Like other great tholeiite sheets of the world, the Midland Valley Sill and Whin Sill were emplaced in thick accumulations of sediments which, at the time of intrusion, were not much older nor had been orogenically deformed. However, the Carboniferous sediments of northern Britain had been deposited under the influence of continuous differential subsidence, so that they were already disposed in syn-sedimentary basins and swells when intrusion began. By drawing palinspastic contours of the sills relative to a late Westphalian marine band (approximating to the surface at the time of emplacement) it is established that the dolerites mainly follow bedding planes, dipping at angles of as much as 5 ° down to the bottoms of the basins. Isopachs of the sills indicate a further relationship, for the intrusions are thickest at the bottoms of the basins. These facts are not explained by any intrusion mechanism previously postulated, though there seems to be broad agreement as to the processes which have given rise to thinning and en echelon lensing at leading edges as well as to step-and-stair transgression of sediment bedding. It is proposed that emplacement must have been controlled in part by gravitational flow down dip from feeder dykes which extended to between 0.5 and 1.0 km below the contemporary surface. Thereafter hydrostatic equilibrium was attained first by accumulation at the bottom of the basins and then by further advance up-dip under pressure of head. The sheets are so similar in aspect to flood basalts that it must be assumed that the sediments offered only low frictional resistance to splitting and flow.

In the type area, and southwest of it, the Capel Curig Volcanic Formation comprises up to three ash-flow tuffs interbedded with marine sediments. The tuffs are generally massive and welded, but pass laterally and upwards into current bedded, ripple marked tuffs. They are believed to have been deposited in a subaqueous environment, after originating from subaerial eruptions farther north where the Formation is represented by an uninterrupted sequence of intensely welded ash-flows. The lower contacts of the submarine ash-flows contrast with subaerial ash-flows in being welded and by locally transgressing the underlying sediments at angles up to 90°, with minor apophyses resembling magmatic intrusions. Eutaxitic foliation, however, remains generally parallel to the regional dip. The sediments adjacent to the transgressive undersurfaces are disturbed and reconstituted. The irregular contacts, together with adjacent discrete pipe-like tuff bodies are interpreted as down-sags analogous to load casts but of unprecedentedly large scale, up to 100 m, formed by the liquefaction and yielding of the sediments during and after the emplacement of the tuffs.

Archival and recent boreholes over an area of c. 3 km2 have revealed complex magma-host interaction at the termination of an olivine–dolerite sill in Fife. The sill interior has zones rich in plastically deformed, vesiculated heterogeneous sediment surrounded by amygdaloidal basalt. Sediments at the contacts have been reconstituted and enclose blebs of chilled vesicular basalt. Intrusion into low rank coal seams has produced multicomponent tuffisite. A vertically nested and laterally fingered sill front is envisaged as having propagated down dip under a thin cover (<500 m) of wet Namurian sediments. Non-explosive bulk interaction of fluidised sediment and devolatilising magma occurred at intrusive contacts. Steam explosivity was more vigorous where lobes of magma repeatedly intruded moist lignite, to produce compositionally banded tuffisite rich in basalt clasts and coal fragments. The hydrovolcanic explosions did not give rise to surface eruptions because the low volumes of porewater and the high permeability and low tensile strength of the lignite prevented a build-up of high pressure steam.

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Summary This paper reviews the state of knowledge of the volcanic activity which accompanied the development of the Caledonide orogen in Britain and Ireland. Unlike the relative abundance of stratigraphical data, geochemical information is scarce in earlier accounts, but in the last decade or so there has been a surge of interest coincident with the introduction of rapid analytical techniques and with the development of methods for distinguishing primary petrographical and chemical characteristics from those of subsequent alteration. Many of the Caledonide volcanic rocks were erupted into or beneath water, interbedded with sediments and then affected by orogenic deformation and metamorphism, yet this has apparently had less effect than might be expected on the gross geochemistry of the volcanic rocks. In the paratectonic Caledonides, submarine weathering and low-grade hydrous metamorphism have been the principal causes of element mobility, and the effects produced are, in many cases, remarkably similar to those which affect present-day eruptives in oceanic environments.