Cases of metamorphosed limestones where the calcium carbonate has been converted by contact with some igneous magma into a calc-silicate rock are by no means rare, one of the best known being the conversion of the Coniston Limestone of the Lake District by the Shap Granite into a rock with various calc-silicate and other minerals such as Wollastonite, Omphacite, etc., as described by Messrs. Harker and Marr. But as far as I know, no case has been recorded where ordinary white chalk has been similarly changed. Dr. Hibsch describes a case of baculite marl containing Foraminifera which has been altered by contact with dolerite. The Foraminifera disappear and the rock becomes a granular limestone with epidote, forming a calc-silicate hornstone. This seems the nearest case to the alteration in co. Antrim I am about to describe.