Summary: Many specimens of plinthite, a species described from Co. Antrim by T. Thomson in 1836 and later from Skye by M. F. Heddle, prove to be mixtures of zeolites and hematite. Both original analyses are those of a ferruginous clay (a mixture of hematite, kaolinite, and montmorillonite). The type specimen (Thomson Collection) is a red clay in the montmorillonite group intimately mixed with hematite and a little analcime.