Increasing dissolved salt concentration decreases the thermodynamic or chemical activity of H 2 O in a solution and as a result lowers the equilibrium water vapor pressure exerted by the solution. The calcium sulfate facies minerals of marine evaporites are precipitated from brines with a H 2 O 0.93-0.76; halite facies minerals from brines with a H 2 O 0.76-0.67; potash facies minerals from brines with a H 2 O less than 0.67. Thus halite can only be precipitated in those areas where the mean relative humidity of the atmosphere is less than 76%. Mean relative humidities of many low latitude coastal and oceanic areas, even in the most arid regions, commonly hover between 70 and 80%. Lower mean relative humidities occur over marine areas surrounded by large expanses of land. Thus only marine basins with a high continentality are likely to precipitate potash facies evaporites. It is suggested that some evaporite basins may be restricted to the calcium sulfate mineral facies only, even if the basin becomes entirely cut off from the world ocean, the controlling factor being a high mean relative humidity.