Analcime has been determined to be a prominent and characteristic mineral in the Popo Agie "beds," a member of the Chugwater formation. It is responsible for the distinctive lithology and physical properties which set out the Popo Agie from other western Mesozoic red beds. The analcime-containing beds are up to 60 feet in thickness and are wide-spread in western Wyoming. Only one other sedimentary occurrence of comparably large quantity of analcime has been reported, parts of the Green River formation. The analcime in the Popo Agie is believed to have originated from the interaction of hydrous aluminum silicate clay minerals with sodium-rich waters of a large interior marsh-lake. It probably represents an aluminum silicate mineral stable under the special saline environment, analogous to certain clay minerals which are stable under soil-forming or marine environments. The analcime-bearing strata constitute a stratigraphic marker within a red bed sequence which, because of insufficient fossil zones or physical datum beds, has been perplexing in stratigraphic studies. It may aid in future stratigraphic work. The large quantity of analcime available invites the attention of industrial mineralogists. Included in its zeolitic properties is the capacity to react with acid and release silica gel and recoverable aluminum compounds.