Evidence is presented for a late Proterozoic, tectonothermal event which affected the rocks of the Moine assemblage in the Central Highland region of the Scottish Caledonides c. 750 Ma ago. This is about 200 Ma before the early Palaeozoic Grampian orogeny, whose effects are superimposed on the Moine rocks as well as dominating the tectonism in the Dalradian Supergroup.

Field and isotopic studies are integrated in zones of ductile thrusting (sliding) which are typified by belts of tectonic schists with related swarms of quartz and muscovite-bearing pegmatite veins. Of particular significance is a ductile thrust (the Grampian slide) which separates deeper crustal rocks (the Central Highland division), interpreted as showing the imprint of the Grenville orogeny, from shallower rocks (the Grampian division) representing a supracrustal assemblage formed between the Grenville and the c. 750 Ma events.

The Grampian slide is the structurally highest member of a system of related, previously unrecorded slides affecting the Central Highland division. New structural, petrographic and Rb-Sr isotopic data, obtained largely from a recently recognised inlier of the Central Highland division at Laggan, bear out that the quartz and pegmatite veins are segregations formed during ductile shearing under amphibolite facies conditions. Muscovites from these veins yield ages between 780 and 730 Ma, and a regression analysis of tectonic schists and the muscovites gives an age of 740 ± 40 Ma. These data substantiate our previous hypothesis that the deeper-seated Moine rocks were affected by a distinct orogenic event at 750 ± 30 Ma.

In the Northern Highlands, similar vein swarms are related to the Sgurr Beag slide and to belts of previously unrecorded tectonic schists in the Glenfinnan division of the Moine assemblage. A new, 755 ± 8 Ma age obtained from such a tectonic schist at Kinloch Hourn, combined with previous, similar age data from lensoid pegmatites, imply that the c. 750 Ma event may have also affected the Morar and Glenfinnan divisions.

A classification is proposed, based mainly on major element analytical data plotted in a coherent set of three simple chemical-mineralogical diagrams. The procedure follows two complementary steps at two different levels. The first is concerned with the individual sample: the sample is given a name (e.g. granite, adamellite, granodiorite) and its chemical and mineralogical characteristics are determined. The second one is more important: it aims at defining the type of magmatic association (or series) to which the studied sample or group of samples belongs. Three main types of association are distinguished: cafemic (from source-material mainly or completely mantle-derived), aluminous (mainly or completely derived by anatexis of continental crust), and alumino-cafemic (intermediate between the other two types). Subtypes are then distinguished among the cafemic and alumino-cafemic associations: calc-alkaline (or granodioritic), subalkaline (or monzonitic), alkaline (and peralkaline), tholeiitic (or gabbroic-trondhjemitic), etc. In the same way, numerous subtypes and variants are also distinguished among the aluminous associations using a set of complementary criteria such as quartz content, colour index, alkali ratio, quartz–alkalies relationships and alumina index.

Although involving a new approach using partly new criteria, this classification is consistent with most of the divisions used in previous typologies. The method may also be used in the classification of the volcanic equivalents of common plutonic rocks.

A structural reinterpretation of Appin Group metasediments of the Dalradian Supergroup in part of the northwestern marginal zone of the Irish Caledonides is presented. No major F1 folds have been recorded although bedding occasionally faces upwards NW in S1. The structure is dominated by a series of NW-facing F2 recumbent folds and associated D2 tectonic slides. The slides, which have thrust the overlying rocks to the NW, are generally oblique to the fold axial planes and commonly cut out major F2 fold hinges. Rotation of the hinges towards the NW-SE (Caledonian) stretching direction in areas adjacent to the slides may have contributed to fold excision: an example of a major dislocated sheath fold is described. DM3 produces major SE-verging inclined folds. Although D4, D5 and D6 are of minor importance in the area D6 intensifies towards the SE to become the principal fabric in a major NE–SW-trending sinistrai shear zone that contains the c. 400 Ma Main Donegal granite. The early major structures of this area are compared with those in Dalradian Supergroup of adjacent parts of Scotland. It is suggested that the absence of a major F1 fold (analagous to the Islay anticline) and the occurrence of D2 recumbent folds and low angle thrusts in NW Ireland reflect the close proximity of the foreland, composed of the Lewisian complex, to the NW; such a situation is analagous to the Moine thrust zone in Scotland. The progressive development of low angle structures and thrusts to the W of the Scottish mainland is attributed to a westward shallowing of the Lewisian basement between the SW Highlands of Scotland and NW Ireland.

A brief account of the stratigraphy of the Westphalian (Upper Carboniferous) of the Ayrshire Coalfield is given. Older rocks are found to be overlain by Westphalian strata of differing age in various parts of the coalfield. A correlation of the coal seams known by numerous local names in various areas of the coalfield is shown. The vertical distribution of species of non-marine bivalves is illustrated and the faunas associated with various horizons discussed. The presence of variants of Anthraconaia pulchella Broadhurst at more than one horizon is noted. The lateral distribution of species at given horizons is examined and it is shown that elements of the overall fauna of an horizon may be absent in parts of the coalfield. The occurrence of marine bands in the succession is noted and their distribution discussed. It is suggested that during the Aegiranum Marine Band incursion the northern and north-western parts of the coalfield were nearer a land mass. A new stratigraphical name, the Bogton Marine Band is proposed. The band occurs within the Upper Similis-Pulchra Chronozone.