An extensive deposit of white, granular, massive gypsum has been discovered in central Thailand. The gypsum, a part of the Mesozoic Korat group, was deposited along with salt by evaporation of sea water in shallow basins extensively developed in Thailand and adjacent parts of Laos. It now occurs in a narrow, deep downfaulted block enclosed within a complex of Paleozoic sedimentary rocks and younger volcanic rocks, and is bounded to the east and west by north-trending high-angle faults. During downfaulting to its present position, the gypsum was altered to anhydrite. The upper part of the deposit has been reconverted to gypsum during the present cycle of erosion to depths of about 80-120 feet. Total reserves may exceed 25 million tons of gypsum with perhaps 10 times as much anhydrite.