Trilobites of upper Ludlow age are described from Member A of the Read Bay Formation on Cornwallis Island. Four new species are represented: Helokybe spio gen. et sp.nov. (Dechenellinae), Encrinurus hyperboreus sp. nov., Kailia? capra sp.nov. (Encrinurinae) and Hemiarges echinatus sp.nov. Other taxa present are: an effaced scutelluid; Cyphaspis sp.; a warburgelline; two more encrinurids; two other forms of Hemiarges. The trilobites occur in sediments indicative of a shallow subtidal environment, with the exception of the warburgelline which occurs in sediments deposited under deeper subtidal conditions below storm wave-base.

Thin coccolith-rich bands, in which coccoliths make up a major part of the rock, have been recorded at 11 horizons in the Kimmeridge Clay of Yorkshire, Lincolnshire, Norfolk, Surrey and Wiltshire and also in cliff sections at and adjacent to Kimmeridge Bay, Dorset. It seems likely that at least nine of these bands form persistent markers throughout southern and eastern England. The coccolithophorid assemblages are discussed and some species of Stradnerlithus are systematically described.

[The following is the text of a lecture delivered in Cambridge on 19 October 1978 to mark the 250th anniversary of the founding of the Woodwardian Chair.]

A new stratigraphic scale of the USSR Precambrian officially adopted by the All-Union Precambrian Meeting in Ufa (1977) and approved by the National Stratigraphic Committee (1978) is discussed. Archean and Proterozoic have been distinguished in the Precambrian, the boundary between being 2600 Ma. Proterozoic is subdivided into the Lower- and Upper-Proterozoic with the boundary between them 1650 Ma. Riphean and Vendian are distinguished in the Upper Proterozoic. More detailed subdivisions of the Upper Proterozoic are Burzyanian, Yurmatian, Karatavian, Kudashian and Vendian. Lithological and palaeontological characteristics of these subdivisions and their radiometric age are given.

Morphological features of brittle fracture in glass, ceramics and rocks include fracture origin, mirror plane, mist, hackle, radial striations and concentric undulations. The plume structure which characterizes many joint surfaces is analogous to striations or ‘grainy steps’ developed on fracture surfaces of ceramics. Joint surfaces with plumose structures result essentially from tensile failure, but may also be affected by shear stresses. The ellipticity of the plumose structure commonly observed reflects a region of secondary tensional strain parallel to bedding developed due to anisotropic elastic properties of the rock as a result of formational loading. Striations and hackles may have similar orientations. A distinction between them is important since hackles on a joint surface indicate a rapid fracture whereas striations can result from a slow fracture. Intense undulations without hackling may indicate a slow fracture. A smooth mirror plane on a joint surface indicates that while the fracture had been formed, stresses had reached a certain maximum and then subsided. On the other hand, a joint surface that shows a mirror plane with hackle around its boundaries indicates that no decrease of stresses occurred during fracture, and fracture was possibly produced under a constant stress. Since AH is a material property, the fracture stress σfresponsible for a rapid fracture may be calculated following a determination of the mirror constant AH of the rock by σfrH½=AH

19 new K–Ar mineral ages of 78-201 Ma and 3 Rb–Sr whole rock isochron ages of 81 ± 10, 127±4 and 181±30 Ma are presented from units of continental crust, mafic complex and island arc assemblage on South Georgia. The Drygalski Fjord Complex, part of the possible floor of the marginal basin in the southern part of the island, includes granodiorite and gabbro plutons of minimum age 180–200 Ma. Together with older metasediments they have been affected by a major thermal event at about 140 Ma, thought to have resulted from the emplacement of a mafic complex (Larsen Harbour Formation) during the initial opening of the marginal basin. Rocks of the Larsen Harbour Formation are cut by the Smaaland Cove intrusion dated by Rb–Sr whole rock isochron at 127±4 Ma. An island arc assemblage exposed to the SW of South Georgia consists of pyroclastic rocks cut by monzodiorite and andesite intrusions, which give radiometric ages of 81–103 Ma. These data suggest that the marginal basin opened during the late Jurassic (pre-140 Ma); that part of an earlier (early Mesozoic) magmatic arc is preserved in continental crust making up part of the floor of the basin; and that subduction continued beneath the island arc until at least the Senonian time. The younger plutons in the arc were emplaced at roughly the same time as turbidite facies rocks at deep levels in the marginal basin were being affected by penetrative deformation and metamorphism. The timing of events on South Georgia agrees closely with that deduced for the continuation of the same island arc–marginal basin system in South America. The 180–200 Ma plutons correlate with an older suite of plutonic rocks reported from the Antarctic Peninsula and southern Andes; they are part of a once-continuous magmatic arc related to subduction of the Pacific plate beneath Gondwanaland during the early Mesozoic.

Whitcliffian and Downtonian macroplants of rhyniophyte morphology are described from Capel Horeb Quarry, Powys, Wales. Both floras contain Hostinetla Stur, Cooksonia Lang and Steganotheca Edwards. They are briefly compared with others of similar age, and the contribution, particularly of the Whitcliffian assemblage, to our knowledge on the early colonisation of the land by vascular plants is discussed.

The blueschist facies metamorphism has always been one of the most difficult to quantify. This is for two important reasons. Firstly, the temperatures and pressures of the facies tend to be inaccessible to experimental analogues of the blueschist indicator minerals: aragonite, glaucophane, jadeite and lawsonite. Secondly, the conditions of blueschist metamorphism are not typical of the earth's crust. For a rock to be metamorphosed under lower temperatures at a given pressure than those of normal geothermal gradients (considered here as negative deviation metamorphism) requires ‘rapid’ downwarping and reuplifting. The movement during metamorphism, combined with hydration rather than dehydration (a feature of this environment) fails to preserve a single metamorphic culmination and hence the mineralogy and textures of blueschist assemblages may fail to match the conventional phase equilibrium metamorphic model. Thus the simple phase equilibrium interpretative basis of metamorphic petrology is recognized as only a special case of a more general theory of metamorphic chemical/mineral interaction.

Studies on the distribution of manganese-bearing rocks in the Birimian and their relationship with gold mineralization are still in progress. Both manganese and gold occur as stratabound lenses within volcano-sedimentary sequence. Evidence of pre- and post-tectonic mineralizations are present. Sedimentological evidence also suggests both may have been contemporaneous with their host rocks in an environment that was strongly Eh and pH controlled. Under present tropical conditions, other elements associated with Birimian gold mineralization, notably copper and arsenic, are highly mobile to remain in residual soils. However, manganese, like iron, remains in residual soils as oxides and could be a useful pathfinder element in the search for gold in tropical lateritic soils derived from Precambrian greenstones.

The four groups of tektites are now widely regarded as the product of splash from a meteoritic or cometary impact melt of terrestrial rock. They include two groups (Moldavites and Ivory Coast tektites) attributed to the Nordlinger Ries and Bosumtwi craters. These craters seem unlikely to be of impact origin, though hyper-explosive. Tektites are, therefore, more probably all wholly terrestrial. For much the largest group, those of the combined Southeast Asian-Australian strewnfield, no satisfactory source is known. An origin in Southeast Asia, even by impact, is improbable. A possible source lies in Central Asia. The hyper-explosive activity is regarded as an extreme form of volcanism, a consequence locally and rarely of the continual combined vertical and ‘horizontal’ movements of continents, here termed tribulation.

The Upper Westphalian succession in the National Coal Board's Eastling Wood Borehole, 1976 ‘TR 6303 1472’ has been analysed in terms of upward-fining and upward-coarsening semicycles, transition probabilities and entropies. This analysis confirms that the succession can be divided into a lower coal facies, which like the earlier Westphalian deposits of Kent was laid down dominantly in deltas but which includes numerous channel-fills, and an upper channel facies laid down in a more upstream fluvial environment.

The Hardy Formation, a sequence of Upper Mesozoic volcanic rocks exposed in Peninsula Hardy (Isla Hoste) in the southernmost archipelago of Chile represents, at least in part, the island-arc assemblage of an island-arc-marginal-basin system related to an eastward dipping subduction zone. This island arc was founded on South American continental crust and is also represented in the island of South Georgia 2000 km to the E. The island-arc assemblage includes pyroclastic rocks, characterized by a high proportion of vitric material, and lava intercalations ranging in composition from rhyolite to basalt. These rocks underwent zeolite and prehnite-pumpellyite facies metamorphism and are gently folded, in contrast with the intense folding exhibited by the rocks exposed to the north of Peninsula Hardy. Silicic volcanics assigned to this assemblage underlie pillow lavas, and are intruded by dolerites and gabbros probably related to a Late Jurassic-Early Cretaceous ophiolite magmatism associated with the generation of a quasioceanic marginal basin. Volcanic turbidites (Yahgan Formation) were deposited into the marginal basin.

It is suggested that in pre-marginal basin times the Hardy Formation interfingered towards the Atlantic with the silicic volcanics of the Tobifera Formation. However, recent geochemical work on the Tobifera Formation suggest an origin by continental crust anatexis in a volcano-tectonic rift zone related to upper mantle diapirism, whereas an island arc origin is favoured for at least the andesitic and basaltic components of the Hardy Formation. Therefore, the geology of Peninsula Hardy as presented here, confirms early assumptions of the splitting apart of a Middle–Upper Jurassic volcanic terrain along the Pacific margin of South America during the generation of a marginal basin. The spreading axis of the latter seems to have been located at the boundary of two somewhat overlapping petrotectonic assemblages: and island arc on the Pacific side and a silicic volcano-tectonic rift zone towards the Atlantic. A probably Cenozoic volcanic complex discordantly overlies the Yahgan and Hardy formations.

Within blueschist facies rocks it is common to find that mineral assemblages represent a series of attempts to arrive at changing states of equilibrium. To interpret such rocks it is necessary to study how the mineral constrains and determines chemical interaction. The gradual process of chemical exchange between minerals, during changes in conditions, is interrupted by the nucleation of a new mineral species. Mineral reactions are instigated by the nucleation of one new mineral species, though when a reaction is seen at an equilibrated completion the reaction path cannot be traced. In low temperature metamorphism the preservation of partial mineral reactions is very widespread. The growth, adaptation and resorption of a mineral are considered in order to demonstrate: (1) how and why a mineral should zone during growth: (2) the competitive reaction paths that may allow a mineral to change composition, even across and internal solvus; and, (3) through the consideration of a few simple parameters, the prediction of a variety of resorption textures. It is proposed that through an understanding of the way in which the mineral can store information on the chemical history of the rock it is possible to obtain a metamorphic analysis without recourse to the conventional phase equilibrium metamorphic model - a frequent misapplication in the blueschist facies.

Sheath and core structure results from secondary hydration and devitrification, initiated at relatively high temperatures during cooling, of the intrusive pitchstone sheets. Less than 0.5% of the water present is an original magma component.

Dionide hectori Reed, 1926, is shown on the basis of new collections from the type area in New Zealand to be a Tremadocian ceratopygide close to Hysterolenus. The species was placed in Taihungshania by Kobayashi (1941). It was provisionally retained in the Taihungshaniidae by Lu (1975) who made hectori type species of a new genus Hectoria (non Hectoria Trechmann, 1918). Ruapyge nom.nov. is proposed here for the reception of hectori. The original length/width ratio is determined by Wellman's method, with little consistency in results. Data from the strain ellipses constructed by Wellman's method are then used to calculate values for the attitude of the line of no finite longitudinal strain. The values obtained graphically do not agree with those derived from a formula given by Ramsay (1967); this contradictory situation is in need of explanation.

Stratigraphical investigations in the Finchley depression (Middlesex depression) are summarized and show that a gravel, laminated deposits and till sequence is present. Compositional study of the gravels indicates that they were derived from S of the present Thames valley. Gravel patches in North Surrey can be subdivided into two terrace fragments formed by a Mole-Wey south bank Thames tributary. The upper unit is correlated with the gravel at Finchley. The lower unit terrace can be shown to grade into the Thames Black Park Terrace Gravel, and the latter is extended through London to Dartford and Chatham. The events leading to the diversion of the Mole-Wey tributary by the Anglian chalky till ice parallel those of the Thames in the Vale of St Albans and suggest that similar events may also have occurred in other S bank tributary valleys to the E.

The Durn Hill Quartzite of Redhythe Point, near Portsoy, can be divided into an eastern shear zone and a western foliated zone. The latter region is typical of most quartzites between the coast and Keith. NW-SE trending flexures, pure shear folding and wrench faults, interfere with the dominantly NE-SW trending folds of the steep limb of the Boyndie Syncline. Possible large scale NW-SE trending monoclines were identified at Redhaven, Aberchirder and Rothes. Such structures bring down the older fold axial traces and isograds towards the NE.

Fabrics in migmatites from Portsoy in the NE Dalradian are compared with those in paratectonic and pretectonic migmatites from deeper structural levels of the Northern Grampain Caledonides. Paratectonic leucosomes, from Portsoy and Mid-Strathspey, have weak biotite fabrics. They lack the biotite foliation which is strongly developed in pretectonic leucosomes from the Upper Findhorn area. In conjunction with the sequence of deformation, intrusion and metamorphism in Banffshire, the fabric data indicate that migmatization at Portsoy was paratectonic with respect to the local F, folding. Partial melts generated in the aureole of the pre-F3 basic intrusions were, in some rocks, still mobile during F3 and completely crystalline only after the formation of small F3 folds. Granitic leucosomes in Mid-Strathspey were formed within the time-span of a deformation episode. Petrographic measurements show that these leucosomes have approximately minimum-melting compositions in Qz-Ab-An-Or, despite some deformation and retrograde alteration. They are comparable with the Portsoy migmatites in grain size and in complexity of leucosome shapes, and they are tentatively interpreted as products of incipient partial melting without dehydration of biotite.

The Wagwater Belt is a fault-bounded structural unit in which more than 3000 m of early Tertiary sedimentary and volcanic rocks are exposed. Geochemical analyses of the volcanic rocks show that they comprise a bimodal suite of plateau-type tholeiitic basalts and calcalkaline dacites. The extrusion of these volcanics is associated with the development of an interarc basin, the Wagwater Basin, at the beginning of the Cenozoic Era. The Wagwater Basin formed as a result of the splitting of a mature late Cretaceous volcanic arc into a frontal and third arc represented by the Blue Mountain Massif and the Clarendon Block respectively. This model for Jamaica can be correlated with tectonic events occurring in the NW Caribbean during the early Tertiary. The creation of the Wagwater Basin and the eruption of the basalts is related to the initial opening of the Cayman Trough. The cessation of dacite volcanic activity in Jamaica signified the separation of the Caribbean Plate from the East Pacific Farallon Plate.

The rare cladid inadunate crinoid genus Lampadosocrinus is presently known from 2 species, L. minutus (Peck) and L. obtusus Strimple and Koenig, of lower and middle Mississippian age (Tournaisian, Lower Carboniferous). Lampadosocrinus frustulum sp.nov. is described from the Lower Pennsylvanian (Namurian B or C. Upper Carboniferous) of Oklahoma, U.S.A. Some difficulties encountered in orientation of the theca of microcrinoids presently classified as cladid inadunates are discussed.