‘Mass extinctions’ and ‘waves of origination’ are defined as peaks in the average probability of family extinction and origination per geologic stage, respectively. A simple neutral model is proposed, assuming only that the average probabilities of family extinction and origination have varied independently and had equal chances of going up and down from one stage to another. As a null hypothesis, this model is not rejected by the available palaeontologic data on marine animal and nonmarine invertebrate families in the Phanerozoic. The only discrepancy between the model and the empirical pattern is due to a systematic bias introduced by Fossil-Lagerstätten. The alleged periodicity of Late Phanerozoic mass extinctions follows inevitably from the model.

The structure of fold and thrust belts has been studied for just over one hundred years and has been the subject of a recent international conference at Toulouse, in May 1984. This article reviews some of the recent advances in thrust tectonics from a personal viewpoint, in the light of the Toulouse meeting. Key tools in correctly interpreting thrust systems are the use of balanced (restorable) cross-sections and the resolution of three dimensional fault systems. These analytical methods are now being widely developed and applied to numerous orogenic belts with many surprising results. While our understanding of foreland thrust belts is now probably better than any other structural environment, there is still great uncertainty as to the emplacement-driving mechanisms of these belts. Future work will try to solve these and other outstanding problems and apply the new analytical methods to the interior parts of thrust belts in efforts to restore entire orogens.

The Hyllingen Series, comprising the southern part of the 160 km2 Caledonian, synorogenic, layered, mafic Fongen-Hyllingen intrusion, southeast of Trondheim, Norway, crystallized from a basaltic parent magma at 5–6 kb. Well developed modal layering strikes directly towards the magma chamber wall at the southern margin. The lithologies of inclusions match those of the adjacent country rock envelope. Eleven sample profiles (274 samples; over 1000 mineral analyses) allow the Hyllingen Series to be subdivided into four stages which generally decrease in thickness from north to south. A sporadically developed Stage I (< 100 m thick) of mostly unlayered ferrodiorites with relatively evolved compositions occurs at the base. Stage II (400–1500m) consists of layered, broadly ferrodioritic rocks. Modal layering is undisturbed by numerous, plate-like, metabasaltic inclusions which occupy about 22% of Stage II. Cryptic variation is slight except at the top where more evolved compositions are developed. Stage III (350–550 m) has fewer inclusions and is characterized by a gradual regression to more primitive compositions. Stage IV (800–2300 m) shows normal fractionation patterns, and a few minor reversals, with extremely evolved rocks in contact with the roof. Occasional metapelitic, platy inclusions occur near the southern margin.

The rocks gradually become more evolved along the strike of modal layering towards the southern margin. Olivine and plagioclase vary systematically over about 7 km from Fo23:An46 to Fo7:An35 along the Stage II/III boundary and from Fo75:An63 to Fo13:An42 along the Stage III/IV boundary. While the apparent angle of discordance between modal and cryptic layering is usually less than 20°, in Stage IV near the wall of the magma chamber they are highly discordant (approaching 90°) over a thickness of about 900 m.

In situ crystallization along an inclined floor took place from magma which became stratified by double-diffusive convection during Stage II. Modal layering developed concordant with the crystallization front while cryptic layering developed essentially parallel to the liquid stratification. Gradual influx of dense hot primitive magma caused elevation of the stratified magma column in Stage III. During this uplift, progressively more primitive liquid came into contact with earlier crystalline products along the inclined crystallization front. Crystallization during uplift of the magma column gave rise to the gradual regression of Stage III. Highly discordant modal/cryptic layering relations in Stage IV require that the magma became zoned with a horizontal, as well as a vertical, component. This may have occurred in response to lateral cooling establishing horizontal thermal gradients within individual magma layers, the resulting tendency towards density increase being compensated by material diffusion to more evolved compositions.

The original roof during the early period of magma chamber evolution is possibly among the inclusions in Stage II. Repeated magma addition during Stage II resulted in the top magma layer having an extremely evolved composition. During the later stages this buoyant, evolved liquid ultimately crystallized to produce the quartz-bearing syenite in contact with the roof.

The relatively well preserved, 42 km diameter lunar impact crater Aristarchus was excavated across the boundary between the Aristarchus Plateau and the mare plains. Previous geological mapping of the different ejecta facies associated with Aristarchus has shown that there are distinct morphological differences between ejecta emplaced on either side of the fault scarp that separates the plateau from the mare. One explanation of this observation is that ejecta behaved in different ways on emplacement depending on the original lithology of the excavated material. To test this proposal, we examine the pre-Aristarchus regional stratigraphy and identify the spectral signatures of each of the material's units. By comparing these data with analysis of the geology and spectral characteristics of the ejecta units, we are able to show the provenance of each of the mappable ejecta facies. The conclusions are consistent with the concept of the crater ejecta asymmetry being related to bedrock characteristics. Using geological mapping, spectral interpretation and present understanding of the impact cratering process, we reconstruct the impact event and suggest how such techniques could be used further to interpret the Moon's composition and history.

A subduction-accretion model incorporating new geophysical data is presented to explain the geology of the Antarctic Peninsula from late Palaeozoic to Cenozoic time. According to the model, the peninsula consists of overlapping accretionary, magmatic and extensional regimes that were diachronous across the peninsula and have built the crust to its present form. The crust, which contains a small proportion of sialic basement, was mainly formed by accretionary and magmatic processes and modified to its present shape by extension. The Gondwanide Orogeny for the Antarctic Peninsula is interpreted in terms of the accretionary processes.

The Tertiary volcanic centre that dominates the constitution of Rhum, one of the Scottish Inner Hebridean group of islands, is immediately provocative to the visiting geologist. The higher mountains are entirely ultrabasic in composition and their near-horizontal, terraced layers remain nearly constant in composition to about 1 km above sea level. Any initial assumption of geological simplicity is quickly dispelled by those and more detailed features. Horizontal layers of ultrabasic rocks with coarsely crystalline texture do not fit happily on a Tertiary landscape, towering high above the adjacent Hebridean lava flows of similar age. And what of the rocks themselves? Thin layers entirely of calcic plagioclase feldspar compete for attention with thick olivine-rich layers, while any day of traversing the Hallival or Askival mountain slopes will reveal a wealth of superbly exposed question marks in the form of spinel–sulphide layers, slump structures, finger structures, ‘harrisitic olivine’ formations, undulatory ‘trough-like’ structures, ultrabasic veins and currently lesser features. Search as one may (and little is hidden from view), there is no marginal gabbroic envelope between the ultrabasics and the country rocks that would lead to a Skaergaard analogy, and no overlying gabbroic layered rocks that would elicit guidelines from the Bushveld or Stillwater intrusions. This is Rhum, and much of what is on that small island is unique and, indeed, still defies full explanation. It is the achievement of this issue, through its several contributors whose voices succeed mine, that the uniqueness of Rhum is welcomed, many of the problems challenged, and many of the secrets unravelled.

Large Dictyonema species, including D. pulchellum T. S. Hall, have been collected from beds in the Uldale Fells (northern Lake District), below strata yielding Didymograptus protobalticus Monsen. They suggest the presence there of graptolitic beds transitional from Lancefieldian to Bendigonian in age (approximately Tremadoc-Arenig) and suggest that the area may yet provide an internationally correlatable base to the Arenig Series in Britain.

Graptolite ultrastructural studies using SEM and TEM are now more than a quarter of a century old and have reached the stage of routine procedure where material of suitable preservation is to hand. (Specimens may be chemically isolated from the rock or, in SEM and STEM studies, may be retained in the matrix.) Description of ultrastructural elements has evolved from the purely empirical to a conceptual/biological classification following (a) the recognition of the role of collagen fibrils in skeletal construction, and (b) the realization that at least two models of skeletal secretion are possible, each essentially opposed to the other. In this paper old terminology is explained with reference to current practice and the latter is tabulated and referred to the increasing number of described ultrastructural elements. Some of the outstanding problems of interpretation are outlined; and a set of concise, in some cases revised, ultrastructural definitions is given together with illustration, where appropriate, of the ultrastructural element defined.

The good exposures of virtually undeformed Callovian and Oxfordian strata along the Djeffara escarpment of southern Tunisia and northwestern Libya have allowed analysis of regional depositional history during this time.

A number of lithostratigraphic problems are considered. In Tunisia, the Foum Tatahouine Formation is subdivided into members and in Libya some of the stratigraphic issues are clarified. A correlation between the two sequences is proposed. The widely claimed aeolian origin for the Libyan Chameau Mort Sandstone is rejected.

The depositional patterns of the Callovian and Oxfordian strata are described in the context of Mid and Late Jurassic sedimentation in the eastern Ghadames basin of the African craton. After a regressive Bathonian sequence, transgressive conditions commenced in Early Callovian time. In a series of continental–marine cycles, this transgressive sequence culminated in widespread shallow, restricted-marine micritic deposition. A regression in Late Callovian time resulted in emergence marked by a thin but widespread calcrete horizon. In Mid? Oxfordian time a renewed transgression brought in open marine, high-energy, shallow-water carbonates. Later, regressive conditions returned, leading to increasing restriction, and latest Jurassic time saw the first signs of the fluvio-deltaic deposition that was to dominate the region in Early Cretaceous time.

Interactively processed Landsat MSS imagery has been used as an aid to studying the regional geology of approximately 10 800 km2 of terrain at the southeast margin of the Archaean Yilgarn Block in Western Australia. The technique proved successful in extending positions of known lithological contacts and lineaments into poorly exposed, inaccessible areas and in revealing new geological features, especially faults, previously unrecognized at ground level. During this investigation the distribution of granitoids and greenstones was more precisely defined, internal greenstone structures highlighted and three main fault trends were identified: (1) NW–NNW and (2) ENE, both within Archaean shield, and (3) NE–NNE within the transition to adjacent Proterozoic mobile belt. In order for the most information to be extracted from Landsat MSS images it is recommended that, whenever possible, image processing should follow ground-based studies as well as precede them, and that field geologist and Landsat specialist should work at the image processing system together.

Lower Ordovician rocks in the Bahçe–Fevzipaşa district of the northern Amanos Mountains, previously divided into three separate formations, are referred to the Seydisehir Formation. Shaly strata that crop out southeast of Bahçe and have been termed Kizlaç Formation, Ayran Series or Bahçe Formation, described variously as being of Ordovician or Silurian age, are referred to the lower shale member of the Bedinan Formation; their sparse fauna includes the trilobites Colpocoryphe, Kloucekia and Nobiliasasphus and is of Caradoc age. A single locality containing Kloucekia phillipsii (Barrande, 1846) is shown to be that recorded by Frech in 1916 and subsequently lost sight of.

Two late Quaternary ash-flow tuffs erupted as single flow units from the Menengai caldera volcano, Kenya, show marked facies variations. Dense welding of the tuffs close to source may have been promoted by their peralkaline trachyte compositions. Coarse, near-vent breccias are partly welded and were emplaced as lithic-rich parts of the body of pyroclastic flows. Non-welded tuff forms the bulk of the outflow sheets which include valley-fill deposits up to 520 m above the present caldera rim, indicating ash-flow generation by collapse of moderately high eruption columns. The flows overran varied topography which controlled development of ash-flow tuff facies. On hilly terrain, the flows became turbulent and separated into a lithic-rich, rapidly deposited part and a fine-grained lithic-and crystal-poor part which was subsequently deposited farther from source. Caldera walls belonging to an earlier caldera probably formed a major barrier to the spread of the second ash-flow from its site of collapse. There are many similarities in the facies variations in these peralkaline tuffs and subalkaline tuffs. Differences in welding characteristics and the shape of ash particles reflect the lower viscosity of peralkaline magma.

The Moine thrust zone of southern Assynt forms part of the northwest margin of the Caledonide belt and has aroused controversy concerning amounts and timing of thrust displacement and sequence of thrust development. Recent mapping shows it to have been a foreland propagating thrust sequence; the uppermost ductile Moine thrust formed first, followed by sequences of imbricates, a major thrust (the Ben More thrust) and then several lower duplex zones. This sequence is clear from new observations that many of the earlier thrusts were folded and/or breached during the development of the underlying structures. A displacement of over 54 km has been estimated for the zone as a whole. An alkaline igneous complex, including the large Borrolan syenite, was intruded during the development of the thrust zone and much of it was carried some 30 km to the west-northwest onto the foreland. Late extensional structures in southern Assynt are an integral part of the Caledonide thrust sequence and probably developed from the collapse of the thrust wedge as it climbed from stronger basement rocks on to a weaker cover sequence on the foreland.

A 22 m thick primitive, basanitoid sill comprises homogeneous olivine teschenite, overlain by picrite and a rhythmically layered sequence of olivine teschenite, salitic teschenite and syenite. Differentiation occurred in two stages by fractional crystallization, in part from melt residual from formation of picrite. Syenitic blebs and hydrated globules, occurring through the layered sequence, formed by the same mechanism rather than by immiscibility. Endothermic cindering of a small coal seam below the intrusion absorbed heat and steam from the sill, removing volatiles from the lower part and establishing a strong downward heat flux. The picrite was restricted to the upper part of the sill by more rapid cooling of the lower half.

Several small outliers of Tertiary lavas and sediments rest with strong unconformity on a buried landscape eroded from Torridonian sediments and Tertiary granophyre. Erosion continued during the period of sediment and lava accumulation. Four formations are recognized; these are, in order of increasing age, the Orval Formation (hawaiite and basaltic hawaiite lavas), the Guirdil Formation (icelandite lavas, interbedded conglomerates), the Upper Fionchra Formation (tholeiitic basaltic andesite lavas, hyaloclastite deposits, basal conglomerate) and the Lower Fionchra Formation (alkali and transitional basalt, basaltic hawaiite and hawaiite lava flows, basal conglomerate); each is separated by an erosional interval. Clasts in the conglomerates reveal a history of erosion of a terrain exposing gneisses, Torridonian sediments, igneous rocks derived from the Rhum Tertiary Central Complex (including allivalites), and Tertiary lavas of local origin but also including, in the oldest conglomerates, tholeiitic basalts not now preserved on or near Rhum. Prior to and during lava and sediment accumulation, erosion on Rhum had cut down to a level similar to that of the present day, although not to the extent that high-grade thermally altered rocks, which are a marked feature of the Central Complex, were being eroded in any quantity. A sequence of east–west trending valleys, possibly initiated on the line of the earlier Main Ring Fault, drained the area of the Central Complex which then, as now, must have been high ground. Small lakes occasionally formed in the valleys allowing the accumulation of fine-grained sediment with plant remains, and promoting the formation of hyaloclastite deposits when buried by later flows. No source for any of the lava formations is preserved on Rhum; they are thought to have come from feeders north of Rhum, possibly near Canna, and to have ponded against the hills and valleys near and in the Central Complex.

The oldest tholeiitic lavas, not now found in situ, were followed by alkali and transitional flows compositionally similar to the Skye Main Lava Series but characteristically feldsparphyric; the most mafic also contain phenocrysts of magnesian olivine (with included Cr-Al-rich spinels) and aluminous spinel. Both the early alkalic/transitional basalts and the youngest hawaiites and basaltic hawaiites equilibrated at pressures < 9 kb; the tholeiitic basaltic andesites and icelandites equilibrated at relatively shallows depths.

Apart from a few N–S to NW–SE-trending basalt dykes, the lava formations represent the youngest Tertiary igneous event on Rhum.

It is proposed that the usage of the term structural facing be extended to include fault structures. Fault facing is defined as the direction in a fault plane which is at right angles to the trace of the bedding and is directed towards the younger beds. Certain types of faulted structures are found to have diagnostic patterns of facing variation.

Young stages isolated from limestone from Girvan indicate that the first two thecae had unsclerotized walls but apertural rings showing that they opened downwards beside the sicula. The development can thus be matched with that of Glossograptus and the two families Glossograptidae and Cryptograptidae merged. The recognition of downward-opening thecae in C. tricornis allows the earlier species C. schaeferi to be more easily accommodated in the genus. Relationships in the family are reviewed.

In southeast Rhum, a Mesozoic/Tertiary sequence is preserved as a fault-bounded and rotated wedge. This is juxtaposed between Precambrian rocks (Torridonian sediments and Lewisian gneiss) and caught up in the complex structure of the Tertiary Main Ring Fault (MRF), which shows three distinct phases of movement. The Mesozoic rocks comprise fossiliferous limestone, sandstone and shale, which show differing degrees of thermal metamorphism depending on their relationships to the Layered Complex. On the basis of faunal and lithological evidence the Mesozoic sediments have been correlated with the Lower Liassic Broadford Beds. The Rhum sediments are overlain by sheared Tertiary basalts, the contact between them probably representing the original landscape unconformity. The presence of these younger strata caught up along the MRF provides crucial evidence for a major phase of central subsidence in the early history of the Rhum Tertiary volcanic centre.