2. FINSBAY HORNBLENDIC GNEISSES AND QUARTZITES
Overall nature of the unit: This rock unit comprises a variety of rock types which are more or less hornblendic. Some of the rock types are almost certainly metasedimentary while others may have been igneous in origin. The unit has been subdivided into the following three groups of rock types:
(a) Amphibolites
Lithology: These rocks consist almost entirely of an amphibole, usually hornblende, with minor plagioclase, quartz and biotite. In hand specimen they are blackish green (hornblendic) or bright green (actinolite rich) with amphiboles up to 8mm long, in radiating clusters when these are actinolites.
Distribution and origin: Such pure amphibolites have a very small areal extent, and most of the exposure consists of the hornblendic variety. Only one outcrop of the actinolite-rich variety was observed, this being on the northern side of the Finsbay - Leverburgh road 700 yards ESE of the southern end of Loch Langavat (therefore just outside the area as defined in Part I).
Often extreme banding in the 'hornblendic gneisses' (discussed below) results in a hand specimen from one part of an outcrop consisting of virtually pure amphibolite, while that from another part of the outcrop consists of virtually pure quartzite. Such bands of amphibolite were observed to be 2 - 650mm wide. Such banding on an even larger scale may result in the observed outcrop-wide exposures of amphibolite.
Some of these amphibolites were probably originally dykes or lava flows which were more or less contemporaneous with sedimentation.
(b) Hornblendic gneisses
Lithology: These rocks are generally well banded, with a parallel orientation of blackish green hornblendes (1 — 3mm long) and black biotites (average length 0.5 - lmm) and a high proportion of light coloured plagioclase and quartz.
Examination of a thin section of a typical rock of this group, collected 950 yards south east of Loch Allityer, showed it to consist of 41% plagioclase, 32% hornblende, 20% quartz, 5% biotite with the remainder mostly magnetite-ilmenite. The plagioclase is virtually unaltered and the quartz was probably a primary mineral. Variations of rock type within this group are principally due to variations in the relative proportions of hornblende and. quartz. To the east of the C79 road such variations are less distinctive in hand specimen than they are to the west of this road. As a result of quartz content increasing, rocks of this group grade into quartzites (discussed below).
Origin: These gneisses are probably metasedimentary in origin, though it is probable that some of the more hornblendic types resulted from the metamorphosis of basic igneous rocks.
(c) Quartzites
Lithology: Rocks of this group vary between very quartz-rich hornblendic gneisses, through 'sandy' quartzites to a massive orthoquartzite. The latter is exposed as a 10 foot thick band on the north eastern side of Cnoc an Sgumain, near Finsbay (figure 3 - NOT AVAILABLE).
The 'sandy' quartzites are the most typical of this group, and in hand specimen they are slightly yellowish fine-grained rocks (grain size lees than 0.5mm), compared to the massive orthoquartzite which is white and relatively coarse—grained (average grain size about 1mm), but there is a range of fairly pure quartzites between these types.
Origin: The quartzites are clearly metasedimentary in origin, with the 'sandy' quartzites representing more feldspathic sandstones than the pure orthoquartzite. The more hornblendic 'quartzites' were probably less quartz-rich psammites originally.
Field relationships of the unit: Figure 3 (NOT AVAILABLE) shows field relationships of rock types within this unit in the Loch an Sgumain - Cnoc an Sgumain area. Here a traverse 'down dip' shows a sequence of rock types that may be due to preservation of a part of the sedimentary succession. However metamorphism and deformation have probably obscured the original sedimentary successions in most of the area.
Age of the unit: This unit is probably older than the 'igneous complex', since the contact of the latter apparently cross-cuts the quartzite - hornblendic gneiss banding (probably closely related to the original sedimentary succession) in the Strath Leetein area. The unit is undoubtedly older than the intrusion of the granite pegmatites, since the latter cut the foliations of rocks of this unit, and one pegmatite vein includes a block of deformed hornblendic gneiss.