AbstractThin beds of pale green and grey ‘claystone’ in the Birkhill Shales (Lower Silurian) of County Down are positively identified as bentonite. In the overlying turbidites, previously regarded as entirely of sedimentary origin, numerous metabentonites demonstrate continuing Silurian vulcanicity.The metabentonites contrast with the enclosing sediments in mineralogy and geochemistry. Illite, with subsidiary chlorite, is the chief alteration product of the primary volcanic glass, and comprises two co‐existing polymorphs. Clay‐grade 1Md mica is derived from montmorillonite, and a coarser 2M variety may be an anchimetamorphic alteration of the former. Some beds additionally contain accessory phenocrysts of orthoclase, oligoclase, biotite or chlorite, and small amounts of apatite and ilmenite. Statistical analysis of the geochemical data indicates that primary alteration of volcanic glass to bentonite was accompanied by relative enrichment of Al, K, Rb, Zr, Nb, and Y, and depletion of Na; additional enrichment of K and Rb occurred with collapse of mixed‐layer clay to illite during Caledonian anchimet