The regional distribution of quantitative data plotted on a map may be tested for randomness. The Chi-square index is used. The determined probability is defined as the probability of randomness (P r ). As an example, Morrison Formation [Jurassic] heavy-mineral data from the Colorado Plateau region is used. The region is objectively subdivided into 4 areas, and the expected values are computed on the basic assumption that the regional distribution is random. A probability of randomness is obtained for each subdivision and for the region as a whole. The use of various geologic or otherwise subjective boundaries makes it possible to test the significance of apparent concentrations of high, low, or group values. The results constitute valuable mathematical evidence which may be used to support, modify, or establish degree of confidence in the geologic interpretation regionally distributed data.