Gismondine is recorded from the Tertiary basalts of the North Atlantic region, from a total of some 40 localities in Antrim and Iceland. It occupies a well-marked position in the sequence of zeolite zones in the lavas, and the characteristic associates are chabazite, thomsonite, and phillipsite. Five new chemical analyses of gismondine from these localities show a negligible content of potash, and indicate plagioclase-type substitution, giving a series ranging from near Ca₈Al₁₆Si₁₆O₆₄.34H₂O to near Na_2·5Ca_5·5Al_13·5Si_18·5O₆₄.34H₂O. Substitution of NaSi for CaAl leads to a decrease in refringence, birefringence, and density.