3050 dead molluscan shells and fragments of calcareous algae were collected from 104 benthic stations at depths from 2 to 160 m in the Firth of Clyde, the Firth of Lorne and on the Orkney shelf, British Isles. The following micro-algal borings were recognised and are described: (i) Rhodophyta: Conchocelis, (ii) Chlorophyta: Ostreobium quekettii, Eugomontia sacculata, Phaeophila sp. and (iii) Cyanophyta: Plectonema terebrans, Hyella spp.

These endolithic algae are bathymetrically zoned. Eugomontia, Phaeophila and the cyanophytes characterize the upper part of the photic zone (2–20 m). Shallow sheltered environments are typified by a high density of algal borings, most of them aligned perpendicular to shell surfaces. The lower photic zone is characterized by Ostreobium and Conchocelis whose borings are parallel to shell surfaces. The photic limit is about 40 m for the open Orkney shelf (59°N), and about 22 m for the partly enclosed firths (56°N).

The Orkney population is dominated by two chlorophyte taxa: rhodophytes and cyanophytes are absent. This may be due to higher tidal energy conditions.

The existence of a basement of granulite beneath the Midland Valley is supported by investigations of inclusions in volcanic rocks and the geophysical studies of the LISPB experiment. To establish age constraints for this basement, a compilation is presented of available Rb–Sr whole-rock, common lead, U–Pb zircon and Sm–Nd radiometrie data for crystalline rocks in Scotland from the earliest recognised crust (c. 2900 Ma) to 380 Ma (“end” of Caledonian orogeny) including xenoliths in volcanic vents and boulders in conglomerates.

For rocks within the Midland Valley, isotopic data provide four lines of evidence. (1) An upper intercept U–Pb age of c. 1700 Ma for detrital zircons from a lower Palaeozoic greywacke from Dalmellington corresponds to a late stage of the Laxfordian orogenic episode (early Proterozoic) with possibly some overprinting during the Grenvillian episode (mid Proterozoic). (2) The common lead composition of the Distinkhorn granite suggests the participation of early Proterozoic basement during granite emplacement. (3) For xenoliths from the Carboniferous Partan Craig vent, one gives a Sm–Nd CHUR model age of 1180 ± 55 Ma, a second yielded a Sm–Nd garnet—potassium feldspar age of 356 ± 6 Ma and an upper intercept U–Pb age from zircons from the third is c. 2200 (± 240) Ma; for xenoliths from other vents, an Rb–Sr whole-rock isochron of 1101 ± 63 Ma and an Sm–Nd model age of c. 1100 Ma arerecorded. (4) A linear array corresponding to an apparent age of 770 ± 180 Ma on a Pb–Pb isochron diagram for Tertiary igneous rocks of Arran points to an underlying basement of late Precambrian orthogneiss.

The existence of basement made of products of the Grenvillian episode, or predominantly so, similar to the basement N of the Highland Boundary fault, is not inconsistent with the available evidence. However, zircons and other rock components appear to have an ultimate Lewisian provenance. At least in parts, there is also a strong late Proterozoic imprint. Further studies are required for an unequivocal solution.

Drepanophycus spinaeformis Göppert is a long ranging Devonian plant. Although reported from numerous localities, it is far from well understood. Observations on specimens from the Emsian Strathmore Group of Scotland have revealed variation in forms of branching including root-like axes on branches interpreted as rhizomes. The anatomy of the xylem is described from uncompressed pyrite permineralisations. Cuticle characteristics are also described and discussed. The systematic position of the plant is debated since, although the morphology of the leaves, leaf traces, roots and xylem indicate an affinity with the lycopods, there is some doubt as to whether sporangial position permits inclusion in this group.

Ten silicified brachiopod taxa have been recovered from the Ben Suardal Limestone Formation (Durness Group) on the Isle of Skye, western Scotland. This Arenig fauna contains a new camerellid genus, Boreadocamara, and new species of each of the orthacean genera Archaeorthis, Fasciculina, and Oligorthis. Representatives of Billingsella, Finkelnburgia, Syntrophia, Syntrophopsis, and an indeterminate orthacean have also been recovered from etched residues of the Ben Suardal Limestone. The Skye fossils show strong affinities with the Scoto-Appalachian or American faunal province, in keeping with the inferred plate tectonic position of the Durness Group along the northwestern margin of the early Palaeozoic ‘Iapetus’ Ocean.

Late Palaeozoic alkalic basalts in and around the Midland Valley of Scotland contain a wide variety of ‘plutonic’ xenoliths. Pyroxene-rich ultramark xenoliths (wehrlites, clinopyroxenites and garnet pyroxenites) may be representative of younger components within a dominantly peridotitic upper mantle represented by ubiquitous magnesian peridotite xenoliths. Glimmerites and other biotite-rich ultramafic xenoliths are probable samples of metasomatised upper mantle facies.

Xenoliths composed mainly of plagioclase, clinopyroxene ± orthopyroxene ± magnetite are widespread. These pyroxene granulites may typify the lower crustal layers. Garnet granulites are rare; such rocks may formerly have been important with loss of garnet occurring through retrograde metamorphism. Anorthositic xenoliths are relatively common. The lower crust may consist largely of anhydrous rocks, of gabbroic to anorthositic composition, ccurring as stratiform bodies of metacumulates.

Other xenoliths of igneous origin include tonalitic and trondhjemitic gneisses. Although these may play some role in the lower crust, they may be more abundant in the mid-crustal domains underlying the deformed upper Precambrian and lower Palaeozoic supracrustal strata. Xenoliths of quartzofeldspathic, granulitic gneisses containing garnet ± sillimanite ± rutile are also of widespread occurrence; many of these are of metasedimentary provenance and are regarded as being derived from the mid-crustal layers beneath the Southern Highlands, Midland Valley and Southern Uplands and their Irish counterparts.

Islay and Colonsay occupy a crucial position in the Caledonian frontal zone S of the Great Glen fault in western Scotland. However, some of the stratigraphical units which comprise these islands are of doubtful age as they have tectonic boundaries and contain no fossils or distinctive lithological components. New information is presented here on the deformational histories and styles of the rocks and this has prompted a reassessment of their stratigraphical position and a major reinterpretation of t0he tectonism of this frontal zone.

In contrast to the conventional view, all the stratigraphical units have been deformed together during a regional shearing event. The Colonsay Group contains intrafolial and sheath folds and has a pronounced linear fabric which is parallel to grain elongations in parts of the Bowmore and Dalradian sequences. Most junctions between the Colonsay Group and basement of the Lewisian complex are highly sheared. The junction between the Bowmore Group and Dalradian Supergroup is also tectonised and is normally referred to as the Loch Skerrols thrust but here as a shear zone. This structure is shown not to be a profound discontinuity, alone representing the Caledonian front S of the Great Glen fault, but one element in a major flat-lying NW-directed ductile shear zone. The Caledonian front therefore lies to the W of the islands.

The Bowmore Group is suggested to be a lateral equivalent of the Crinan Grits in the Dalradian Supergroup, lying in the overturned sheared limb of the Islay anticline. The age of the Colonsay Group remains enigmatic; the identification of previously unrecognised early structures which may precede shearing in the Grampian episodes of the Caledonian orogenic cycle allows the possibility of a pre-Caledonian deformation of the Group.

Twelve localities of Lower Carboniferous strata in Scotland (Loch Humphrey Burn, Glenarbuck, Pettycur, Oxroad Bay and the Berwickshire localities of Cove, Burnmouth, Gavinton, Edrom, Foulden) and in France (Esnost, Roannais, Montagne Noire) have been investigated with particular regard to their anatomically preserved floras. New data on the composition and preservation of the assemblages, their geological setting and stratigraphical age, using palynology in particular, are presented. Present data suggest that four successive groups of floras obtained from these localities can be recognised: from the Montagne Noire (mid to late Tournaisian), from the Berwickshire localities and Oxroad Bay (late Tournaisian to early Viséan), from Loch Humphrey Burn and possibly Glenarbuck (mid Viséan) and from Roannais, Esnost, and Pettycur (late Viséan). The similarities and differences between the floras are discussed with particular emphasis on stratigraphical as opposed to ecological controls. Comparisons are made with the New Albany Shale floras of the U.S.A., the Saalfeld and Glätzish-Falkenberg floras from the German Democratic Republic. The data suggest that the ferns and pteridosperms show the most striking changes through the Lower Carboniferous strata.

Twenty-seven species assigned to 20 genera of trilobites are described from Feilaishi in Guizhou Province, the type section of the Shihtzupu Formation in S W China. They occur in association with a sparse graptolite fauna including Glyptograptus teretiusculus. Eleven taxa are recorded here for the first time. Much new morphological information is provided regarding previously known species and 3 lectotypes are selected. The trilobites are largely endemic and indicate a quiet and comparatively shallow water environment

Charnockite and homophanous leptynite are two quartz–feldspar neosomes in the late Archaean granulite-migmatite Eastern Ghats terrane of Orissa, India. Three phases of deformation and metamorphism are recognised to have preceded a late phase, or phases in which deformation was not intense. An early granulite facies event (D1–M1) produced basic granulites and khondalite (sillimanite-garnet aluminous paragneiss). A second event (D2–M2) involved some retrogression but temperatures and pressures remained high (possibly c. 750°C—>5 kb). D3–M3 was associated with the development of open folds (F3) with axial planar shear zones in pinched antiforms; temperatures and pressures were also falling (c. 650°C— 4kb), but the shear zones acted as a control on the development of firstly homophanous leptynite, and secondly charnockite. In both cases, neosomes of quartz and potassium feldspar have myrmekitic textures and intergranular occurrence. The palaeosome became strained and garnet–biotite and garnet–biotite–clinopyroxene ± orthopyroxene mineral assemblages were formed in association with the development, respectively, of homophanous leptynite and charnockite. The persistence of straining in the palaeosome and the presence of poikilitic textures indicate that this typical assemblage does not represent one in thermodynamic equilibrium. This has important consequences for geothermobarometry.

The structural control on neosome development reflects a regional characteristic where shear zones are the site of peralkaline plutons, fenitisation, carbonatites and epigenetic graphite development. The peculiar features of charnockite are seen as just one of several aspects related to the fluxing of hot CO2-dominated fluids from either the deep crust or mantle during the late Archaean development of the Indian craton.

Oxroadopteris parvus gen. et sp. nov. is described from a single incomplete curved stem, 12 cm long and 8 mm diameter, having a cylindrical exarch protostele and sparganum outer cortex. Attached are 8 incomplete bifurcate petioles in probably 2/5 phyllotaxy. Each petiole has a swollen base with sclerotic nests. Petiole lengths increase upwards. Each petiolar trace is small, U-shaped in cross-section, bipolar, and divides to form similar smaller bundles in two secondary rachises.

U–Pb isotopie data for zircons from a granulite-facies xenolith from a volcanic vent at Partan Craig in the southeastern part of the Midland Valley of Scotland display evidence of an early Proterozoic (>2·0 Ga) component. The xenolith is probably relict metasediment and the data therefore indicate old detritus in the sedimentary precursor. The exact ages of sedimentation and metamorphism are uncertain but these must be compatible with Sm–Nd data on similar xenolithic fragments from the same locality which constrain the ages to c. 1–0 Ga or younger.

The origin of a suite of ultrabasic and basic rocks from the Letovice crystalline complex in the eastern part of the Bohemian massif, Czechoslovakia, is discussed on the basis of new geological and geochemical data. Strongly serpentinised spinel peridotites, plagioclase peridotites and pyroxenites, together with metagabbros, amphibolites and minor dykes of metadolerite, make up about 70% of the complex; metasediments, mainly mica schists, make up the remaining 30%. The ultramafic and mafic rocks constitute parts of a dismembered ophiolite considered to have been formed in the Cadomian episode. They show effects of polyphase deformation and were variously metamorphosed under conditions of greenschist to garnet amphibolite fades at 400–600°C and 2–5 kb. Geochemical features of the ultrabasic suite are similar to those of other ophiolite complexes, such as those of Bay of Islands, Oman, Papua, Pindos, Sarmiento and Troodos and are likely to be products of a marginal sea environment.

Graphite bodies of economic importance occur in the Eastern Ghats migmatite complex of Orissa, India, which consists of sillimanite-rich gneiss (khondalite), calc-silicate granulite, basic (pyroxene) granulite and acidic gneiss palaeosomes and a variety of quartzofeldspathic neosomes and pegmatite veins. There is more graphite in migmatised khondalite and calc-silicate granulite than in their unmigmatised counterparts and it is common at pegmatite-country rock margins. Lodes of graphite are structurally controlled by fold hinge zones, necks of boudins and along the dominant foliation in rocks that show the effects of polyphase deformation. Faults, fractures and joints also act as structural controls. The richest lodes are located where there is the coincidence of several lithological and structural features.

The graphite occurs in graphite-rich schists and gneisses, as veins and as disseminated flakes along grain boundaries and in microfractures and cleavages. It is present in all rock types except pegmatites emplaced after the D4 deformational phase, and is localised in structures developed during the D1, D2, D3 and D4 phases. Most of the bodies of graphite are lenticular and show pinch and swell characters. Fourteen textural and microstructural habits have been recognised and are described with many of the features of fabric, such as intergrowths, inclusions and kinks being particularly well shown under reflected light.

Methanation of carbonate minerals is considered the most likely initial stage in the formation of the carbon with subsequent pyrolysis of the methane being the process of graphitisation. While the possibility of the carbon being either of sedimentary derivation orfrom igneous-derived CO2 cannot be ruled out, there is no convincing evidence for such sources. Remobilisation of graphite following its formation by pyrolysis of methane in structurally controlled low pressure zones is the dominant mechanism of concentration. Thefabric features point to the graphite having moved as a solid phase along open spaces inthe gangue minerals while still maintaining its crystallinity, and having been deformed in the process. The effects of remobilisation apparently have obliterated evidence relating to other methods of concentration.

Ultramafic xenolith lithologies representative of the mantle beneath the Midland Valley of Scotland comprise magnesian peridotite (predominantly spinel lherzolite) and cumulate wehrlites and clinopyroxenites. The lherzolites are typical of the worldwide type I (Cr-diopside) xenolith suite; their textures and mineral chemistry record a complex thermal and deformational history. The petrographical and mineralogical features of the wehrlite-clinopyroxenite suite can be interpreted within the context of a sequence of cpx + ol ± sp cumulates that have undergone a protracted period of subsoli dus re-equilibration. Mineral compositions are similar to those of type II (Al-augite) ultramafic xenolith suites.

Although xenolith populations imply widespread lower crustal heterogeneity, meta-igneous basic granulites form a major component of this region beneath the Midland Valley. They are principally composed of pi + cpx + mt ± opx ± ap; a modal continuum exists from clinopyroxenite to anorthosite, skewed towards plagioclase-rich lithologies. Garnet is rare. Locally, evidence indicates re-equilibration from garnet granulite precursors. Rock densities range from c. 2·8 to c. 3·2 gm . cm−3, implying P-wave velocities in the range 6·5–7·5 km . s−1 consistent with the known seismic properties of the lower crustal layer beneath the Midland Valley. Phase relations are also consistent with equilibration of primary assemblages at depths of 20–35 km. Retrograde reactions indicate that portions of the crust may have had a complex pressure-temperature-time evolution. Major element compositions of the granulites are broadly basaltic, ranging from ne to hy normative with the mean corresponding to alkali olivine basalt. They are distinct, chemically, from basic granulites of the Lewisian complex of NW Scotland.

The lead isotope compositions of 61 galenas from central and southern Scotland vary markedly between different regions. Most galenas from the southern Grampian Highlands yield isotope ratios (206Pb/204Pb 17·77 ± 0·25, 207Pb/204Pb 15·47 ± 0·05, 208Pb/204Pb 37·63 ± 0·26) less radiogenic than those from Midland Valley galenas (18·22 ± 0·12, 15·55 ± 0·05, 38·13 ± 0·14) whilst galena lead from the Southern Uplands (18·28 ± 0·12, 15·56 ± 0·03, 38·21 ± 0·18) is more radiogenic than that from the southern Midland Valley (18·12 ± 0·06, 15·52 ± 0·02, 38·06 ±0·10). The change in isotopie composition across the Highland Boundary fault reflects the presence or absence of Dalradian rocks which included a magmatic component of lead. Galenas from the Dalradian sequence in Islay, where igneous rocks are lacking, have a composition (18·14±0·04, 15·51±0·01, 37·90±0·02) more like Midland Valley galenas. In the Southern Uplands, galenas yield lead isotope ratios similar to those of feldspars from Caledonian granite (18·30 ± 0·14, 15·57 ± 0·04, 37·96 ± 0·15) analysed by Blaxland et al. (1979). The similar ratios reflect the incorporation of Lower Palaeozoic sedimentary rocks into the granite magma, rather than a granitic source for the mineralisation. The granites were then thermal-structural foci for later mineralising fluids which leached metals from the surrounding rocks. Within the Midland Valley, galenas hosted in Lower Devonian-Lower Carboniferous lavas are notably more radiogenic (18·31 ±0·12, 15·58 ± 0·06, 38·20 ± 0·16) than sediment-hosted galenas (18·14 ± 0·07, 15·52 ± 0·02, 38·08 ± 0·10). The Devonian lavas at least may have inherited lead from subducted (? Lower Palaeozoic) rock incorporated in the primary magma.