The main structural and metamorphic features of the Lewisian rocks of the Outer Hebrides are outlined and compared with those of the Lewisian on the Scottish mainland. In each area three structural zones are recognized, characterized by folding of different styles and periods. The three zones of the Outer Hebrides are closely analogous to the Scourian and Laxfordian zones of the mainland. Evidence of three periods of folding and metamorphism in varying degrees of intensity may be seen in the Outer Hebrides. The first and second periods were separated in time by the intrusion of a non-orogenic suite of basic sills and dykes of tholeiitic affinities, possibly associated with the plutonic centre in South Harris. This basic suite is regarded as equivalent to the Scourie dykes of the mainland. The first period of folding and (granulite-facies) metamorphism is recognized in the gneisses above the Outer Isles thrust-plane and in a narrow belt along the Sound of Harris, and is correlated with the Scourian of the mainland. After the intrusion of the basic suite, the rocks were affected by a second period of folding and granulite-facies metamorphism which converted the basic intrusions to garnet-pyroxene-amphibolites. The Scourie dykes of the mainland retain evidence of this metamorphism in the form of corona garnets, garnetiferous border zones, and clouded feldspars. The second folding and metamorphism is regarded as Early Laxfordian because the folding has the same trend as that accompanying the third and last period of metamorphism, which is a retrograde type associated with migmatization and intrusion of granites and pegmatites and is known from absolute age-determinations to be equivalent to the Lax-fordian of the mainland. The positions of the Lewisian metamorphic and structural zones on the opposite shores of the Minch do not coincide, and the offset can be adequately explained only by later wrench-faulting. Evidence is presented to support the existence of the Minch fault, a sinistral wrench-fault with an approximate displacement of 77 miles, comparable in direction and magnitude with the Great Glen fault.

The metamorphic petrology and history of the Lewisian rocks of South Harris are described. The paragneisses in each of the two main belts are divided into three lithological and possibly stratigraphical units, the disposition of which indicates folding in tight plunging isoclines along Nw–Se (Early Laxfordian) axes. Relic textures indicating an early (pre-Lax-fordian) metamorphism and folding are recognizable in the paragneisses and also in the pyroxene-granulites which appear to intrude the paragneisses. This early metamorphism is probably Scourian in age. The pyroxene-granulites are earlier than and are not comagmatic with the igneous complex. The igneous complex, which is a metamorphosed differentiated (and possibly originally layered) anorthosite–gabbro–tonalite mass, together with the metamorphosed basic intrusions in its immediate vicinity and the widespread metamorphosed basic dykes and sills in the Outer Hebrides, constitute an essentially non-orogenic suite of post-Scourian pre-Early Laxfordian age. The geochemistry of this suite shows it to be tholeiitic in character and probably closely related to the Scourie dykes of the Scottish mainland. Early Laxfordian granulite-facies metamorphism associated with the main Nw–Se folding of the area followed the emplacement of the non-orogenic igneous suite. A south-easterly plunge and north-easterly overturning characterize the folds to the north-east of the igneous complex; to the south-west the plunge is north-west, and the overturning is to the south-west. A Late Laxfordian injection-complex situated to the north-east comprises extensive areas of migmatization and granitic intrusion and is associated with a front of hydrous retrograde metamorphism which strongly affects the Langavat paragneisses and parts of the igneouscomplex. The sinuous trend of the Langavat paragneisses is attributed to cross-folding of thisdate. All the earlier rock-types are cut by a suite of Late Laxfordian granite pegmatites containing uranium, thorium, and rare-earth minerals.

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The central zone of the Outer Hebrides is composed of a gneissic (‘grey-gneiss’) complex containing evidence of the earliest as well as many of the later events in the history of the Lewisian. In northern South Uist, an old metamorphic complex is intruded by pegmatites which were strongly deformed by a tectogenesis equated here with the Scourian of more than 2600 m.y. ago. It is concluded that much of the original material of the greygneiss may represent the products of a pre-Scourian orogenic cycle of possible Katarchaean (>3000 m.y.) age. Widespread post-Scourian basic (tholeiitic) dykes which cut the grey gneiss are thought to be equivalent to the Scourie dykes of the Scottish Mainland. These intrusions were subjected to granulite-facies metamorphism, regarded as Early Laxfordian, followed by retrogressive amphibolite-facies metamorphism with local folding and migmatization, all of Late Laxfordian date (1500 to 1600 m.y.). A number of dykes described from Benbecula and South Uist illustrate stages of Laxfordian deformation in which perfectly preserved cross-cutting relationships are progressively obliterated, the final products in the southern zone of South Uist being highly deformed and disrupted amphibolite masses with concordant margins in mobilized Laxfordian migmatitic gneiss.

Sills of olivine-microgabbro and olivine-dolerite 0.5–180 m thick intrude Lower Liassic (Hettangian–Sinemurian) sediments in the Fastnet Basin, a SW extension of the N Celtic Sea Basin, about Lat. 50°N, Long. 10°W. The evidence of 3 wells and 2300 km of seismic reflection profiles indicates 6 major sills and sill-complexes, and 3 possible igneous centres or plugs. A high-level sill may intrude Lower Cretaceous sediments, and one plug apparently deforms Cretaceous reflectors. K-Ar ages on separated biotites from a thick sill intruding Liassic sediments gave 170 ± 4 Ma (Bajocian). This sill contains 30% olivine (FO 70-73 ), 20% clinopyroxene (magnesian salite), 45% plagioclase (An 60-78 ), &, with accessory magnetite, ilmenite, edenitic hornblende, Ti-phlogopite, apatite, zircon, spinel (in olivine) and sulphides. Alteration to serpentine minerals, chlorite, sericitic mica and leucoxene is sporadic. The magmatism in the Fastnet province thus appears to have been active from the mid-Jurassic to the early Tertiary, related initially to the time of initial rifting between the European and American plates, and subsequently to their final separation between Greenland and Rockall and associated Thulean magmatism.