Data on the carbon and oxygen isotopic composition of the carbonate sediments, limestones and calcite cements are compiled and reviewed, and an elementary exposition of the factors that control them is given. Typical compositions and diagenetic trends are displayed on δO 18 –δC 13 scatter diagrams, and an attempt is made at an isotopic categorization of environments of carbonate lithification. Many limestones pass through several diagenetic environments, which are recorded isotopically by cement generations of distinct isotopic composition. Most limestones contain essentially 'marine' carbon, but extreme compositions can result from organic reactions, especially those involving methane; some of these may be important in petroleum exploration, but their volumetric abundance is probably quite small.

In interpreting the results of a petrographic and isotopic study of concretions, a range of subjects is discussed including the original texture of the Oxford Clay sediment, Jurassic palaeotemperatures, the diagenetic history of pore‐waters and the palaeo‐hydrology of central England. The concretions are all composed predominantly of calcite. They include precompactional, pyrite‐rich concretions that later suffered an eposide of brecciation, and others that only commenced to form after compaction had crushed ammonite shells included in the bituminous clay sediment. Petrographic, chemical, and especially carbon isotope data demonstrate a dominantly organic source for the carbon in the early formed concretions. Oxygen isotopes indicate formation at the same temperatures (13‐16°C) at which benthic molluscs were living. Concretion growth in pelleted, anaerobic mud proceeded concurrently with bacterial sulphate reduction and pyrite precipitation. Cracking of the concretions started at this stage: in a few concretions, the cracks were also partially filled with brown calcite. During post‐compactional growth, δ13C increased and pyrite content decreased, showing waning organic influence; δ18O decreased. The brecciated concretions were intruded by clay in which baryte crystals grew; finally, most remaining voids were filled with strongly‐ferroan calcite of δ18O about—7 PDB and δ13C about O PDB. This must indicate strong depletion of the pore waters in 18O. Mechanisms that might lead to this are reviewed. It is concluded that the sequence of mineralogical and chemical changes is most readily explained if originally marine porewaters, first modified by bacterial activity, were flushed from the compacting clays by water of ultimately meteoric origin. This had its source in palaeo‐aquifers beneath the Oxford Clay. Speculative attempts are made to relate this history to the geology of the region.

Fossil assemblages can give quantitative estimates of palaeotemperatures, by comparison with modern biota, only in the recent geological past. Oxygen isotopic palaeotemperatures on calcareous or phosphatic fossils are potentially available for the whole Phanerozoic. Their reliability is limited by physiological effects (generally believed minor), preservation (for which criteria are available), and by uncertainty in the isotopic composition of ancient seawater. The latter is greatly affected by glaciation. In the Cenozoic, the relative contribution of ice-volume change and temperature change in producing isotopic variations can largely be resolved by analysing planktonic and benthic foraminifera in deep-sea cores. For earlier times only continental shelf deposits are available. In the Mesozoic, reasonable assumptions about ocean isotopic composition lead to palaeotemperature estimates that suggest generally higher temperatures than at present, particularly for mid- to high latitudes. This agrees with estimates based on biotic distributions. Late Palaeozoic glaciation is reflected in variable isotopic compositions in high palaeolatitude areas. In the earlier Palaeozoic, well-preserved fossils indicate either oceans enriched in 16O compared to today's or generally higher temperatures; controversy continues about the relative importance of the two effects.

Pyrite occurs both in normal clays and shales with a benthic fauna (Oxford Clay, England, and Lias ε, Germany) and in highly bituminous shales (Lias ε, Germany). In normal shales it is present in small quantities as early framboids, but more conspicuously as internal moulds of fossils, especially ammonites. The pyrite in these is petrographically varied; several types of internal sediments and chamber linings are described and illustrated by reflected‐light and scanning electron microscopy. Most striking are pyrite stalactites, suspended from the roofs of ammonite chambers, which were later filled by calcite or baryte. Pyrite formed in reducing micro‐environments, while the sediment generally was not wholly anoxic. Most pyrite pre‐dates compaction of sediment, breakage of fossils and solution of shell aragonite. Variable rates and conditions of reduction of sea water sulphate are reflected in δ34S values ranging from −55 to +44. Stalactites probably started to form when the ammonite chambers were partially gas‐filled. In the bituminous Lias ε shales pyrite occurs abundantly as early framboids and micro‐nodules. Larger nodules show a variety of forms, some of which post‐date compaction of the sediment. Pyrite is not associated with the abundant flattened ammonites. δ34S values in shales are grouped about a mode near −20. Pyrite formed over a long time‐span, and throughout the sediment, not just in protected cavities. Contrasts in pyrite types can be related to differing depositional environments and organic contents of the shales. Pyrite is an important mineral in diagenetic mineral parageneses which can be deduced by studying fossil void‐fillings and concretions, and which help define the diagenetic history of a shale.

Synopsis Revised lithostratigraphical terminology for the Great Estuarine Group in Skye, Raasay, Eigg and Muck is proposed. The group comprises the Cullaidh Shale Formation, Elgol Sandstone Formation with Kildonnan Member and Lonfearn Member, Valtos Sandstone Formation, Duntulm Formation, Kilmaluag Formation and Skudiburgh Formation. The type sections of these formations are defined, new measured sections of 5 of them are illustrated and their lateral variations described. The Group is predominantly Bathonian in age, having its base in the Garantiana Zone (Upper Bajocian) and its top in the Macrocephalus Zone (Lower Callovian). The mainly argillaceous formations show wide lateral continuity indicating that a single depositional basin must have occupied the Inner Hebrides—Minch area during the Bathonian.