As a well-known phosphatized Lagerstätte, the Ediacaran Weng'an biota in central Guizhou Province of South China contains diverse acanthomorphic acritarchs, algal thalli, tubular microfossils as well as various spheroidal fossils. These fossils provide crucial palaeontological evidence for the radiation of multicellular eukaryotes after the termination of the Neoproterozoic global glaciation. While the Weng'an biota is generally considered as early Ediacaran in age on the basis of phosphorite Pb–Pb isochron ages ranging from 572 Ma to 599 Ma, the reliability and accuracy of these age data have been questioned and some geologists have proposed that the Weng'an biota may be younger than 580 Ma instead. Here we report a SIMS zircon U–Pb age of 609 ± 5 Ma for a tuffaceous bed immediately above the upper phosphorite unit in the Doushantuo Formation at Zhangcunping, Yichang, South China. Litho-, bio- and chemostratigraphic correlations suggest that the upper phosphorite unit at Zhangcunping can be well correlated with the upper phosphorite unit at Weng'an, which is the main horizon of the Weng'an biota. We therefore conclude that the Weng'an biota could be as old as 609 ± 5 Ma.

An Ediacaran assemblage dominated by an unidentified species of CyclomedusaSprigg 1947, along with species of Ediacaria?Sprigg 1947, BeltanellaSprigg 1947, HiemaloraFedonkin 1982, and Nimbia?Fedonkin 1980, is described for the first time from the Innerelv Member of the Stappogiedde Formation exposed in coastal outcrops west of Tanafjorden on Digermul Peninsula, in northeastern Finnmark, northern Norway. The fossil assemblage is dominated by discoidal forms which share certain affinities with the cosmopolitan genera. Cyclomedusaand Ediacaria. However, our specimens differ from these and other discoidal Ediacaran fossils in the absence of radial sculpture. This, along with a basically concentric organization, are characteristics shared with Kullingiafrom the Dividal Group of northern Scandinavia, the White Sea, Podolia, and northwestern Canada, along with undescribed discoidal remains from the Charnian Supergroup, Charnwood Forest, Leicestershire, England, and the Conception Group, Avalon Peninsula, Newfoundland.

Our discovery of an Ediacaran-type assemblage within the Middle Innerelv Member provides support for previous suggestions of a late Vendian age for this sequence. This general conclusion is consistent with the occurrence of early Cambrian taxa, including the trace fossil Phycodes, and the problematical forms Vendotaenia and Sabellidites, in basal portions of the Lower Breivik Formation, within the same stratigraphie section. The lowest formally-proposed faunal zone in northern Scandinavia is the Kullingia Zone, based on the occurrence of the fossil medusoid Kullingia concentrica in Member III (Middle Sandstone C) of the Dividal Group, northern Scandinavia. Kullingiais a distinctly chambered form that was probably pelagic. In contrast, Cyclomedusa,and related genera of the so-called Cyclomedusaplexus, comprise an informal grouping of intergrading, probably benthic, taxa that possess radial and/or concentric organization. In light of the intergradational nature of taxa, present difficulties in taxonomic interpretation and correlation, and the abundance of cyclomedusoids in many Ediacaran assemblages, we suggest that the concept of the Kullingia Zone, as originally defined for northern Scandanavia, be broadened to include the common form genera of the Cyclomedusa plexus, inclusive of the occurrences in the Innerelv Member described herein. It is our hope that additional fieldwork will provide a basis for more refined taxonomic evaluations and biozonation.

Very large volume (>1000 km3 of magma) crystal-rich dacitic ignimbrites that lack pronounced evidence of fractional crystallization or vertical zonation erupt in some continental magmatic arcs (e.g. the Lund Tuff of the Great Basin and the Fish Canyon Tuff of Colorado in western USA). Apparently, their magma chambers were only modestly heterogeneous and not systematically zoned from top to bottom. These ignimbrites have 40 to 50% phenocrysts set in a high-silica rhyolite glass. Mineral assemblages and mineral compositions suggest pre-eruption temperatures were 730 to 820°C and water and oxygen fugacities were relatively high. We have speculated that these very large volume ignimbrites are unzoned because crystallization and convection in slab-shaped magma chambers inhibited separation of crystals from liquids and resulted in a chamber filled with compositionally heterogeneous magma that lacked systematic chemical zonation or strong fractionation. However, many other very large volume silicic ignimbrites are strongly fractionated and may be vertically zoned (e.g. tuffs related to the Yellowstone hotspot). These rhyolitic tuffs typically have few phenocrysts, anhydrous mineral assemblages, low oxygen fugacities, crystallization temperatures of 830 to 1050°C, and a strong imprint of fractional crystallization. Yet these Yellowstone-type rhyolites are derived from chambers 40 to 70 km across which have sill-like shapes (depth/diameter ratios much less than 1). Thus, factors other than chamber shape must be important for establishing the degree of evolution and nature of zonation in silicic magma chambers. Here, the role of crystallinity-dependent viscosity on the evolution of these two types of contrasting magmas is explored. Calculated magma viscosities for the hot, dry, crystal-poor rhyolites are significantly lower than for the cooler, wetter, crystal-rich dacites. Perhaps these hot rhyolites had low enough crystal contents and viscosities to allow efficient crystal–liquid separation, probably by a combination of unhindered crystal-settling, floor crystallization (including compaction), and crystallization on the walls of large chambers. Clean separation of melt from residual solids at their sources may have been promoted by their high temperatures and low viscosities (<104.5 Pa s). In contrast, monotonous dacitic magmas may never have been crystal-free near-liquidus magmas. Their large magma chambers may have developed by progressive growth at a shallow level with repeated input of intermediate to silicic magma. Crystallization of the water-enriched dacitic magmas occurred at lower temperatures (<800 °C) where crystallinity and hence magma viscosity (>106.5 Pa s) were significantly higher. These characteristics inhibited all forms of crystal–liquid separation, hindered development of systematic vertical zonation, and promoted quasi-equilibrium crystallization in small domains within large heterogeneous magma chambers. Eruptions of these crystal-rich dacites may only occur if the roof fails over a growing magma chamber that is becoming increasingly molten.

Granulites including a charnockite suite, mafic granulites, pelitic granulites, metanorite dykes and their retrograde varieties occur as discontinuous shear zone-bounded bodies within the Archaean basement comprising a granite gneiss–amphibolite–metasedimentary rock association in the central part of the Aravalli Mountains, northwest India. The entire suite, named the Sandmata Complex, preserves a complex history of tectonothermal evolution. Except for their strongly foliated margins, the granulite bodies are largely massive. Partial melting in the ‘country rocks’ led to the development of migmatite gneisses close to the contact of the granulite, a feature not as common in the rocks further away from the granulite contact. Geothermobarometry of massive granulites indicates Tmax>900°C and Pmax∼7.5 kbar. The retrograde granulites, which formed at lower amphibolite/upper greenschist-facies conditions, experienced channelized hydration reactions concomitant with shearing. These rocks locally appear as hornblende–biotite-bearing foliated granulite with or without Cpx or Opx. The rocks seem to have followed an inverse PTt path and have undergone an earlier phase of near-isobaric cooling. Our single zircon Pb–Pb ages indicate that the exhumation of granulites to the shallower amphibolite-facies levels with concomitant melting in the country rocks took place between 1690 Ma and 1621 Ma. Assuming that the granulite-facies metamorphism took place at around 1725 Ma, we relate the entire process of granulite metamorphism and exhumation covering an age range between 1725 and 1621 Ma to the rift basin opening stages of the Delhi Orogenic cycle that culminated at c. 1450 Ma.

A rich fauna dominated by trilobites and calcified chordates has been collected in Shropshire from the Arenaceous Beds, the highest member of the Tremadoc Shineton Shale Formation, and hitherto regarded as poorly fossiliferous. This fauna shows that shelf conditions persisted longer in Shropshire than has been supposed. It is likely that even younger Tremadoc is cut out at the unconformity below the Caradoc in the Shineton Inlier. The correlation of the later Tremadoc is reviewed, and the nomenclature of British Tremadoc biozones is revised. The trilobites described here are a mixture of previously known and new forms. The name Shumardia (Conophrys) salopiensis Callaway is revived for British material traditionally assigned to the Scandinavian species Shumardia pusilla (Sars), from which it is distinct. The new taxa Litagnostus meniscus sp.nov., Apatokephalus sarculum sp.nov. and Skljarella cracens sp.nov. are described. The type species of Asaphellus, A. homfrayi and Leptoplastides, L. salteri, are redescribed, and Geragnostus callavei, Pseudokainella impar, and Parapilekia sp. are recorded. New information on the ontogeny of S. (C.) salopiensis and A. homfrayi is given. Litagnostus and Skljarella are recorded from the British Isles for the first time.

An episode of remarkable biotic, climatic, sea-level and facies changes took place during the late Viruan (Caradoc) epoch in the Baltoscandian area. We studied the carbon isotopic composition of carbonate sediments from this period. Data on the stable carbon isotopic composition of whole-rock carbonates from three south Estonian core sections, together with those on ostracode, distribution are presented. In two core sections, a positive δ13C shift of 2 ‰ was revealed in the upper part of the Keila Stage (mid-Caradoc). The comparison of isotope and ostracode data in the sections suggested the occurrence of a gap of late Keilan age in the marginal area of the basin equivalent to the North Estonian Confacies Belt. The beginning of the excursion approximately coincided with climatic and sea-level changes in the shallow shelf area. The peak of the late Keilan excursion preceded the biotic crises and maximum black shale accumulation in the Baltoscandian palaeobasin. An approximately synchronous carbon isotopic event has been reported from North America, referring to a possible global oceanographic event in the Caradoc epoch. Although the positive carbon isotopic excursion and related environmental events of the late Keilan age have some unique features, they show more similarities to the end-Ordovician and Silurian events, characterized by oceanic change from stratified state to thermohaline circulation state, than to warm anoxic events related to eustatic sea-level rise.

Sections exposing the Cambrian–Ordovician Boundary interval at Broom Point in western Newfoundland have been proposed earlier for a global systemic boundary stratotype. These lie within the Cow Head Group, a late Middle Cambrian to early Middle Ordovician allochthonous unit of limestone, shale, and conglomerate deposited at the toe of the ancient continental slope and on the adjacent continental rise. Several recent studies have further investigated the stratigraphy, sedimentology, and palaeontology of the Cow Head Group and others are under way on magnetostratigraphy and chemostratigraphy. These aspects are reviewed for six key boundary sections representing proximal to distal facies: Cow Head Ledge, Broom Point South, Broom Point North, St Pauls Inlet Quarry, Martin Point, and Green Point. In particular, new data are presented from 260 conodont samples that yielded 15500 conodonts. This intense sampling has allowed the discrimination of minor hiatuses in the proximal to intermediate facies where conglomerates have eroded and cannibalized underlying strata. New conodont data from Broom Point North have lowered the base of the C. lindstromi Zone into unit 74 conglomerates, thereby making this section unsuitable as a boundary stratotype. New collections from Green Point have yielded abundant conodonts and over 9400 conodonts have been recovered from 77 samples.

The conodont, graptolite, and trilobite biostratigraphy through the boundary interval is documented allowing accurate correlation between sections and more precisely revealing small hiatuses in the proximal and intermediate facies. The sequence of conodont zones is: Eoconodontus notchpeakensis, Cordylodus proavus, C. caboti, C. intermedius, C. lindstromi and C. angulatus. These can be correlated with trilobite zones established from both in situ and clast faunas from the proximal to intermediate facies and with graptolite assemblages (of Cooper 1979) especially in the intermediate to distal facies. Three new species of Cordylodus are described (C. andresi, C. hastatus and C. tortus) and the full apparatus of Iapetognathus preaengensis is illustrated.

The criteria for selecting a global boundary stratotype and point (GSSP) are reviewed in terms of the Cow Head sections. The Green Point section is shown to meet, and largely surpass, the prerequisites required of a stratotype. The Green Point section is proposed to be the global boundary stratotype with the base of the Ordovician System defined at the base of unit 23, which is the base of the Broom Point Member, Green Point Formation, at a level coincident with the base of the Cordylodus lindstromi Zone. In addition to an abundant and superbly preserved conodont fauna, this section preserves the best sequence of earliest planktic graptolites through a 40 m interval; the first nematophorous graptolites (of Assemblage 1) occur in unit 25, 6.9 m above the base of the C. lindstromi Zone. This level can be readily correlated into the proximal facies where both deep and shallow water trilobites (in situ and in clasts, respectively) show the base of the C. lindstromi Zone to lie within the Symphysurina brevispicata trilobite Subzone.

From a study of silicified fossils, and in particular the microstructure of brachiopods, from the Wilmington Sands (Upper Cretaceous) of Devon, a model of skeletal silicification is proposed. Three distinct morphologies of silica were formed, controlled by the relative rates of silica supply and carbonate dissolution: (a) a fine-scale replacement of the original shell microstructure where silica was abundant; (b) a concentric ring morphology called ‘beekite’ where silica supply was limited, and (c) a granular white crust formed where carbonate dissolution was restricted. Silicification occurred during early diagenesis as a result of bacterial decay of organic matter intimately associated with skeletal fragments, within a sediment of restricted permeability. A build-up of CO2 probably caused dissolution of skeletal carbonate, and bicarbonate released from this caused local precipitation of silica. The proposed mechanism is belived to be of general applicability to micrite-rich carbonate sediments.

Sedimentological evidence suggests that sea-water chemistry has oscillated between ‘Aragonite’ and ‘Calcite’ seas during the course of Phanerozoic time. During ‘Calcite’ sea periods aragonite was soluble and dissolved on the sea-floor. Here we propose, using a survey of familial diversity, that during times of such corrosive seas bivalved molluscs which secreted their shells entirely of aragonite were at a selective disadvantage compared to those with calcitic outer shell layers. This is the first case in which it has been suggested that there was a dynamic link between sea-water chemistry and the evolution of a particular taxon throughout Phanerozoic time.

Cathodoluminescence (CL) examination of Recent biogenic carbonates shows that they are often luminescent regardless of their mineralogical composition (calcite v. aragonite), habitat (marine v. fresh water), way of life (sessile v. vagile) or environment (hyper- v. hyposaline water). Thus, the presence of luminescence in biogenic particles is not a reliable indicator of diagenetic alteration as some authors have suggested. In addition, CL can reveal variations in the mineralogy of shell material (e.g. regenerated calcitic v. primary aragonitic) and can highlight growth-related structures. Manganese (Mn2+) is the most likely activator of this luminescence, and its content in the shells of benthic organisms seems to be linked to growth rate, ontogeny, open sea conditions, bathymetry and salinity. In neritic environments the Mn2+ content and the CL of molluscs and foraminifera appear to increase with decreasing salinity. This study indicates that CL may be an important tool for the determination of environmental and ontogenetic parameters in biogenic carbonates in addition to its current use indiagenetic studies.

Arising from detailed reassessment of field work in the area of Ny Friesland and Olav V Land from 1938 to 1965, a revised stratigraphical scheme is proposed for the Hecla Hoek succession which is known to range downward from Lower Ordovician and Lower Cambrian (1 km.) into a relatively unbroken sequence of Pre-Cambrian rocks about 19 km. thick. The metamorphosed older rocks, previously grouped as Lower Hecla Hoek, are reinterpreted as a succession 12 km. thick with a large volcanic component and a distinctive tillite horizon. Correlation is attempted anew with rocks elsewhere in Spitsbergen and overseas. The Hecla Hoek suite contributes to an interpretation of the development of the Caledonian tectonic province in late Pre-Cambrian and Lower Palaeozoic time.

Carbon and oxygen isotope data from a succession of Cretaceous (Barremian–Cenomanian) age recovered from the Pacific Ocean (DSDP site 463) are presented. The carbon isotope curve reveals a large isotope excursion within the early Aptian where δ13C values reach ~4.8‰ in the L. cabri–G. ferreolensis foraminifera zone. A decrease in δ13C values is observed at the base G. algerianus zone, before a return to more positive values at the top of the G. algerianus–T. bejaouaensis zone. The pronounced early Aptian positive event is preceded by a large negative isotope excursion, confined to the G. blowi zone. Synchronous with this excursion are increased total organic carbon values and increases in Mn and Fe concentrations. Integrated biostratigraphic and magnetostratigraphic data, together with the carbon isotope profile, suggest that the organic-rich units of site 463 are correlatable with Oceanic Anoxic Event 1a. The input of isotopically light volcanic CO2 in concert with the intensification and upwelling of intermediate water enriched in 12C could account for the observed trends. A potential trigger may have been the destabilization of the water column and the prodigious CO2 emissions associated with hydrothermal activity and the emplacement of the Ontong Java Plateau. Coupled with faunal evidence, the subsequent positive carbon isotope excursion is interpreted to be resulting from high, but decreasing, productivity and possibly increasing ocean stratification resulting in strong carbon isotopic gradients and 13C-enriched surface waters. The decrease in δ13C within the G. algerianus zone is coincident with more positive δ18O values. If these are interpreted in terms of temperature this interval may be characterized by a period of cooling and possibly a waning of Corg cycling. A return to lower δ13C values during the middle Albian is considered to be related to the increased influence of upwelling, as opposed to a waning of Corg cycling. Upwelling introduced isotopically light carbon to the surface, arresting the stratified oceanic conditions.

Granitoids in the Gabal El-Urf area in Eastern Egypt consist of a monzogranite pluton, belonging to the Younger Granite province, emplaced in granodioritic rocks. Whole rock Rb–Sr dating indicate ages of 650±95 Ma and 600±11 Ma for the granodiorites and monzogranites, respectively. The granodiorites (65–70% SiO2) are calc-alkaline and metaluminous with low Rb/Sr, Th and Nb contents, moderate enrichment in the LILE (K2O, Rb, and Ba) and display most of the chemical and field characteristics of syn-to late-tectonic I-type granitoids described elsewhere in the Arabian–Nubian Shield. The monzogranites (72–77% SiO2) are metaluminous to mildly peraluminous, highly fractionated and depleted in Al2O3, MgO, CaO, TiO2, Sr and Ba with corresponding enrichment in Rb, Nb, Zr, and Y. They can be correlated with the undeformed post-orogenic granites in the Arabian–Nubian Shield that chemically resemble A-type granites emplaced in extensional settings. The mineralogical and chemical variations within the granodiorites and monzogranites are consistent with their evolution by fractional crystallization. The granodiorites have a low initial 87Sr/86Sr ratio (0.7024) and high ∈Nd values (+6.9–+7.3) and are significantly different from those (initial 87Sr/86Sr ratio=0.7029, ∈Nd values=+5.2–+5.8) of the monzogranites. These data suggest a predominant mantle derivation for both granite types and demonstrate that they originated from different source materials.

The granodiorite melt was most probably generated through vapour-saturated partial melting of an early Neoproterozoic depleted mafic lower-crust reservoir due to crustal thickening associated with orogenic compression and/or arc magma underplating. The mineralogical and geochemical data of the A-type monzogranites are consistent with their derivation as a residual granitic liquid from a LILE-enriched mafic magma through crystal-liquid fractionation of plagioclase, amphibole, Fe–Ti oxides and apatite. The parental mafic magma was originated in the upper mantle due to crustal thinning associated with extension in the late stage of the Neoproterozoic crustal evolution of north-eastern Egypt.

Arc-flank volcaniclastic sedimentation in the Murihiku Terrane of New Zealand lasted about 120 million years from Late Permian to Early Cretaceous time. Despite the effects of pervasive zeolite-facies alteration, whole-rock geochemical parameters for sandstones, siltstones and tuffs record changes in source-rock composition, both in time and along the length of the depositional basin. Sandstones are considered to give a more reliable indication of the state of evolution of the source volcanic arc than do the siltstones. The siltstones commonly contain detrital white mica flakes that are generally lacking in the sandstones, and are possibly of distal continental origin. Some also contain very fine felsic ash particles. Average abundances and normalized multi-element diagrams are used to estimate proportions of three model end-member source constituents, average upper-continental crust (UCC), high-K rhyolite (RHY) and basaltic andesite (AND). Sandstone provenance for the Southland Syncline sector changed from a predominantly basaltic-andesite source in Late Permian to early Middle Triassic time, for example, UCC:RHY:AND = 0:17:83 in the Early to early Middle Triassic, to highly felsic in the Middle to Late Triassic, reaching UCC:RHY:AND = 2:74:24 in the Late Triassic Oretian Stage. A UCC component became increasing significant from latest Triassic upward and the proportion of mafic to felsic volcanism increased again, with UCC:RHY:AND = 15:30:35 in the Middle Jurassic Temaikan Stage. Mix modelling suggests that along-arc source proportions varied, with greater mafic and upper continental crust contributions in the northern Kawhia segment than in the Southland segment. These patterns may be explained by deposition at an oceanic Aleutian-type arc margin, with transition to a continental oceanic arc character induced either by arc evolution and dissection, forearc sliver translation, or underplating of rafted microcontinental fragments.

From a locality in North Staffordshire. Microspore assemblages are described from the Six Inch Mine coal seam, the immediately overlying carbonaceous shales, and the marine shale bed. Marked differences occur within this thin sequence in the proportions of certain genera present; in particular, the dominance of Florinites in the marine shales is discussed. Whilst the proportionate numbers of genera vary, the actual species present are more or less the same, though a greater variety exists in the shales. Three new genera and seven new species are described.

The Svecofennian Domain of the Fennoscandian Shield constitutes a considerable volume of Palaeoproterozoic crustal growth, 2.1–1.86 Ga ago, in between the Archaean craton in the NE and the 1.85–1.65 Ga Transscandinavian Igneous Belt (TIB) in the south and west. The Bergslagen area is a classical ore province located in the southwestern part of the Svecofennian Domain of south-central Sweden. Its northern part is dominated by volcanic and plutonic rocks of a magmatic arc with continental affinity, while the SE part is made up by a sedimentary basin. The Bergslagen area shows a metamorphic zonation from lower to middle amphibolite facies in the north to upper amphibolite facies and locally granulite facies in the south; a small greenschist area exists in the west. Identifying the age spectra of inherited components, magmatic crystallization, as well as metamorphic episodes, provide important constraints on the geodynamic evolution of this centrally located piece of the Shield.

U–Pb zircon SIMS data presented in this paper complement the previous, regionally scattered TIMS data from this area. Magmatic zircons from two felsic metavolcanic rocks and two amphibolites (metagabbros) yield 1888±12, 1892±7 and 1887±5, 1895±5 Ma, respectively; i.e. within the 1.91–1.86 Ga range previously obtained for Early Svecofennian magmatism in Bergslagen. An augen gneiss from southern Bergslagen, assigned to the earliest TIB generation, yield an intrusive age of 1855±6 Ma. Metamorphic monazites from the same rock indicate that deformation and elevated thermal activity prevailed 1.83–1.82 Ga ago (TIMS). Metamorphic zircons in high-grade metasedimentary rocks from the south and west yield ages of 1793±5 and 1804±10 Ma, in accordance with ages for regional peak metamorphism and migmatite formation found elsewhere in the southern Svecofennian province of Sweden. More importantly, a few zircon crystals and overgrowths in rocks from the north indicate an early metamorphic episode at c. 1.87 Ga, indicating that Bergslagen has experienced two major metamorphic events. Detrital and inherited zircons span the range 2.78–1.90 Ga, with an apparent gap at 2.45–2.1 Ga, which further emphasize previous observations of a major juvenile (<2.1 Ga) and a minor Archaean provenance. This, and in particular the 1.94–1.91 Ga crystals present in the c. 1.89 Ga amphibolites, support the suggestion of a former Palaeoproterozoic pre-1.91 Ga crust in the Bergslagen area.

BY those accustomed to the hopeless confusion of the Greywacké of the earlier geologists, the publication of Murchison's grand work on the “Silurian System” was hailed with feelings of the most profound relief and satisfaction. His clear and brilliant presentation of the physical and palæontological proofs of an orderly sequence among the Palæozoic Rocks below the Old Red Sandstone, as originally set forth in all their force and harmony in his magnificent volumes, naturally astonished and dazzled the majority of his scientific contemporaries, and secured for his nomenclature of these ancient deposits an almost universal acceptance. His subsequent abuse of this advantage to strengthen and consolidate his own system at the expense of that of his equally - illustrious co-worker—the less fortunate but more cautious Sedgwick —was a gallant but unscrupulous defence of this original nomenclature, which by that time he must have felt himself almost powerless to disturb. His later extension downward of the limits of his system, till it embraced all the rocks between the supposed Azoics and the Old Red Sandstone—though, in a measure, forced upon him from without—ought perhaps to be regarded in part as a very natural return to the ideas of his early teachers, who had always held the practical unitly of the rocks of the Transitionalperiod. In this wey, however, Murchison unwittingly destroyed many of the most beneficial results of his won labours; in a sense, spending his old age in the attempted re-erection of the very edifice it hed been the pride of his manhood to destroy—the early years of his scientific career being devoted to the worthy task of proving the marvellous variety of the Lower Palæozoics; his later years to demonstrating their integrity, unity, and indivisibility.

The Lower Silurian Osmundsberg K-bentonite is a widespread ash bed that occurs throughout Baltoscandia and parts of northern Europe. This paper describes its characteristics at its type locality in the Province of Dalarna, Sweden. It contains mineralogical and chemical characteristics that permit its regional correlation in sections elsewhere in Sweden as well as Norway, Estonia, Denmark and Great Britain. The <2 μm clay fraction of the Osmundsberg bed contains abundant kaolinite in addition to randomly ordered (RO) illite/smectite (I/S). Modelling of the X-ray diffraction tracings showed the I/S consists of 18% illite and 82% smectite. The high smectite and kaolinite content is indicative of a history with minimal burial temperatures. Analytical data from both pristine melt inclusions in primary quartz grains as well as whole rock samples can be used to constrain both the parental magma composition and the probable tectonic setting of the source volcanoes. The parental ash was dacitic to rhyolitic in composition and originated in a tectonically active collision margin setting.

Whole rock chemical fingerprinting of coeval beds elsewhere in Baltoscandia produced a pronounced clustering of these samples in the Osmundsberg field of the discriminant analysis diagram. This, together with well-constrained biostratigraphic and lithostratigraphic data, provides the basis for regional correlation and supports the conclusion that the Osmundsberg K-bentonite is one of the most extensive fallout ash beds in the early Phanerozoic. The source volcano probably lay to the west of Baltica as part of the subduction complex associated with the closure of Iapetus.

In some recent papers dealing with Tithonian ammonites, the writer included zonal schemes of the higher Jurassic, up to what he considered the topmost horizon, namely the privasensis zone. The correlation of the Mediterranean ammonites with those of the “boreal province” or Pavlow's “Aquilonian” was also discussed. Since this author had recorded “Aquilonian” Craspedites and Garniericeras from the Speeton Clay and Spilsby Sandstone, the inquiry naturally led to a revision of the ammonites of the Neocomian of Yorkshire and Lincolnshire. A critical examination of the ammonite horizons at the Jurasso-Cretaceous border-line seemed specially invited because in our most recent textbooks Yorkshire strata obviously well up in the Cretaceous are still included in the Jurassic; and it was also deemed useful to link up the table of Tithonian ammonite zones, above referred to, with that of the Aptian, given by the writer in a paper on the “Ammonite Horizons of the Gault and Contiguous Formations”.

Accessory chrome spinels are scattered throughout the serpentinite masses in two allochthonous thrust sheets belonging to the Penjween–Walash sub-zone of the northwestern Zagros Suture Zone in Kurdistan. Based on field evidence, the serpentinites are divided into two groups: (1) highly sheared serpentinites (110–80 Ma), which occupy the lower contact of the ophiolitic massifs of the Upper Allochthonous sheet (Albian–Cenomanian age), and (2) ophiolitic mélange serpentinites of mixed ages (150 and 200 Ma) occurring along thrust faults on the base of the volcano-sedimentary segment (42–32 Ma) of the Lower Allochthonous sheet. The Cr-spinels of both groups show a wide range of YCr (Cr/(Cr + Al) atomic ratio) from 0.37 to 1.0, while the XMg (Mg/(Mg + Fe2+) atomic ratio) ranges from 0.0 to 0.75. Based on the Cr-spinel compositions of the entire dataset and in conjunction with back-scattered electron imaging, from core to rim, three spinel stages have been recognized: the residual mantle stage, a Cr-rich stage and a third stage showing a very narrow magnetite rim. These three stages are represented by primary Cr-spinel, pre-serpentinization metamorphosed spinel and syn- or post-serpentinization spinel, respectively. The chemical characteristics of primary (first-stage) Cr-spinels of both serpentinite groups indicate a tectonic affinity within a fore-arc setting of peridotite protoliths. The second stage indicates that Cr-spinels have undergone subsolidus re-equilibration as a result of solid–solid reaction during pre-serpentinization cooling of the host rock. Here the primary Cr-spinel compositions have been partly or completely obscured by metamorphism. During the third stage, the Cr-spinels have undergone solid–fluid re-equilibration during syn- or post-serpentinization processes. Both the second and third stages point to diachronous metamorphic paths resulting from continuous tectonic evolution influenced by either slow or fast uplift of mantle protoliths. In the fast metamorphic paths, the primary chrome spinels are flanked by a very narrow magnetite rim. The presence of two groups of distally separated serpentinites with different emplacement ages and fore-arc tectonic affinity could indicate that the closure of the Tethys Ocean culminated in two fortuitous subduction processes.