, and where portions of Olenellus were first found by 

 Mr. Macconochie, occurs in a small stream on the eastern side of the 

 valley, joining the Loch an jSTid river about half a mile to the east 

 of Aehneigie. Bising in the drift-clad plateau formed by the 

 eastern schists, the Allt Bigh Ian furnishes, in the lower part of 

 its course, a continuous section of certain members of the Durness 

 Series. From the materials brought down during heavy floods a 

 great cone has been formed at the mouth of this burn on the right 

 bank of the river. 



Joining the burn at the head of this cone, the passage-beds 

 between the basal quartzites and the overlying ' Pipe-Bock ' are 

 exposed. Ascending the section, all the various sub-zones of the 

 ' Pipe- Bock ' ^ are met with, followed in regular order by the ' Fucoid 

 Beds.' Here the burn has cut a small gorge in the brown dolomitic 

 shales and bands of rusty dolomite forming the lower part of this 

 zone. Just above the point where the burn is crossed by the 

 hill-road between Dundonnell and Aehneigie, the upper portion of 

 the 'Fucoid Beds' is seen. Owing to a slight fall in the slope of 

 the ground, the stream has carved for a short distance above the 



^ See Vertical Section II. of Durness Series, Quart. Journ. Geol. Soc. vol. xliv. 

 (1888) p. 406. • 



