Pre-Palaeozoic basement of the Scottish Midland Valley

THE tuffs and agglomerates associated with Carboniferous volcanism within the Scottish Midland Valley comprise predominantly basic volcanic and unmetamorphosed sedimentary fragments. For the most part, the former can be confidently ascribed to the contemporary basaltic volcanism whereas the sedimentary materials seem to be derived from Carboniferous and possibly Devonian strata. In spite of the fact that the bulk of the clasts within the volcanic vents are undoubtedly of shallow crustal origin and are unlikely to have had a vertical travel of more than a few thousand metres, others—in particular the spinel lherzolite clasts known from several vents—have probably arisen from sub-crustal levels. In the Elie Ness vent, high pressure pyroxenes and pyrope garnets indicate rapid evation from depths in excess of 60km (ref. 1). It would be surprising if such violent, gas-fluidised eruptions, capable of carrying mantle materials, entirely failed to sample lower levels of the crust. We have to call attention to a previously undescribed assemblage of metamorphic rocks at the Partan Craig vent [NT 567852] on the southern side of the Firth of Forth, some 1.5km east of North Berwick Harbour. The locality is situated roughly midway between the lines of the Southern Upland Fault and the extended line of the Pentland Fault.