The St David's Head Intrusion, exposed in North Pembrokeshire, is a tholeiitic sill up to 570 m thick, comprising principally mafic gabbros, which are in part layered. Layering varies from the centimetre to the metre scale. A range of gabbroic compositions is present, defining seven major petrological types, which are cut by thin silicic (aplitic) veins.

Log–log plots of incompatible elements from the various lithological units indicate that all of the rocks in the intrusion are petrogenetically linked, although a variety of processes has been operative. Roach (1969) considered the quartz gabbros and dolerites, which form an envelope around the other units, to relate most closely to the parental composition. However, the least evolved compositions in the intrusion are from the xenolithic laminated olivine gabbros, although these mafic compositions are due primarily to the presence of abundant, mafic, cognate xenoliths. These xenoliths are thought to relate to an earlier episode of crystal accumulation in a high-level magma chamber. The various laminated gabbros reflect crystal accumulation in situ after magma emplacement, leading in certain layers to extreme enrichments in Fe, Ti, and V, related to high modal proportions of cumulus ilmenite. Further in situ crystallisation led to differentiation of the residual liquid, producing more silicic gabbros with well-developed granophyric textures, the granophyre reflecting the silicic residuum. Extreme differentiation, possibly combined with expulsion of silicic residual liquid during crystal accumulation and compaction, resulted in the cross-cutting aplite veins.

Three different types of layering are present in the intrusion. Firstly, preferred orientation of tabular minerals in the laminated gabbro units is thought to result from discrete sedimentation episodes from a convecting magma chamber. Secondly, macrorhythmic modal layering up to 1 m thick consists of an alternation of relatively ilmenite-rich and ilmenite-poor layers in the laminated gabbro units, although the reason for the modal variation is not certain. Thirdly, a centimetre-scale felsic–mafic microrhythmic layering is present in the envelope quartz gabbros and dolerites, which is similar to the inch-scale layering in the Stillwater Igneous Complex. This layering is thought to relate to metasomatic reaction in the gabbro in the presence of water at a late magmatic stage. Overall, these various lithological units themselves define a large-scale layering in the intrusion.

Combined, the petrological and geochemical data suggest that the St David's Head Intrusion was not emplaced in a single event. Rather, a series of magma pulses, of contrasting compositions but petrogenetically linked, was intruded. Some of the chemical variations now seen existed prior to emplacement, indicating the former presence of high-level crustal magma chambers, while other variations developed as a result of in situ crystallisation processes and related chemical differentiation.

Cyclotron lapworthi (Groom, 1902), one of the few British Cambrian bradoriid ostracod species known from a large number of specimens, is redescribed. It has an interdorsum, thus indicating a hesslandonid affinity. The ontogeny of C. lapworthi does not demonstrate a clear distinction of its individual moult stages but does show marked changes in the development of nodes and lobes. C. lapworthi shows wide variation in carapace shape possibly indicating that its valves were thin and relatively flexible. In England and Canada C. lapworthi is restricted to the Upper Cambrian Olenus Zone. C. lapworthi may also occur coevally in the Upper Cambrian of Sweden.

The Lower Palaeozoic tectonic history of central and eastern Europe is poorly understood because of extensive Variscan and/or Alpine reworking. The trace of the Tornquist Sea, the SE arm of the Lower Palaeozoic Iapetus Ocean, extended from NE Britain to Asia Minor. The site of this ocean is constrained by the tectonostratigraphy and faunal provinciality of Lower Palaeozoic inliers in northern Czechoslovakia, and southern Poland. In this paper, the collage of contrasting tectonostratigraphic histories of terranes in the Lower Palaeozoic of Poland is reviewed. Fossil evidence demonstrates that the Holy Cross Mountains and the Krakovian Belt display Lower Ordovician and Lower Devonian angular unconformities. Faunal data suggest that the Tornquist Suture Zone must lie south of the Holy Cross and between Upper Silesia and the Barrandian of the Czech Republic. Between these areas, in the Sudeten Mountains, a continental scale sinistral mylonite zone (along the line of the Intra-Sudetic Fault) was periodically active between the Middle Ordovician and the Upper Triassic. Various dismembered ophiolite, island arc and batholith terranes from alongside the Intra-Sudetic Fault have Ordocivian and Silurian magmatic and metamorphic zircon isotopic and fossil ages. Thus the often stated view that deformation in the Sudetes is Variscan (i.e. post-Middle Devonian) must be called into question. It is proposed instead that the Tornquist Suture is located within the Sudeten mountains, and as in the Holy Cross Mountains, much of the observed deformation is post-Cambrian and pre-Gedinnian in age, i.e. Caledonian.

Critical collecting, measurement of sections and taxonomic reassessment through the Climacograptus wilsoni Zone (basal Lower Hartfell Formation) of the southern Scottish graptolite zonal sequence suggests that, although only locally developed, it is an easily recognisable and useful biostratigraphic interval. The North Cliff of Hartfell Spa is defined as type section for both the base of the Lower Hartfell Formation and that of the C. wilsoni Zone. The zonal assemblage is figured and described, with particular emphasis being placed on the distinctive species C. wilsoni which has not been redescribed since the beginning of this century.