Marble and limestone briquettes were placed at National Acid Precipitation Assessment Program (NAPAP) test sites in North Carolina, Washington, D.C., New Jersey, and New York to determine mineralogical changes that might be attributed to acid deposition. Samples have been examined after exposures of 1 and 2 yr, and the most significant change is the development of a gypsum-rich "spot" on the sheltered side of the briquettes. X-ray and SEM analyses reveal that gypsum plus calcite is present within the "spot" area, but outside this area and on the upper surface of the briquettes, only calcite is detected. A model, based on the sequence of salts observed to crystallize from a progressively more concentrated solution, is presented to explain the presence of the "spot" on the undersides of the briquettes. In the models, the CaCO 3 -saturated solutions filling the pore space in the stone continuously precipitate calcite during the drying period after the rain event; gypsum is precipitated only after evaporation is nearly complete. As evaporation proceeds, the solution migrates by gravity to the lower surface of the briquette and the last residual liquid precipitates gypsum and produces the gypsum-rich "spot". It is proposed that the most significant stone damage is due to salt build up on and within the stone rather than due to stone removal through dissolution.--Modified journal abstract.