Summary: The type locality for erionite has been identified and specimens compared with Eakle's original material. New determinations of the physical properties and a chemical analysis have been made. Correlation of X-ray powder and fibre rotation data with electron diffraction of single crystals has shown that the unit cell is hexagonal with a 13·26, c 15·12 Å., space-group P31c, P¯31c, P6₃mc, P¯62c, or P6₃/mmc, giving a calculated sp. Gr. of 2·07 and atomic cell contents (Ca, Mg, Na₂, K₂)_4·5Al₉Si₂₇O₇₂.27H₂O. A structure is proposed for the aluminosilicate frame, with space-group P6₃/mmc, which gives satisfactory correlation between observed and calculated structure factors for the X-ray fibre rotation pattern.