The overprinting of the high grade rocks of the Scourian episode, during the Inverian and Laxfordian episodes, has so far prevented a reliable estimate being made of the age of the Scourian granulite–facies metamorphism in the Central Region of the Lewisian of northwestern Scotland. In an attempt to solve this problem the U–Pb isotopic systems of zircons from two samples of hypersthene granulite from the Kylesku group have been examined. These zircons have discordant apparent ages complicating the interpretation of the results. However, by measuring a number of size fractions from each of the two zircon suites it has been possible to demonstrate that a major isotopic event or events took place approximately 2700 m.y. ago in these granulite–facies rocks. It has also been shown that the two discordance trends found for the two zircon populations can be explained in terms of episodic disturbances of the U–Pb isotopic systems at times which correspond to the known ages of ‘Laxfordian’ and ‘Caledonian’ events.

An earliest Llanvirn age is established on micropalaeontological grounds for the interbedded lava–pelite sequence at the base of the Binsey Volcanic Formation in the northern Lake District. The northern volcanic sequence, here termed the Eycott Volcanic Group, is earlier than, and chemically distinct from, the main Borrowdale Volcanic Group. The evidence precludes a regional unconformity between the Skiddaw Slates and the overlying volcanic rocks, but is compatible with the hypothesis that the vulcanicity occurred in an island arc environment.

Various small-scale tectonic structures which deform the first, regional folds and cleavages are described. The strain sequence in the rocks near Aberystwyth is shown to resemble that established elsewhere in the paratectonic Caledonides.

The stretched crystals of tourmaline occur in the Orange Group gneisses of the Lucomagno massif and in the quartzite of Piano Croce Group in the Molare region, Ticino, Switzerland. They were stretched as a result of an extension acting in the plane containing the tourmaline crystals by the tectonic strain of F3 phase of deformation. The crystals show extension parallel to the F3 fold axes. The computation of the values of the finite strain from stretched tourmaline crystals indicates a constriction-type ellipsoid for the tectonic strain of F3 phase of deformation.

The paper comprises descriptions of a new genus and species of multiramous dichograptid from the Bendigonian (Be 4) and a new species of Tetragraptus from the Lancefieldian (La 2).

Measurements of en-échelon veins cutting small chevron folds in Carboniferous greywackes, near Hartland in Devon, allow the following new conclusions about naturally occurring second-order fractures to be stated. 1. Veins may be initiated between 10° and 46° to the shear zone in which they occur. Previous studies of en-échelon fractures have generally classified them as either tension gashes formed at 45° to an array, or Riedel shears at 15° to an array. 2. Veins at about 20° to 40° to an array probably occupy surfaces which are transitional between shear and extension fractures. 3. Proof of a component of shear, in addition to dilation, on many of the veins at less than 40° to an array is provided by crystal fibres at acute angles to vein margins, and third-order en-échelon veins in arrays which are parallel to and part of a set of second-order veins.

The cumulates and crescumulates in the Crescumulate Zone of the Rognsund gabbro are described. The olivine and olivine-clinopyroxene crescumulates contain large dendritic crystals of olivine due to rapid upward growth from cumulus nucleii on the temporary floor of the magma chamber. Crescumulate growth was accompanied by the bottom accumulation of primocrysts, some of which were enlarged by the normal adcumulus process. The crescumulates are thought to have originated when cumulus crystals sank from a labile environment near the roof of the body into a metastable environment on the floor.

A method of contouring pole distributions on Equal Area stereograms is presented; the advantage of this method is a combination of speed, convenience and maximum accuracy.

Potassium–argon dates from hornblende-bearing pyroxene granulites in a large xenolith near the margin of the charnockitic Ernabella Adamellite, central Australia, are interpreted in the light of oxygen isotype studies of associated mineral phases which give a detailed thermal history of the Adamellite and associated rocks.

Hollow, often ellipsoidal, clusters of small manganiferous garnets are described. They occur as discrete entities in the normal garnet–quartz coticule fabric of the Garnetiferous Beds, an important group of metamorphosed manganese-rich sediments found within the aureole of the Leinster Granite. The clusters are thought to represent small original sedimentary concretions, subsequently completely reconstituted.