The ultrabasic rocks of the southern portion of the Early Tertiary Cuillin Igneous Complex, Isle of Skye, are recognised as forming a Peridotite Series s.l. and have been separated into six distinct structural–lithological units. These units range from almost pure dunite (Unit 1, at the lowest structural level), through to feldspathic peridotites and allivalites (Units 5 and 6, at the highest structural levels). Detailed field and mineralogical studies indicate that both cumulus and postcumulus processes involving ultrabasic (picritic) magmas may be identified, and that the latter processes have significantly modified many of the primary features of these rocks.

Layering, both modal and phase, is present within all six units, although it is more prominent within the higher units, especially Units 5 and 6. Differing orientations of fabrics defined by cumulus spinel and intercumulus plagioclase layers within Unit 3 indicate the important role of compaction and intercumulus melt migration. Unit 4 is extremely heterogeneous, involving material ranging in composition from peridotite to allivalite, and provides clear evidence for postcumulus melt movement, magma-mixing, disruption and brecciation. Units 5 and 6 developed with a more porous cumulus framework, giving rise to dendritic growths involving cumulus olivine and poikilitic plagioclase.

It is concluded that postcumulus melt movement, injection and magma-mixing, involving ultrabasic magmas, were significant processes in the formation of the ultrabasic rocks of the Cuillin Igneous Complex.

Detailed mapping of the Trallval-Ruinsival area of the ultrabasic complex of Rhum has revealed the relationship between the intrusive peridotites, which form a major part of the complex, and the layered ultrabasic rocks. The layered rocks on Trallval are correlated with Units 8 to 15 of the Eastern Layered Series as seen on Hallival and Askival, and part of an additional unit, Unit 16, has also been identified. The layered rocks of Ruinsival and the western part of Trallval form part of the Central Series and have been subdivided into six cyclic units. They are associated with four sets of intrusive breccia zones which converge towards a central region. Four cylindrical plugs of intrusive peridotite occur within the area of outcrop of the Central Series. Criteria are listed for the distinction of intrusive peridotite from conformable peridotites formed in situ. Distinctive textures and structures are described; these include vertical feldspathic streamers, which are interpreted as direct evidence for the upward expulsion of interstitial fluid, and loading structures at the contact between a peridotite layer and underlying allivalite are ascribed to the intrusion of a peridotite sill. A comparison of the Eastern Layered Series and the Central Series suggests that three stages were involved in the formation of the ultrabasic rocks: (i) the accretion of rhythmically layered unconsolidated olivine cumulates; (ii) the upward expulsion of intercumulus liquid; and (iii) the formation of either allivalite (as in the Eastern Layered Series) or ultrabasic breccia zones (as in the Central Series) from the expelled intercumulus liquid, depending on whether the tectonic environment was quiescent or extensional.