Summary: A stevensite occurring as an alteration product of pectolite in amygdale-like structures within the Whin Sill is described. Data given for the stevensite include optical properties, stain reactions, chemical analysis, X-ray powder data, and D.T.A. curves. The close similarity between many of the properties of sepielite and stevensite is indicated and D.T.A. is shown to be the most satisfactory method of distinguishing between the two minerals.