The reference of Isograptus manubriatus (T. S. Hall) to the genus Maeandrograptus Moberg by Skevington (1968) is argued to be invalid. A new genus, Pseudisograptus has been introduced to which the manubriate isograptids I. manubriatus, I. hastatus, and I. dumosus are referred.

Within the Tethyan Zone of Central West Nepal, the Nilgiri Limestone is the dominant unit. The significance of the separation of the Larjung Formation is questioned in the light of new outcrops examined, and the Nilgiri Sandstone is described. Recumbent fold structures are correlated between Annapurna, Nilgiri and Dhaulagiri. Between these mountains the Thakkhola graben faults are continued southwards, and estimates are made of the magnitude of their throws.

Precambrian rocks throughout most of the southern part of the Bighorn Mountains, Wyoming, are dominated by a biotite-bearing plagioclase–quartz gneiss containing hornblende-rich or microcline-rich layers locally. Compositional and/or optical data are presented for plagioclase, biotite, and hornblende as well as optical and X-ray data for microcline and preliminary morphologic data for zircon in the gneiss. These data, supplemented by field and petrographic observations, suggest that the gneiss was formed under conditions of the low amphibolite facies, that late-occurring potash metasomatism accompanied the metamorphism, and that the gneiss was derived from a relatively chemically uniform parent rock.

A new structural diagram derived from the equal-area net is proposed that helps in conversion of pitch data of folds into amount and direction of plunge, and in precise classification of the fold orientations. A new convention for indexing of folds and the use of subscripts for concise description of inclined folds are suggested.

Occurrences of penecontemporaneous folds of varying style and amplitude are described from low-metamorphic flysch sediments from an area in the Central Norwegian Caledonides. A short discussion of the mode of formation of these folds is presented, and from geometrical considerations it is suggested that, in several cases, both foundering and slumping have contributed to the overall deformation. The significance of the presence of these structures in the stratigraphical sequence is outlined, and the inherent danger of their possible misrepresentation as tectonic folds in higher metamorphic areas is briefly discussed.

Bouldery outcrops of coarse-grained olivine-free lava, containing abundant orthopyroxene and clinopyroxene, mostly in glomerophyric aggregates, occur over a small area among the Tertiary to Recent volcanics of the Jos Plateau, Nigeria. This is the only rock of tholeiitic affinity in an otherwise completely alkaline volcanic province. Petrographically it does not greatly resemble tholeiitic occurrences reported from such provinces elsewhere, but that is believed to be due more to a relatively complex crystallization history than to any fundamentally different process of magma generation. It is not regarded as representing a tholeiitic parent for the alkaline lavas of the Jos Plateau region.

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.

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.