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 Kinlochewe ; 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®^ 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. 
