Sulphur isotopic data are presented for the four horizons of stratabound mineralisation in the Argyll Group Dalradian (Vendian) of the Central Highlands of Scotland. The styles of mineralisation reflect the stratigraphic and tectonic evolution of the Dalradian basin. The SEDEX-type Ba + (Zn + Pb) Loch Lyon Horizon has δ34S values for pyrite of +17% and for baryte of +27%. The baryte sulphur source was Dalradian sea-water although the values were modified by isotopic exchange with an H2S-rich metamorphic fluid derived from the surrounding graphitic schists. The sulphur source for sulphide in the horizon was probably the underlying strata; sulphide values were only slightly affected by metamorphism. Sulphides in the Pyrite Horizon are isotopically indistinguishable from those in the remainder of the volcanogenic Ben Lawers Schist Formation (0 ± 4%); these values suggest that, at least in the Tyndrum area, sulphides in the Pyrite Horizon are of igneous origin. VMS-type Cu + Zn + Pb mineralisation in the Ben Challum Quartzite Formation has a narrow range of values around + ll%. The likely source of this sulphide sulphur is reduced sea-water sulphate, the isotopic values remaining consistent owing to the buffering effect of anhydrite in the underlying calcium-rich rocks.

Dolomites form in a range of environments by processes able to drive large volumes of magnesium-rich waters through existing carbonate sediments or rocks. These fluids need not be of unusual composition but there are kinetic barriers to crystallisation which is influenced by factors such as the Mg/Ca ratio, salinity, temperature, the /Ca2+ ratio and time. Dolomite is able to form at near-surface temperatures and pressures within a few thousand years.

Textures in dolomitic rocks are controlled by their conditions of formation. A large proportion are replacive but few of these are mimetic, preserving primary structures. Crystals vary from euhedral to anhedral with boundaries ranging from planar to consertal. Solution chemistry and temperatures influence the density and distribution of nuclei together with growth rates and crystal morphology. There is still doubt whether irregular crystal faces are products of high or low temperatures or saturation. Dolomite cements are more important than has previously been realised in massively dolomitised rocks. Differential dissolution of aragonite, calcite, or evaporite minerals leaves space for these cements to occupy. Dolomitisation may also be allied to compaction, generating stylolitic rocks which are progressively enriched in dolomite. Dolomite may be replaced by calcite or it may be dissolved and the resulting pores filled with a calcite cement.

There is no general correlation between any set of petrographic features and particular geological models for dolomitisation. Similar physicochemical conditions are reproduced in a range of environments and the most effective guides to origin are in the geometry and regional petrographic variation of dolomite bodies.

A new taxon of probable early gymnosperms is described from permineralised specimens showing excellent preservation of the tissues, from the late Lower Carboniferous (Dinantian), Brigantian, deposits of the Midland Valley of Scotland. Stanwoodia gen. nov., represented by the single species S. kirktonensis sp. nov., shows the combination of a primitive protostelic organisation with a dense secondary wood. It closely compares to the enigmatic genus Bilignea which was formerly referred with some reservations to the Cordaitales. Another interest of the new plant concerns the excellent preservation of the phloem with at least three different types of constituent cells including fibres. Stanwoodia also exhibits the oldest well-documented evidence of a sequent periderm or rhytidome in fossil plants.

The subfamily Koneprusiinae is revised as part of a larger study of the Odontopleuridae, and Laethoprusia gen. nov. is described. The type species L. salax sp. nov. is from the Wenlock of Gotland, Sweden, and the genus is known also from the Wenlock of the Montagne Noire in south France, and from the Lower Devonian of Australia. The type species is most unusual among trilobites in possessing large fenestrae perforating the exoskeleton on segmental boundaries from the cephalic posterior margin to within the pygidium. The fenestrae were probably covered with soft tissues. Their function is enigmatic. Laethoprusia gen. nov. is the sister taxon to Isoprusia and/or Koneprusia. These genera are revised, and a review of all species assigned to them is given. A cladistic analysis of the best known taxa of Laethoprusia, Isoprusia, and Koneprusia is presented. Laethoprusia is the most derived genus of the subfamily, although stratigraphically it is the earliest.

The systematics of parts of the Odontopleuridae are revised using character analyses tracing homologous structures, and a computerised cladistic analysis. The choice of outgroup is shown to affect the result of the analysis. Several synapomorphies place Selenopeltis, the type genus of the Selenopeltinae Hawle & Corda, 1847 as sister genus to Dicranurus. The latter's close relationship to Miraspis, type genus of the Miraspidinae Richter & Richter, 1917 is confirmed. The Selenopeltinae therefore becomes a senior subjective synonym of the Miraspidinae. The Selenopeltinae includes about 90 of the over 380 named odontopleurid species known to date (excluding accepted synonyms). The Ceratocephalinae is recognised, including about 30 species. The paired, large pygidial border spines present in most odontopleurids are in some species not homologous, and a falsifiable hypothesis is proposed for the homology of the ‘true major border spines’. This structure is the posterior pleural spine of the tenth postcephalic segment in selenopeltines, ceratocephalines and odontopleurines, and in acidaspidines and apianurines it is the serially homologous spine of the eleventh postcephalic segment. The spine belongs to the first pygidial segment in all taxa except ceratocephalines, where it is on the last thoracic segment. The homology in selenopeltines of the progressive restructuring of the cheek border is reviewed, explaining the supramarginal appearance of the genal spine. The presence and homology of the sublobation of L1 in odontopleurids is discussed, and it may be homologous with the sublobation in lichids. The strongly differentiated thoracic segmental lengths (exsag.), with maximum length reached in the anterior part of thorax, and the much reduced length of the posterior segments, are aspects of a single character-complex, uniquely derived within the Selenopeltinae. The ontogenetic origin of the anterior and posterior pleural spines is reviewed. The presence in Ceratocephala of two instars in one meraspid degree is discussed. A cladistic analysis of selenopeltine and ceratocephaline genera does not entirely resolve the topology of the phylogenetic tree of these taxa, but it confirms Selenopeltis as sister taxon to Dicranurus. These two genera belong in a clade also including Miraspis, Selenopeltoides, and Ceratonurus. Two new genera are erected, Ceratocara and Archaeopleura, the latter with type species A. kazakhensis sp. nov.