Lewisian Gneiss Analysis

The relationships between the lithologies in the Lewisian Gneiss complexed can be obsereved best at Loch na Fiacail, NC 233 486 (Field Notebook page 42).
There are three main lithologies found which are the Lewisian gneiss, amphibolite and pegmatite. The gneiss was wasformed first as it is cross cutted by bothe the pegmatite and the amphibolite. The amphiboliote mafic protolith was intruded next and the youngest event was the intrusion of the pegmatite which cross cuts all the ither lithologies.
Gneiss
The gneiss are generally dipping to the noth-east. The are distributed mostly to the north of the mapping area. There is a small area of gneiss south east of the mapping area which is not as well exposed.
The Lewisian gneiss can be subdivide

It is heterogenous as it is massive but also has areas which show relict igneous textures. The ophitic texture between pyroxene and plagioclase feldspar ( the former enclosing the latter) shows that the pyroxene crystallised from the magma before the plagoclase feldspar. The presence of hornblende in the rock shows retrogression from granulite facies to amphibolite facies The hornblende retrogresses from the pyroxene* .
Pegmatite
Pegmatite is quartzo-feldspathic in composition and has a homogenous pegmatitic texture. The thickness of the pegmatite varies (pinches and swells), it ranges from mm-scale to m-scale.
Torridon Sandstone
Most of the data for this strata is collected in the north-west direction of the map as this is where it is best exposed. The bedding planes are shallowly dipping to the south. The thickness of the strata decreases from the south-west to the north-east of the map.
There are two main lithologies in this group.
Breccia
The breccia is polymictic and extraformational with fragments of the Lewisian Gneiss. It is in direct contact with the Lewisian gneiss at the unconformity .
Arkosic

The Glencoul Thrust uplifted the Lewisian gneiss complex upwards to the west on top of their cover rocks. [M. Coward et al, 1980]. The pipe rock is on the downthrown side of the fault.
The Sole Thrust uplifts Basal Quartzite onto the top of the cambro-ordovician rocks, [M. Coward, 1985].Fucoid Beds are on the downthrown side.
This also creates an unconformities between the Basal Quartzite and the Lewisian Gneiss and between the Basal Quartzite and Fucoid Beds.
There is an angular unconfomity (non-conformity) between the Lewisian Gneiss and the Torridon sandstone. The gneiss dips vertically while the Torridon sandstone is relatively horizontal (shallow dip).
There is an angular unconformity between the Torridonian Sandstone and the basal quartzite. The Basal quatzite oversteps (truncuates stratigraphic boundaries along the unconformity surface [Lovely, 1948]) both the Lewisian Gneiss and the Torridonian sandtone.
The change in thickness of the pegmatite in the Lewisian Gneiss Complex is due to a process called boudinage which forms boudins . It occurs by extension and deformation of competent