Rhyolites in the vicinity of Snowdon (North Wales) are intimately associated with the evolution of the Lower Rhyolitic Tuff Formation (LRTF) caldera of Ordovician (Caradoc) age. They occur as deep-seated dykes, sills and small stocks, shallow-level intrusive domes, and domes extruded within a predominantly shallow-marine environment. Extrusion occurred during three main phases, indicating the episodic availability of rhyolite magma. The rhyolites can be divided on their trace element ratios (e.g. Nb/Zr) into five main groups. Extrusive representatives indicate that each group correlates strongly with a single phase of rhyolite extrusion. Within each group, the distribution and variation of intrusive form with stratigraphic level suggests that geochemically similar rocks were emplaced at approximately the same time. Consequently, the groups represent discrete magma compositions tapped from the evolving Snowdon subvolcanic magma system. Differences in distribution of the groups reflect changes in structural controls of emplacement before and after development of the LRTF caldera.

Microscopic soft-sediment deformation structures in a 30 × 25 × 12 mm hand specimen of glaciogenic silty mudstone from the Permo-Carboniferous Dwyka Formation of northern Natal in South Africa are illustrated by serial sections. The processes these structures imply are interpreted using palinspastically restored sections and isopachytes reconstructed from them. The sedimentation of silts, muds, and microrhythmites is found to have been punctuated by episodes of hydraulic activity which resulted in thirteen bodies with the mixed fabrics of micromelanges. These are divisible into five melange types: (1) subconcordant intrusive; (2) disconcordant intrusive; (3) subconcordant extrusive; (4) near-surface with asymmetric internal structures, and (5) conformable near-surface with internal structures symmetric about the palaeovertical.

Remarkable similarities between isopachs for pairs of micromelanges at different levels in the specimen suggest that intrusive melanges at depth fed contemporaneous extrusive melanges on the sea floor. Each couplet of melanges with matched isopachs is linked by faults which are interpreted as having acted as hydraulic vents despite only parts of them being infilled by intrusive melange. A significant proportion of the succession could consist of sediment recycled from depth by hydraulic extrusion. Repeated hydraulic intrusions along, and extrusions from, the same disturbed interface suggest that this interface acted as the distal leaking end of one of the hydraulic sills recently described by von Brunn & Talbot (1986). Pulses of pressurized water were transmitted through a prograding marine slope of quickclay by the tidal pumping of a marine ice sheet periodically grounding upslope.

The results of this analysis are extrapolated to processes operative in subduction complexes. All four fabrics described from large-scale hydraulic melanges in modern and ancient accretionary prisms are matched on a much smaller scale in our sample from a proglacial submarine slope. Dismemberment by faults is the only fabric element described from accretionary complexes which is missing from the micro-analogues described here.

Durvilleoceras woodmani of debated Late Permian or Early Triassic age is accompanied by other ammonoids, deemed to belong to Araxoceratidae, Xenodiscidae, and Goniatitida, of chiefly Late Permian age. The ammonoids are found principally in the Deserter Bay and especially Tongue Point Members (new names) of the newly named South Arm and Wells Arm formations of the Greville Subgroup, which are overlain, apparently in normal stratigraphic succession, by the Waiua Formation and newly named formations of the Te Mokai Group, with three successive faunas of Permian appearance.

Two new genera and species of Middle Cambrian sphinctozoans, Jawonya gurumal and Wagima galbanyin, are described from the Tindall Limestone of the Daly Basin, Northern Territory, within continental platform successions of the northern Australian craton. They are ambisiphonate sebargasiids, coeval with the island-arc shelf fauna from the Ordian Stage of western New South Wales. Additionally, a new Early Cambrian (Atdabanian equivalent) species, ?Jawonya tiro, displaying some sphinctozoan and some archaeocyathan features, is documented from the Ajax Limestone of the Arrowie Basin, South Australia. It is tentatively regarded as the oldest known sphinctozoan, but raises the possibility of the derivation of sphinctozoans from archaeocyaths. A brief review charts the Early–Middle Palaeozoic history of the sphinctozoans. The use of filling tissue type as a family-level criterion in sphinctozoan classification needs some reappraisal, as some types are of secondary origin.

Archaean sedimentary and volcanic successions in the Lalla Rookh Basin and c. 2950 Ma old Whim Creek Belt in the Pilbara Block, Western Australia, were deposited in basins with roughly the same configuration as their present outcrop. Basins were fault-bounded and developed in an ensialic setting, overlying older (3500 to 3300 Ma old); deformed and metamorphosed supracrustal rocks and granitoids. The basin margin faults are now part of a pattern of strike–slip faults which were active during the later stages of regional batholith emplacement. In both cases, structural patterns and style of basin filling are similar to younger basins related to strike–slip faulting. The Lalla Rookh Basin was dominated by coarse clastic sedimentation, comprising alluvial–fan, braided–stream, fan–delta and lacustrine facies. The Whim Creek Belt contains bimodal volcanics and clastic sediments, which comprise alluvial, subaqueous fanglomerate, submarine-fan and basinal facies. Regional strike–slip faulting and the development of the Lalla Rookh Basin and Whim Creek Belt, in response to externally imposed deformation, records an important step in the cratonization of the Pilbara Block. Late Archaean sedimentary basins, dominated by coarse clastic facies and situated adjacent to major strike–slip faults, in other cratons may have a similar origin.

The Castores and probably the Santa Rosa plutons of north-west Isla Navarino, southern Chile, have been emplaced by in situ diapirism into metasedimentary rocks of the Upper Jurassic (?)–Lower Cretaceous Yaghan Formation. For the former, this model is consistent with the concentric foliation paralleling the margin of the pluton and the foliation and stratification planes in the metamorphic aureole. Only the southern part of the Santa Rosa Pluton is preserved, and it has some structures similar to those of the Castores Pluton, which can also be interpreted as produced by an inflating diapir. The main intrusive rocks of these plutons are quartz-monzodiorites and quartz-diorites with synmagmatic foliation. They were preceded by minor bodies of hornblende gabbros, and followed by dykes and small bodies of non-foliated granodiorites. Non-foliated to weakly foliated granodiorites, forming the centre of the Castores Pluton, probably represent a younger intrusive pulse.

Twelve K–Ar mineral dates from 10 specimens of plutonic rocks, interpreted as near crystallization ages, span the period 80–90 Ma. These dates do not show the sequence of intrusion of the different rock-types, which may suggest that all of them were intruded and cooled in a short period of time. The timing of emplacement of these plutons in relation to tectonism is difficult to determine; however, a post-tectonic emplacement for at least the Castores Pluton, is proposed.

The way in which probability can enter the interpretation of geological data is outlined. Probabilistic models are introduced in data synopses to allow statistical inferences about samples and population parameters. Inferences of this type are a special application of mathematical theory and have little to do with testing scientific generalizations. The logic of statistical inference, as used in operations research, is contrasted with that of geological inference. Geology is a cumulative science; its methodology is not that of experimentation with formal replication; inferences are developed from observations and tested and refined against new data, often by different investigators at different times and in different places. Because of the way in which inferences are drawn, sampling in geology is purposive. Random sampling, necessary for inferences based on sampling theory, is unattainable in most geological contexts. Different classes of geological measurements require careful consideration as to their appropriate form of synopsis, and, particularly, as to whether a parametric probability model is applicable.

The new terms ‘equal fold’ and ‘unequal fold’ are defined by whether the ratio of dips of limbs, measured in degrees, is less or greater than 1½.