Abstract
Synopsis: On the basis of recent chemical analyses the nomenclature of the Tertiary intermediate lavas in Antrim is reviewed and the overall petrogenic relationships between all the Tertiary volcanic rocks in Antrim are assessed. The conclusion is drawn that the intermediate and acid lavas result from differentiation of the basalts along a tholeiitic trend via silica-saturated intermediate compositions. Therefore the use of the terms mugearite and trachyte as previously applied to the intermediate derivatives in Antrim should be discontinued and a tholeiitic nomenclature substituted.