Many explosive eruptions of dacitic magmas have occurred on St Lucia during the late Quaternary. These have produced widespread aprons and fans of pumice flow and ash flow deposits radiating around the central highlands, with co-eruptive air-fall and surge layers interbedded with palaeosols and epiclastic deposits. Vents in the highlands have not been located because of the dense tropical jungle but we suspect they are now plugged by lava domes surrounded by aprons of block and ash flow deposits. Young magmatically related dacitic lava domes have been extruded in the Qualibou depression. The pumice succession can be divided into older quartz-poor deposits forming the Choiseul Pumice and younger crystal-rich deposits with abundant large quartz which are called the Belfond Pumice. The Choiseul Pumice groups together scattered remnants of the products of many eruptions of different low-silica dacitic magma types. The Belfond Pumice is the product of several eruptions of a high-silica magma type and 14C ages have dated these between 20900 to 34200 years B.P.

The pumice flow deposits occur as small-volume valley fills. A granulometric study of Belfond pumice flow deposits shows them to be strongly depleted in finer ash and vitric components. It is suggested that the narrow, winding and vegetated valleys on the island locally induced turbulence and the flows moved with large, highly fluidized and inflated heads, resulting in substantial loss of fine vitric ash. One ash flow deposit which is extremely rich in crystals and carbonized vegetation is highly depleted in fines and shows enhanced vitric losses. This flow may have been a much more violent ash hurricane or blast which surmounted topography ingesting large amounts of lush vegetation. Ignition of this released the large quantities of gas needed to elutriate most of the fines.

A model is suggested for the recent volcanic activity on St Lucia in which separate batches of silicic magma, each having a distinctive petrological and chemical character, rose into high level chambers over a large area. Eruptions of volatile-rich magma led to highly explosive pumice-forming activity from vents in the central highlands. Degassed and more crystal-rich magma was extruded later from the same vents or in the attenuated flank of the Qualibou depression to from lava dome complexes.

The Tremadoc Series and series of the Ordovician System are widely used in international correlations, and this review explains and summarizes how the base of each of these series is recognized in Britain. No type section in which the base of any one of them is displayed has ever been selected and formally designated, hence the basal horizon of each series is a matter of general understanding rather than precise definition. This general understanding is of particular biozones, or of assemblages of fossils and their relative ages. The geographical separation between type sections of series, and differences in facies, complicate the understanding of relative ages of biozones. Brachiopod and trilobite faunas of the Llandeilo, Caradoc and Ashgill Series have been studied in detail in recent years, and biozones proposed. These biozones have proved difficult or impossible to recognize outside their type areas in Britain, and brachiopod–trilobite faunas of the Tremadoc, Arenig and Llanvirn Series are likewise highly provincial. Thus for correlation within and outside Britain attention has been focused on the ‘standard’ graptolite biozones, particularly since such conodont faunas as have been found have not proved useful. Yet there are problems associated with these ‘standard’ biozones; for example, the Tetragraptus approximatus Biozone is unknown in Wales or England, the relationship between the base of the Glyptograptus teretiusculus Biozone and the earliest Llandeilo shelly faunas requires clarification, and at what level the base of the Caradoc Series may lie in the Nemagraptus gracilis Biozone is unknown.

Thus to bring precision to definitions of Ordovician Series in Britain the identification of continuous, fossiliferous sequences that will serve as stratotypes, is required. Such a section for the basal portion of the Tremadoc Series has recently been proposed. Systematic studies related to particular biozones are needed, for example early Caradoc and early Ashgill brachiopods, and graptolites of the Didymograptus bifidus and Glyptograptus teretiusculus Biozones. The stratigraphical ranges and abundances of species in particular sections require documentation, so that the type of biozone (assemblage, acme, concurrent-range, etc.) may be recognized. Such work could lead towards chronostratigraphy, the provision of a standard stratigraphical scale in Britain for the Tremadoc Series and Ordovician series.

Recent interpretations of Cenozoic arc systems and collision belts facilitate reinterpretation of some aspects of British Caledonide evolution. End-Cambrian ‘Grampian’ collision of the passive ‘Dalradian’ foreland following southeastwards subduction beneath an island arc was accompanied by initiation of the Highland Boundary Fault as a high-angle south-directed oblique-slip thrust. Mid-Ordovician to early Devonian northwestward oblique subduction of the Iapetus Ocean beneath the Grampian orogen resulted in a continental margin magmatic arc, back-arc thrusting and development of an accretionary prism, while southeastward subduction led to arc magmatism and back-arc extension followed by initiation of the Rheic Ocean as a back-arc marginal basin; this syn-subduction N–S asymmetry of the Iapetus Ocean margins was analogous to the E–W asymmetry of the modern Pacific. Closure of Iapetus was diachronous, earlier in the northeast: during end-Silurian collision the southern Caledonides behaved as a passive foreland; post-collision foreland thrusting resulted in deposition and deformation of Lower Old Red Sandstone foreland basin deposits in Wales, and probably in northwest-directed back-thrusting in the region of the Longford-Down accretionary prism. Subsequent dextral movement in the suture zone juxtaposed the southern Caledonides with Scotland and northern Ireland, beneath which northwestward subduction had continued into the early Devonian.

Cumulative curves of grain-size against frequency often show segmented shapes. These shapes have been interpreted as resulting from a combination of normally distributed populations each of which should reflect a mode of transport. The present study deals with samples in motion as bedload, pure saltation load and suspended load. All of these exhibited segmented shapes on cumulative curves. An explanation of this is that the populations are better described as log-hyperbolic distributions rather than as a mixture of log-normal distributions. The log-hyperbolic distribution when plotted on probability paper has a shape which can be misinterpreted as consisting of segments.

The earliest stage of magmatic activity within the Palabora carbonatite was marked by the intrusion of phosphate-bearing pyroxenite. In good exposures in the north, large-scale (2m diameter) orbicular structures are found. These consist of regularly-spaced alternating dark layers (phlogopite-rich) and light layers (diopside-plus apatite-rich) which, in hand specimen, are very similar to the ‘inch-scale’ planar layering which has been described in layered mafic instrusions. One of the purposes of this paper is to describe these unique features which are currently being destroyed by mining, as they form economic phosphate concentrations.

The resemblance of the Palabora orbicules to Liesegang rings has led to the development of a qualitative model whereby the orbicules are regarded as having been formed by concentric periodic precipitation around central cores within dynamically quiet, isolated pockets of largely liquid magma. The controlling parameters are interpreted as being the rates of growth of the constituent minerals and the rates of diffusion of the elements crucial to their growth, i.e. K+, Al3+ and (OH)- for dark layers, and Ca2+ and P5+ for light layers.

The presence of these spectacular structures with their delicate layering indicates that at the time of crystallization of the pyroxenites relatively non-turbulent conditions prevailed, and diopside, phlogopite and apatite crystallized essentially coevally. Hence, the vertical large-scale banding in the pyroxenite may also be a function of diffusion controlled processes rather than being caused by separate magma pulses.

Some 880 m of grey, green and purple argillites are estimated to be present in complexly deformed Upper Devonian successions which occur in several overlapping coastal sections. Four lithostratigraphical formations are formally defined, in order of decreasing age: the Marble Cliff Limestone; Longcarrow Cove Slate; Harbour Cove Slate; and Polzeath Slate Formations. Improved correlation and biostratigraphical control is available for the two younger formations which are studied in detail. The formations range in age from lowest Frasnian to mid Famennian. Despite the intense deformation there are no major structural breaks in the successions.

Measurements of the size and shape of the grey reduction spots and of the variation of iron content across the spot–host rock interface have been made on samples from the Enville Group. The red coloration due to Fe3+ is thought to reside mainly in the matrix material cementing the mineral grains of the rock. The reduction to Fe2+ is ascribed to carbonaceous particles included in the sediment. To account for the sharp boundary of the spots it is suggested that the outward movement of the boundary is due to concentration dependent inward diffusion of oxygen which converts the carbonaceous material to carbonate.