234 BATHYMETRICAL SURVEY OF 



by the post-Cambrian movements there are masses of displaced 

 gneiss, of which the most important lies immediately to the north 

 of Kiiilochewe; others appear further north on Mullach Coire Mhic 

 Fhearchair, and far to the south on Glas Bheinn, on Torr na h-Iolaire, 

 and at Coulags in Glen Carron. Over much of the region they form 

 lofty ground and give rise to prominent peaks, as, for example, Beinn 

 Lair (2817 feet), Beinn a' Chaisgein Mor (2802 feet), and Beinn Airidh 

 Charr (2593 feet), all north of Loch Maree. 



Throughout this area there is a remarkable development of those 

 types of Archaean rocks that have affinities with plutonic igneous 

 products, consisting mainly of massive and foliated, pyroxenic, horn- 

 blendic, and micaceous gneisses. Along the northern margin of this 

 district, between Loch na Sheallag and Gruinard Bay, the original 

 characters of the rocks that enter into the fundamental complex are 

 well displayed. The various stages in the separation of the ferro- 

 magnesian from the quartzo-felspathic constituents, and the gradual 

 development of mineral banding in the massive gneisses are there clearly 

 shown. In that area, also, the intrusive character of the basic dykes 

 traversing the gneiss in a west-north-west direction is proved beyond all 

 doubt. Passing southwards to the tract lying south of Poolewe, both 

 the gneisses and the intrusive dykes have been thrown into an anticlinal 

 fold, which is represented on the Geological Survey Map (Sheet 91). 

 Here we find that, under the influence of mechanical stresses, there 

 has been differential movement of the rock constituents, and linear 

 foliation has been developed in the basic dykes the foliation being 

 parallel with the pitch of the folds. Further south in the Torridon 

 district biotite gneisses prevail, which are traversed by bands of horn- 

 blende-schist representing the original basic dykes. 



Of special interest is the development of crystalline schists, that 

 have affinities with rocks of sedimentary origin, north of Loch Maree 

 and near Gairloch. The prominent members of this series are quartz- 

 schists, mica-schists, graphitic-schists, limestones, and dolomites, with 

 tremolite, garnet, and epidote, which are there associated with a massive 

 intrusive sheet of hornblende-schist. Lithologically some of these 

 crystalline schists closely resemble the altered sediments in the Eastern 

 Highlands. The quartz-schists, mica-schists, and limestones are well 

 exposed in various folds between Letterewe and Glen Tulacha, west of 

 Lochan Fada, pierced by the great sill of hornblende-schist forming 

 Beinn Lair and Beinn Airidh Charr (B G on map). The original 

 relations of these altered sediments to the gneisses that have affinities 

 with plutonic igneous rocks have been obscured by subsequent earth 

 stresses. But along their outer margin they are bounded by gneiss 

 apparently underlying them, and they are visibly overlain by gneiss 

 with basic dykes, the whole series being affected by a common system of 

 folds. 



