Identification of terranes is frequently based on the apparent disparity of source area for sediment supply, palaeontological evidence and the presence of faults whose sense and amount of displacement is sometimes conjectural. As an example of such superficially unrelated stratigraphies, the Silurian successions of the Midland Valley of Scotland and the northern part of Ireland are compared. A diachronous late Llandovery (early-middle Telychian) transgressive episode, which is earlier in the Midland Valley inliers, is common to many of the successions. The transgression resulted in the development of comparable sedimentary facies and fossil assemblages. The clast compositions of Llandovery and Wenlock conglomerates suggest erosion of a common provenance of volcanic rocks founded on a metaquartzite basement. Areas of active volcanism appear to have migrated in a north-westerly direction with time; a similar sequence is evident in Canada. It is suggested that the common features of the Scottish-Irish successions may be accommodated in a unified basin model for Silurian sedimentation and volcanism. Such a basin may have formed in an intra-arc environment and been controlled by oblique-slip fault mechanisms although evidence for the presence of a Silurian arc is equivocal.

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