Sediments, mainly sandstones, conglomerates and shales, accumulated in small turbidite fans along the northern arc–trench margin of the Iapetus Ocean from middle Ordovician to Silurian time. These fans, together with the underlying pelagic facies and part of the oceanic crust, were sliced and accreted northward resulting in the Lower Palaeozoic accretionary prism which forms the Scottish Southern Uplands and the Longford-Down inlier in Ireland. North Down is the continuation of the Northern belt of the Southern Uplands of Scotland into Ireland, bounded to the S by the Orlock Bridge fault. Lithological and petrographical comparison with the rest of the Northern belt indicates closer affinities with the Southern Uplands of Scotland than with the western end of the Longford-Down inlier. Major ENE—WSW-trending Caledonian strike faults define five blocks, in which new formations of Caradoc and ? Ashgill age are defined. Pillowed spilitic rock, interpreted as a fragment of the ocean-floor, is only recognised in the Ballygrot block. Pelagic and hemipelagic black shales and cherts are overlain by arenaceous sediments in all blocks.

Calc-alkaline lamprophyres are porphyritic dyke-rocks, richer in amphibole, biotite, ultramark elements (Mg, Cr, Ni) and incompatible elements (K, F, P, Rb, Sr, Zn, Nb, Ba, REE, Th, U) than other rocks of comparable colour index (35–67) or % SiO2 (46–57). Field and petrological criteria are suggested for identifying them uniquely. New average compositions, based on some 600 screened analyses, confirm the chemical variability but possible heteromorphism of vogesites, kersantites, spessartites and some minettes. Calc-alkaline lamprophyres probably crystallise from volatile-rich, crystal-laden fluids. Microdioritic ‘porphyrites’, K-rich syenites and quartz-feldspar porphyries commonly form from these fluids by acidic hybridisation, and more rarely by in situ differentiation. Calc-alkaline lamprophyres have close compositional equivalents among, for example, shonkinites and absarokites, but not among kimberlites or common plutonie or volcanic rocks. They are compositionally more ‘crustal’ than lamproites and leucitites, despite some overlap. They are far more widespread than other K-rich or alkaline rocks. Three genetic petrological associations are confirmed: with calc-alkaline granitoid plutons (A), with shoshonitic volcanic and subvolcanic suites (B), and with appinite—breccia-pipe complexes (C). Most calc-alkaline lamprophyres, from association A, perhaps form by crustal modification of primary lamproitic or leucititic magmas; a very few, carrying rare mantle-type xenoliths, may represent relatively unmodified, but otherwise similar, primary magmas. Those of association B may form merely by volatile enrichment of shoshonitic magmas during subvolcanic crystallisation. Different origins for minettes in these associations are suggested by compositional differences, revealed by discriminant analysis.

Svecokarelian migmatites in the Jussarö area of the southern Finnish archipelago comprise tonalitic gneiss, amphibolite, metagabbro, microgranite, the metamorphosed derivatives of sediments, including carbonates and graded argillites, together with later-emplaced potassic granite, and basic and acidic pegmatites. The rock assemblage shows evidence of repeated deformation; a succession of 15 sets of folds (and fold-related structures) is identified and documented. There are two sets of early intraf oliai isoclinal folds (Faa and Fab), isoclinal folds (Fbr), recumbent folds (Fx), upright open folds (Fy), tight upright folds (Fbb), two sets of asymmetrical folds (Fc and Fd), ‘scar’ folds (Fb1, Fcb Fd1), slip folds Fb, conjugate shear folds (Fe), angular folds (Fn), open warps (Fw) and tight ductile shears (Fds). From isotopie evidence reported elsewhere, the evolution of this crustal segment of the Svecokarelides is known to have spread over a period of approximately 200 m.y. from c. 1900 m.y. to c. 1700 m.y.