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LEWI SI AN GNEISS 
115 
group, arc placed togetlier for purposes of description. Tlicy arc all foliated, 
some liat-ing tlie aspect of mica-schists, others being typical augen-gneisseJ^, 
or liglit grey gneisses with abundant oligoolase and inclusions of microlitic 
epidote. 
The rocks of each of these types are usually restricted to relatively 
small areas, and they succeed each other with much irregularity all the way 
from Skye to Cape Wrath. Their chemical and mineralogical composition 
proves them to have' decided aiiiuities with the plutonic igneous masses of 
the earth’s crust. 
The only exceptions to this prevalent igneous type occur in the 
districts of Gairloeh and liOch Carron, where the gneiss appears to be 
associated with a group of mica-schists, graphitic-schists, quartzites and 
siliceous graiiulites, limestones, dolomites, chlorite-schists and other schists. 
That these are altered sedimentary formations can hardly be doubted. 
What their precise relations to the fundamental complex of the gneiss may 
be has not yet been satisfactorily determined. They are certainly far older 
than the Torridon sandstone which covers them uuconformably. Possibly 
they may represent a sedimentary formation still more ancient than the 
gneiss. 
Save these obscure relics of a pre-Torridouian system of strata, tbe 
gneiss never presents any structure which suggests the alteration of clastic 
constituents. Everywhere its mineral composition points to a connection 
with the sul)terrauean intrusions of different igneous magmas, while the 
nianuer in which its different rock-groups are associated together, and the 
internal structure of some of them, still further link it with phenomena 
which will be described in succeeding chapters as parts of the records of 
volcanic action. 
An interesting feature of the fundamental complex, as bearing on the 
origin of the gneiss, is to be found in the occurrence of bosses and bands 
which are either non-foliated or foliated only in a slight degree. These 
Comparatively structureless portions present much of the character of bosses 
or sills of true eruptive rocks. They occur in various parts of Sutherland 
O'Od Ross. Their external margins are not well defined, and they pass 
insensibly into the ordinary gneiss, the dark basic massive rocks shading- 
off into coarse basic gneisses, and the pegmatites of quartz and felspar 
which traverse them merging into bands of grey quartzose gneiss. 
So far, therefore, as present knowledge goes, the main body or funda- 
niental complex of the T.ewisian gneiss in the North-west Highlands of 
Scotland consists of what may have been originally a mass of various 
eruptive rocks. It has subsequently undergone a succession of deformations 
from enormous stresses within the terrestrial crust, which have been 
investigated with great care by the Geological Survey. But it presents 
structures which, in spite of tlie abundant proofs of great mechanical 
deformation, are yet, I venture to think, original, or at least belong to the 
time of igneous protrusion before deformation took place. The alternation 
of rocks of different petrographical constitution suggests a succession of 
